GeoJournal DOI 10.1007/s10708-014-9564-8
Assessing the impact of demographic characteristics on spatial error in volunteered geographic information features William F. Mullen • Steven P. Jackson Arie Croitoru • Andrew Crooks • Anthony Stefanidis • Peggy Agouris
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Ó Springer Science+Business Media Dordrecht (outside the USA) 2014
Abstract The proliferation of volunteered geographic information (VGI), such as OpenStreetMap (OSM) enabled by technological advancements, has led to large volumes of user-generated geographical content. While this data is becoming widely used, the understanding of the quality characteristics of such data is still largely unexplored. An open research question is the relationship between demographic indicators and VGI quality. While earlier studies have
W. F. Mullen (&) S. P. Jackson A. Croitoru A. Stefanidis P. Agouris Department of Geography and GeoInformation Science, George Mason University, 4400 University Drive, MS 6C3, Fairfax, VA 22030-4444, USA e-mail:
[email protected] A. Croitoru e-mail:
[email protected] A. Stefanidis e-mail:
[email protected] P. Agouris e-mail:
[email protected] A. Crooks Department of Computational Social Science, Krasnow Institute for Advanced Study, George Mason University, 4400 University Drive, MS 6B2, Fairfax, VA 22030-4444, USA e-mail:
[email protected]
suggested a potential relationship between VGI quality and population density or socio-economic characteristics of an area, such relationships have not been rigorously explored, and mainly remained qualitative in nature. This paper addresses this gap by quantifying the relationship between demographic properties of a given area and the quality of VGI contributions. We study specifically the demographic characteristics of the mapped area and its relation to two dimensions of spatial data quality, namely positional accuracy and completeness of the corresponding VGI contributions with respect to OSM using the Denver (Colorado, US) area as a case study. We use non-spatial and spatial analysis techniques to identify potential associations among demographics data and the distribution of positional and completeness errors found within VGI data. Generally, the results of our study show a lack of statistically significant support for the assumption that demographic properties affect the positional accuracy or completeness of VGI. While this research is focused on a specific area, our results showcase the complex nature of the relationship between VGI quality and demographics, and highlights the need for a better understanding of it. By doing so, we add to the debate of how demographics impact on the quality of VGI data and lays the foundation to further work. Keywords Volunteered geographic information OpenStreetMap Spatial analysis Spatial data quality Demographics
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Introduction Recent years have been characterized by a significant shift in the way geographic information is produced, specifically through the rise of volunteered geographic information (VGI; Goodchild 2007), which has been fueled through Web 2.0 technologies (Hudson-Smith et al. 2009). While this shift has resulted in an increase in the volume and richness of geographic data, it is also posing some significant challenges with respect to evaluating the quality of such data. The fact that volunteers with minimal (if any) geographic training are contributing such information (Mooney et al. 2010), as well as the mechanisms that govern the contribution process, brings to question the quality of such data (Sui 2008). An exemplar of this trend is OpenStreetMap (OSM), an open source collaborative mapping project that aims to generate an editable global map database (Mooney and Corcoran 2012). Earlier studies addressing the quality, in particular positional accuracy, of VGI with respect to OSM have pointed out the lack of homogeneity (e.g. Hochmair and Zielstra 2013; Koukoletsos et al. 2012). In order to understand these issues, researchers have explored the relationship of VGI data and its quality with various geographical and demographic indicators. Studies addressing the relation between population density and VGI have indicated that in densely populated urban areas more contributions can be expected, which may ultimately lead to higher quality (Haklay 2010). At the same time, Girres and Touya (2010) noted that population density is not the only factor controlling the positional accuracy of VGI, indicating that areas with higher income and younger population are characterized by higher number of contributions. Further efforts to identify discernible spatial patterns in VGI quality (e.g. by comparing the topology of volunteered road network data and commercially available data) did not find statistically significant correlation (Neis et al. 2011). A common assertion that has emerged from such studies is that the demographic characteristics of the contributing volunteers may impact the distribution of the positional and shape errors in VGI (Fairbairn and Al-Bakri 2013). Certain demographic indicators that have been explicitly suggested as potentially contributing to data quality patterns, include race and economic status (e.g. Tulloch 2008; Elwood 2008; Graham 2005; Zook and Graham 2007a, b; Crutcher and Zook 2009). However, these earlier studies were
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primarily qualitative in nature and were not accompanied by rigorous quantitative analyses. Motivated by this gap, this paper aims to examine the quantitative relationships between VGI quality—in particular positional accuracy and completeness—and demographic properties. We do so through a case study in Denver, Colorado (CO), where we use VGI data and contrast it with local demographics from the United States (US) Census Bureau. The remainder paper is organized as follows: in section ‘‘Background and motivation’’ we provide background information with respect to the demographic characteristics of VGI. In section ‘‘Data and methods’’ we present the background and rationale for our approach to assess the impact of demographic variation on the quality of VGI. In section ‘‘Results and discussion’’ we present the results of our analysis, and conclude with our summary and outlook in section ‘‘Discussion’’.
Background and motivation When attempting to assess the quality of VGI content we need to be cognizant of the particular nature of the volunteering process that differentiates VGI contributions from the traditional and established processes through which geographical data had been collected. While VGI was enabled by technological advancements, it gained popularity primarily because it addressed the general public’s growing need to access geographical data for a constantly increasing array of activities. This is the reason why OSM, the prototypical example of VGI, emerged and grew in the United Kingdom, where geographical data were not as freely distributed by government agencies (Haklay and Weber 2008). In contrast, the US government policy has leaned more towards openly sharing much of its geographical data. Up to this point, efforts to assess the quality of OSM content have focused primarily on road networks, as OSM was intended after all to be a ‘street map’. Such studies have compared for example OSM road data to a reference data set, in order to assess OSM relative to an authoritative standard such as the U.K. Ordnance Survey road network data sets or comparable products (Brown and Pullar 2012; Haklay 2010; Hochmair and Zielstra 2013; Koukoletsos et al. 2012; Neis and Zipf 2012; Neis et al. 2011). However, as OSM has evolved well beyond streets we are now in need to assess the accuracy of other types of features as
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well, in order to gain a more thorough understanding of quality issues with respect to VGI. This need is further emphasized by the fact that large volumes of road data (e.g. TIGER/line files), which were made freely available by the U.S. Census, have been bulk-uploaded into OSM since 2009 (OSM 2013b). Accordingly, road content in OSM (especially in the US) does not comprise solely VGI contributions, but a hybrid aggregate of VGI and authoritative data. Therefore, a study of non-road features (especially ones that do not include bulk uploads of authoritative content) will complement the current body of work and enhance our understanding of the accuracy of OSM contributions, and this is one of the contributions of this paper. Furthermore, there is a lack of understanding with respect to the spatially-driven motivations for VGI contributions: why do people contribute for some areas and not others? For example Zielstra and Zipf (2010) contrasted the differences between VGI and commercial data sources in Germany against population density, noting that the completeness of the VGI degraded considerably as the distance from the urban core increased. Conversely, rural areas with low population densities are less likely to attract VGI contributions. However, our understanding of how this participation varies among locations with similar population density remains largely unexplored. For example, Goodchild and Li (2012) found that Linus’ Law is not as effective for geographic facts as it is for other information like Wikipedia, and that population density alone is not sufficient to explain the trends in the data error. A study of the quality of contributions as it relates to the demographics of the place will therefore advance our understanding of the relation between population characteristics and VGI quality, and this is the second contribution of this paper. In the following two subsections we provide review of accuracy and completeness in the context of OSM (section ‘‘Positional accuracy and completeness in OSM’’), followed by a discussion of the characteristics of the contributors to VGI—both in terms of their motivation and their demographics (section ‘‘The motivation to contribute to open source initiatives’’). Positional accuracy and completeness in OSM In the context of this paper the term accuracy is used to refer to positional accuracy, namely the closeness of
the coordinate values of a VGI feature (e.g. a point) to its corresponding authoritative equivalent feature based Euclidian distance. The term completeness refers to the extent to which features are included or omitted from a dataset, again in comparison to the authoritative equivalent. In that sense, if one were to consider schools, accuracy would refer to how close a VGI record of a school is to its corresponding authoritative record, whereas completeness would refer to the percentage of schools that have been mapped in a VGI dataset. Both terms are indicators of the overall quality of a dataset, a term which in this paper is used to refer to the overall fitness for use of a dataset. Past efforts to assess the quality of VGI contributions have primarily focused on positional accuracy. Haklay (2010) compared OSM data and the United Kingdom’s (UK) Ordnance Survey road centerline data, finding that OSM data road centerlines are displaced on average by 5.83 m for selected areas within London. Girres and Touya (2010) compared French road data and reported an average displacement of 6.65 m, which is consistent with Haklay (2010). These studies however, did not examine any spatial variations of accuracy. In an effort to address this issue, Al-Bakri and Fairbairn (2010) performed a localized study in the UK, assessing the accuracies of nodes and linear segments of polygonal VGI entries. They found that node accuracy degraded between urban and the more rural ‘peri-village’ areas, noting errors of 9.6 and 11.0 m respectively. For linear segments, the authors measured an average displacement from the surveyed results of 1.5 m using uniformity of buffers established for both the reference and OSM data. The spatial heterogeneity of VGI errors was also pointed out by other researchers who noted that inaccuracies were often localized to specific areas (e.g. Girres and Touya 2010; Haklay 2010; Zielstra and Zipf 2010). In an effort to understand these accuracy variations, Haklay et al. (2010) showed that there was no observable correlation between the number of contributors and quality of VGI data once that number reaches a certain level. Considering the lack of discernible spatial patterns of accuracy variations, the research community turned its attention to the motivation behind VGI contributions, as a potential explanation for these variations.
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The motivation to contribute to open source initiatives Open source initiatives exceed the purview of the geographical community, with similar efforts seen in a very broad range of activities, ranging for example, from Wikipedia and open source software development projects to citizen science. Nevertheless, regardless of the topic, there exist certain general motivating factors that drive participation in such efforts. For example, Oreg and Nov (2008) identified the three primary motivational elements for contribution, ranked by priority of importance, as: self-development, altruism, and reputation building. Kuznetzov (2006) argued that for Wikipedia, perhaps the ‘poster child’ for crowdsourcing efforts, the key motivation is a sense of community and accomplishment. Vickery and Wunsch-Vincent (2007) noted that the motivations driving these contributions of user-generated content are primarily technological, social, economic, and institutional or legal considerations. In essence, if it’s not difficult, illegal, or inconvenient to contribute the information and there is some motivation in the form of intrinsic social or economic return achieved by the contributor, then there will be information contributed (Nov et al. 2011). While the above drives public participation, the very nature of public participation introduces biases in volunteered content. These biases result in variations in the patterns of contribution and the accuracy of the contributed content itself. Such biases stem from four key areas: Internet access, knowledge of language, available time, and adequate technical capability to support the editing functions required of the contributors (Holloway et al. 2007). As these areas are closely associated with demographic properties, it is only natural to move towards studying VGI content relative to the corresponding demographic information of an area, which is a direction that we pursue in this paper. This is also consistent with earlier work by Porter and Donthu (2006) and Longley and Singleton (2009), who also identified the correlation between Internet usage and several demographic indicators (e.g. age, education, income and race). When we consider the above under the lens of VGI contributions in particular, an argument emerges about the relation between VGI contributions and demographics. Elwood et al. (2013) note that differences in social processes associated with VGI can impact the
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content and quality of the contributed data. Studies of OSM contributions have also noted that as the population density decreases so do OSM contributions (e.g. Zielstra and Zipf 2010; Girres and Touya 2010; Haklay 2010; Zielstra and Zipf 2010). However, population density itself is not the only factor affecting content and quality (Girres and Touya 2010; Schmidt and Klettner 2013) as it is not uncommon to have information gaps in highly populated areas (Cipeluch et al. 2010). Girres and Touya (2010) and Haklay (2010) noted a significant decrease in contributions for areas that were economically deprived or disadvantaged. This suggests that socioeconomic factors, such as income and educational achievement may affect OSM contributions, leading to complex spatial patterns of participation (Elwood 2008, 2009; Ghose and Elwood 2003; Sieber 2006; Tulloch 2008). Graham (2005) along with Zook and Graham (2007a, b) noted similar impacts as spatial queries and ‘softwaresorting’ techniques influenced by cultural differences which can create or enhance a bias in digital presentation or ‘perception’ of a place. Furthermore, Crutcher and Zook (2009), in a study of geographical data in the context of reporting the impacts of the 2005 Hurricane Katrina in New Orleans, Louisiana, stated that racial inequities were a key factor affecting access and use of digital technologies. However, while there have been a number of suggestions regarding the impact of the demographic element on the contribution patterns and quality of VGI data, this direction remains understudied, and in this paper we make a contribution by addressing this issue.
Data and methods The problem of exploring possible relationships between population demographics and VGI quality gives rise to significant challenges, both in terms of the required data and in terms of the analysis methods that are utilized. In terms of data, the study of demographics and VGI quality requires the availability of detailed demographics data that can be spatially related to a given study area. At the same time, it is also necessary derive VGI quality measures for the same study area. To derive such measures both VGI and reference data are required so that both positional accuracy and completeness could be calculated. At the analysis level, several key aspects should be considered. First,
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Fig. 1 The analysis workflow
the derivation of VGI quality measures requires a reliable method for conflating (i.e. matching geographic features between) the VGI data and the reference data. Second, as the possible relations between demographics and VGI quality can be considered as both non-spatial and spatial phenomena, both non-spatial and spatial analysis methods should be applied. When studied as a spatial phenomenon, the possible relations between demographics and VGI quality can emerge both at the local and the global geographic scales, requiring both types of spatial analysis. Finally, the high multidimensionality of demographics data and its type heterogeneity (e.g. data can include ranked-scale variables or intervalscale variables) requires both the ability to reduce such dimensionality and account for the various data scales. In view of these challenges, it is necessary to employ multiple data sources as well as an array of both nonspatial and spatial analysis methods. An overview of the data sources used in this study as well as the workflow that combines the different analysis steps and methods is shown in Fig. 1. In section ‘‘VGI quality and demographics data’’ below we describe the data sources (reference, VGI and demographics) used in this workflow, and in section ‘‘Analysis methods’’ we describe the methods used to explore possible relations between demographic indicators and VGI quality.
VGI quality and demographics data In order to explore the relation between demographics and VGI quality we build upon the previous work of Jackson et al. (2013), who proposed a methodology for quantifying the completeness and accuracy of point datasets in VGI data. In particular, given a VGI data set and a reference data set, the proposed methodology assessed completeness through a multi-step matching process that attempts to find for each feature in the reference data set a matching feature in the VGI data set. This results in matched features (for which a match in the VGI dataset was found) and unmatched features (for which a match in VGI dataset was not found). The rates of matched and unmatched feature in the VGI dataset can then be transformed into completeness rates based in the reference data set. However, that paper did not explore the relation between demographics and VGI quality. Consequently, the goal of research presented here is to investigate this relationship. In order to accomplish our objective we leverage the same datasets used in Jackson et al. (2013) and augment it with demographic information. The study area covers a large percentage of the City and County of Denver, including downtown Denver, extends into portions of the surrounding Arapahoe, Jefferson, and Adams Counties as shown in Fig. 2. Specifically the following datasets were used:
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Fig. 2 The study area
Oak Ridge National Laboratory (ORNL) data, which geographically located a Department of Education list of schools using the street address information. The ORNL data locates a point for each school at the street address. School locations from the Points of Interest (POI) layer of OSM. The POI layer represents each specific feature as a node. In OSM, it is common practice to represent area features as points (Over
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et al. 2010) and the guidelines provided within OSM indicate that the node should be placed in the middle of the site (OSM 2013a). Data from the United States Geological Survey (USGS) OSM Collaborative Project (OSMCP— 2nd Phase; Poore et al. 2012). We consider the ORNL data as the authoritative dataset in our analysis while the remaining two
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datasets represent different types of VGI data. The OSM POI school data represents ‘classic’ VGI while the OSMCP dataset represents a variant of VGI data in the sense that it introduces limited authoritative oversight to the VGI process, in the form or peerdeveloped quality control feedback to the volunteers (e.g. university students) as well as USGS feedback (Poore et al. 2012). This is part of a larger effort by the USGS to augment the National Map (2014) with VGI data pertaining to manmade structures (e.g. schools, hospitals, post offices, police stations etc.). As one might expect, the OSMCP effort would likely fall in the Expert Professionals range of the scale proposed by Coleman et al. (2009) since the final review of the data is conducted by USGS personnel. This has also been shown in Jackson et al. (2013), who indicated that OSMCP data was of higher quality both in terms of completeness and positional accuracy compared to OSM data. In order to assess the relationship between demographics and VGI quality the above-mentioned datasets were augmented with demographic data. Such data can be drawn from census and provide us with the respective characteristic properties (e.g. age, race, ethnicity, economic status, or gender) of the population under study (McKechnie 1983) at a specific point in time. In the US, the US Census Bureau has the responsibility to compile and publish the official demographic data, including conducting the decennial Census, at a variety of spatial scales (with Census blocks being the finest). Within our study we use Census tracts rather than Census blocks, as the latter lacks certain demographic information (e.g. ethnicity) due to privacy concerns. For example, the Census blocks could potentially lead to identification of individual households or businesses (US Census Bureau 2012). At the Census tract level demographic information such as: age, sex, education, employment, ethnicity, immigration, income, marital status, population, poverty level, and race are made available through a spatial join operation. Consequently, census tract data provides us with a rich source of georeferenced demographic data, which we can use for assessing how demographics impact on the quality of VGI data. As discussed above, prior research suggests that four broad demographic categories (i.e. population, economic status, educational achievement, and race/ethnicity) should be used to assess the impact of demographics on VGI accuracy and
Table 1 Census tract level demographic indicators considered in our study across four categories: population, economic status, education, and race/ethnicity Population
Education
Total population
Percent without high school (HS) diploma
Population density
Percent with HS diploma (and over 25 years old)
Median age
Percent with BA degree (or better) (and over 25 years old)
Percent male
Race/ethnicity
Percent female
Percent white
Economic status
Percent white—not Hispanic
Median household income
Percent African American
Median home value
Percent American Indian
Percent below poverty line
Percent Asian
Percent of homes receiving food stamps
Percent Hispanic
completeness. From among these categories we selected a set of 18 demographic indicators to use in our study as shown in Table 1. In conjunction with these indicators, the completeness and positional accuracy of the OSM and OSMCP were evaluated. For this purpose, the method described in Jackson et al. (2013) was used to derive the positional accuracy and completeness of each of OSM and OSMCP datasets with respect to ORNL dataset (which was considered as the authoritative source). This analysis resulted in completeness rates for each dataset (89 % for OSMCP vs. ORNL and 72 % for OSM vs. ORNL) and positional error distributions (47 m ± 50 m for OSMCP vs. ORNL and 190 m ± 314 m for OSM vs. ORNL). Furthermore, 59 % of the time OSMCP schools were closer to their ORNL reference entries, compared to their OSM counterparts, suggesting that that OSMCP outperforms OSM. These results, together with the demographics indicators, were then used to analyze possible relations between VGI quality and demographics. Analysis methods Our analysis is carried out in two modes: non-spatial and spatial. While in the non-spatial analysis mode we focus on the descriptive statistical characteristics of the relationship between demographics and quality, in
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the spatial analysis mode we focus on the geographic properties of this relationship. Below, we provide a concise description of each analysis mode (non-spatial and spatial) with a particular emphasis on the spatial analysis methods used. Non-spatial analysis In the non-spatial analysis mode we explore the relations between VGI quality and demographic indicators in four steps. First, we compare the distribution (i.e. histogram) of quality measured with respect to the distribution of the demographic indicators in order to determine whether accuracy errors or completeness errors are generated in tracts that fit specific demographic profiles. As some demographic indicators are continuous variables, a method for binning the data is required. In this study we approximate the distribution of the demographic indicators using histograms using a binning process. A number of statistical approaches have been developed to bin data, which generally fall into four categories: natural (Jenks); quantile; equal-interval; and standard deviation (Longley et al. 2010). While each of these approaches could be considered, no substantial differences in the results were observed between them in our case study. Consequently, natural (Jenks) breaks within each data element are used to establish five ranges within each of the selected demographic indicator to enable the exploration of emergent patterns in the distribution of a VGI quality measure across a demographic parameter (e.g. OSM completeness vs. median age). Following this initial analysis, Principal Component Analysis (PCA; Press and Wilson 1978) and discriminant analysis (Davis 1973) is applied in order to identify the demographic indicators that would best explain quality variations in the two VGI sets. Through this analysis, PCA can potentially lead to a reduction in the dimensionality of the demographic indicators, while retaining as much as possible of the variation present within the original dataset. This is achieved by transforming the original dataset to a new set of variables, the principal components (PCs), which are uncorrelated but are ordered so that the first few retain most of the variation present in all of the original variables (Andrews et al. 1996). As PCA can be sensitive to the scale of the variables, it may be necessary to rescale the demographic indicators.
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However, most of these indicators are already captured as percentages, therefore such scaling is not necessary. Specific non normalized indicators, such as total population, population density, median age, and median household income, must be scaled to fit within the range of 0–1 prior to the application of the PCA. The transformation to compute these new scaled values is straight-forward: the minimum and maximum attributes are determined, and then the minimum is subtracted from the maximum to establish the range, finally the minimum is subtracted from the actual value and the result is divided by the range, yielding scaled value for that attribute. PCA is particularly useful as a starting point for additional detailed analysis, such as linear regression, in that the PCA results may identify multi-colinearities within the data that can then be used to reduce the data dimensionality prior to additional analysis. By eliminating such colinearities, linear regression can be simplified. In order to explore and quantify possible correlations between demographic indicators and VGI quality measures both linear regression and discriminant analysis are used. Specifically, linear regression is used to assess the correlation between VGI positional accuracy and demographic properties (Burt et al. 2009), and discriminant analysis (Davis 1973) is used to assess the correlation between VGI completeness and demographic properties. In the linear regression analysis positional accuracy is used as the dependent variable and the demographic properties as the explanatory variables, and R2 values are calculated for both the OSM and OSMCP datasets. In the discriminant analysis completeness is used as the discriminate function, demographic properties are used as the explanatory variables, and Wilks’ lambda (Huberty 1984) is used to estimate the significance of the discriminant function. Though the combination of linear regression and discriminant analysis we are therefore able to assess how both positional accuracy and completeness relate to the various demographic indicators in the study area. Spatial analysis In the spatial analysis mode we analyze the spatial properties of the completeness and positional accuracy that were derived earlier. This analysis is carried out in two steps. In the first step we explore whether spatial patterns emerge in the VGI quality measures, and if so,
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whether such patterns can also be found in the spatial distribution of the studied demographic indicators. If found, the existence of spatial patterns in both data sets would provide supporting evidence to the possible association between VGI quality and demographic indicators. However, if patterns exist in one dataset and not in the other then such association would be unlikely. In this analysis, the term pattern refers to a non-random spatial distribution of the studied variables that is statistically significant (based on a relevant significance test). To accomplish this we utilize Nearest Neighbor analysis (NN; Clark and Evans 1954), Moran’s Index of Spatial Autocorrelation (Moran’s I; Moran 1950), and Local Indicators of Spatial Associations (LISA; Anselin 1995) to test for statistically significant spatial patterns. In the second step, we utilize regression to model any spatial associations between the VGI quality measures and demographic indicators. Specifically, we use exploratory regression as well as Geographically Weighted Regression (GWR: Fotheringham et al. 2002) to determine whether a statistically significant model (global or local) can be derived. We briefly describe each of these analysis methods below. NN analysis utilizes the average distance to the nearest neighbor data point to determine whether a point distribution is spatially random, clustered, or dispersed compared to a random point pattern. Based on this, the determination whether a given dataset is randomly distributed is made through a z-score statistic: absolute score values above 2.58 indicate a 99 % chance that the data is not randomly distributed, while values above 1.96 indicate a 95 % chance. Using the NN analysis, both completeness and positional accuracy can be analyzed for the OSM and OSMCP to determine whether any spatial patterns can be detected based on VGI completeness rates. While the NN analysis addresses the question of whether global spatial patterns exist in the quality of the two VGI datasets studied here, it does not address the question of whether such patterns exist at the local scale. To address this, we utilize LISA in order to identify areas where local clusters of higher or lower than expected values exist (Anselin 1995). Once a dataset is shown to exhibit spatial autocorrelation, LISA can identify significant areas within the overall dataset that generate such spatial autocorrelation. Accordingly, within our analysis LISA is used to determine which, if any, locations drive the spatial autocorrelation values that
are observed. Similarly to the NN analysis, the positional accuracy and completeness measures from both OSM and OSMCP will be used. In conjunction with the NN and LISA analyses for the VGI quality measures, similar analysis is carried out for the demographic indicators in order to explore whether any spatial patterns emerge. Specifically, we are interested in determining whether demographic indicators exhibit any significant spatial autocorrelation (Burt et al. 2009) in the study area. As the demographic indicators are associated with aerial features (i.e. tracts) we utilize Moran’s I to evaluate the global spatial autocorrelation between tracts. Similarly to the NN analysis, Moran’s I analysis determines whether a set of features and their attribute values are spatially randomly distributed, clustered, or dispersed. In addition to considering the attribute values of features to calculate autocorrelation, Moran’s I takes into the spatial relationships between features in the form of a spatial weights matrix, which can express both topological and metric spatial relations (Wong and Lee 2005). The statistical significance of Moran’s I is evaluated using a z-score test. After exploring the existence of spatial patterns in VGI quality measures and demographic indicators in the study area, our analysis turns to explore whether VGI quality could be modeled, either at the global or the local scale. At the global scale we utilize in this research exploratory regression (de Smith et al. 2007; Braun and Oswald 2011) and GWR at the local scale. Exploratory regression was carried in order to identify an appropriate regression model that utilizes the distribution of the regression errors to assess the overall regression quality. Building on Ordinary Least Squares (OLS), exploratory regression considers nonspatial regression quality indicators (e.g. R2 and residuals distribution) as well as the spatial autocorrelation within the residuals in order to evaluate the quality of a given regression model. The premise behind this approach is that a regression model should exhibit both good fit to the explanatory variables, normally distributed residuals (evaluated using the Jarque–Bera p value), and a lack of spatial autocorrelation between residuals (evaluated using a p-value condition on global Moran’s I test). In addition to the R2 and the p-values criteria, additional threshold conditions on the quality of the regression can be applied, for example a minimum number of model variables and a maximum Variance Inflation Factor
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GeoJournal Table 2 Summary of the exploratory regression criteria values
Non-spatial analysis results
Criteria
Threshold
Histogram analysis
Minimum adjusted R-squared
[0.50
Maximum p value
\0.05
Maximum VIF value
\7.50
The statistical distribution of the VGI quality measures can be analyzed through the study of histograms in an effort to determine whether positional accuracy or completeness are generated in tracts that is characterized by a specific demographic profile. Figure 3 illustrates this process using a sample histogram the percent white (non-Hispanic) demographic indicator versus completeness rates (i.e. percent of matched features) for both the OSM and OSMCP data sets. This histogram, in which natural breaks bins were used, provides a qualitative yet fast way to gain some initial insights with respect to any noteworthy trends in the relation between a given quality measure and a demographic indicator. Additionally, differences between the OSM and OSMCP data sets with respect to a given demographic indicator can be observed using such a histogram: when the difference between bars of the same bin is small, then the two datasets exhibit a similar behavior. Conversely, large differences between bars of the same bin imply that the demographic characteristic being examined may cause a difference between the results in the comparison. A pattern can then emerge if the overall behavior of one histogram across the bins is different from the behavior of the second histogram across the same bins. As can be seen from the example in Fig. 3, while there are some differences between the two histogram bars, it is difficult to identify a clear pattern. In our analysis, the process of creating the histogram was repeated for each demographic indicator in an attempt to identify any visible trends or patterns. A summary of the results of this process is provided in Table 3 for completeness rates in both the OSM and OSMCP data sets. An inspection of these results shows that, generally, none of the demographic indicators considered exhibit a clear trend or pattern.
Minimum Jarque–Bera p value
[0.10
Minimum spatial autocorrelation p value
[0.10
(VIF; Wheeler and Tiefelsdorf 2005). The exploratory nature of this approach stems from its iterative implementation, in which different combinations of the explanatory variables are tested until the best model is found. This analysis was implemented for OSM and OSMCP quality measures using the modeling thresholds in Table 2. At the local scale, GWR is utilized to study spatially localized regression models of the relation between VGI quality measures and demographic indicators. While based on principles similar to the global exploratory regression, GWR explores local regression models by allowing the regression coefficients to vary based on location in the study area. This is accomplished by a location-dependent weight matrix that expresses the spatial relations between data elements. As a result, we are able to explore whether there are any significant local associations between VGI quality and demographic indicators.
Results and discussion Following the workflow and analysis methods described in section ‘‘Data and methods’’, in this section we outline and discuss the results that were obtained through each analysis method. In particular, the results of the non-spatial analysis mode is described in section ‘‘Non-spatial analysis results’’, and the results if the spatial analysis mode is described in section ‘‘Spatial analysis results’’. The different analyses were carried out using the SPSS1 statistical analysis package (for non-spatial analysis) and the ArcGIS 10.1 Spatial Statistics Toolbox.2
1 2
http://bit.ly/1pFDgnd. http://bit.ly/1oTz1EL.
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Principal component analysis (PCA) As described earlier, PCA is utilized in this study as a tool for reducing the dimensionality of the data potentially simplify the linear regression analysis. Accordingly, PCA was used to analyze the relationship between the eighteen demographic indicators presented in Table 1 to determine whether all of them
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Fig. 3 Analyzing the histogram of percent white (non-Hispanic) versus completeness rates (percent of matched features from the ORNL data set) in the OSM and OSMCP data sets
Table 3 Summary of the histogram comparisons completeness in the OSM and OSMCP data sets (NV indicates no value) Demographic indicator
OSMCP completeness rates
OSM completeness rates
Bin 1
Bin 2
Bin 3
Bin 4
Bin 5
Bin 1
Bin 2
Bin 3
Bin 4
Bin 5
Total population
80.0
91.8
87.6
90.6
87.5
55.0
74.5
67.4
76.4
72.9
Population density
91.1
90.7
92.2
74.4
50.0
65.6
74.8
73.0
71.8
50.0
Median age
NV
81.0
92.0
91.6
92.3
NV
72.4
70.3
69.7
79.5
Percent male
NV
92.1
88.0
85.7
NV
NV
65.9
74.7
71.4
NV
Percent female
NV
85.0
87.7
90.8
84.2
NV
80.0
73.5
71.0
52.6
Median household income
80.5
92.2
89.1
92.5
91.7
74.4
69.9
68.3
75.0
91.7
Median home value
100
87.2
87.4
94.7
91.7
83.3
75.9
69.5
66.7
66.7
Percent below poverty line
90.8
92.0
90.4
81.6
75.0
72.4
70.4
71.3
72.4
75.0
Percent of homes receiving food stamps
91.1
88.6
90.4
81.0
100
70.4
70.7
74.7
71.4
100
Percent without HS diploma
89.9
90.3
88.2
87.2
88.5
66.4
73.1
71.8
69.2
96.2
Percent with HS diploma
NV
87.5
87.6
90.7
89.9
NV
77.3
71.1
73.2
66.4
Percent with BA degree
86.0
89.5
85.7
94.9
91.3
74.0
71.4
73.0
61.0
78.3
Percent white
100
80.0
91.9
89.0
89.8
77.8
75.6
83.8
67.1
67.7
Percent white—not Hispanic Percent African American
90.0 89.6
81.5 86.9
90.9 89.3
89.2 87.5
91.7 92.3
90.0 73.8
81.5 65.5
90.9 67.9
89.2 68.8
91.7 76.9
Percent American Indian
92.5
89.0
86.6
87.4
97.0
73.6
72.0
62.5
76.7
81.8
Percent Asian
92.1
90.4
81.6
87.0
100.0
69.5
77.7
66.7
68.5
93.3
Percent Hispanic
91.9
86.7
92.8
84.5
86.4
74.8
63.8
67.5
75.9
84.1
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provide unique information or whether they were mostly redundant. The 401 features (schools) in our data set, i.e. both features for which a match was found and for which a match was not found, were assessed. The results of the PCA identified five factors from the eighteen input properties as able to describe approximately 75 % of the variance. However, the analysis did not find any of the 18 demographic indicators to be redundant, and therefore none could be excluded. Following this, the rotated component matrix was computed using Varimax and the members of each component were evaluated in an effort to understand the particular relationships between the demographic properties. In this analysis the first factor included eleven of the eighteen demographic properties from across the four demographic categories identified in Table 1. The other four factors identified included fewer demographic properties; however, the groupings of the components for each of the factors were composed of a mixture of demographic indicator categories presented in Table 1. Consequently, the PCA did not indicate specific demographic categories (i.e. Population, Economic Status, Education, Race/Ethnicity) that drive the variability in VGI completeness. Linear regression Linear regression analyses were conducted on the quality measures of both OSM and OSMCP in order to assess whether they can be related to the demographic properties associated with their location. In the regression model positional accuracy was used as the dependent variable and the demographic properties as the explanatory variables on the 288 matched features from the ORNL-OSM feature matching process and the 357 matched features from the ORNL-OSMCP feature matching process (section ‘‘VGI quality and demographics data’’). Based on the results of the regression standardized residuals were computed and were checked for a possible linear relationship between positional accuracy and the model residuals. The results of this process are shown in Fig. 4 for both the OSM and the OSMCP data sets. As can be seen, both datasets show high correlations (0.92 and 0.95) respectively, indicating a violation of the homoscedasticity. It should be noted that while three observations in the OSM and OSMCP data sets were initially suspected as outliers, the overall behavior of the standardized residuals suggests that the regression
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Fig. 4 A plot of positional accuracy versus standardized residuals for the OSM and OSMCP data sets
model would not be valid even if these suspected observations are removed due to the strong linearity of the standardized residuals, particularly in the case of the OSMCP dataset. Given these results, further investigation of the demographics indicators was carried out in order to determine whether any multicolinearities exist between demographic indicators in Table 1, as such multicolinearities may result in the standardized residuals behavior that was observed in Fig. 4. This analysis revealed that the following demographic indicators were identified as multicolinearities: percent females, percent males, percent with and without HS diploma, percent white, and percent African American. These indicators were then removed and the linear regression correlation matrix results for the OSM and OSMCP datasets were recalculated. However, no significant correlation was observed between the positional accuracy and any of the remaining demographic indicators, suggesting that these indicators do not explain the positional accuracy error behavior in our datasets. It is worth noting that low correlation was observed between positional accuracy and the population of Asians across the study area.
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Discriminant analysis
Table 4 Z-scores of matched features calculated from a NN analysis
Discriminant analysis was used to assess the relationship between completeness and demographics in the OSM and OSMCP data sets. This was carried out by analyzing the differences in the demographic indicators for those features that were matched against the unmatched features for the two datasets. This analysis yields an interesting result, in that within the same dataset, there is a statistically significant difference between those features that were successfully matched within the study area and those features that were unmatched. The results were consistent for both the OSM and the OSMCP datasets indicating that there is a statistical difference between the demographic properties of the matched features and those of the unmatched features.
Data set
Data
Z-score
OSM
Matched features
-2.54
OSMCP
Matched features
-1.99
ORNL
All school features
-3.72
Spatial analysis results Nearest neighbor (NN) analysis Nearest neighbor analysis was performed in order to explore whether a spatial pattern (clustering or dispersion) in positional accuracy and completeness could be identified for both the OSM and OSMCP data sets. As described in section ‘‘Spatial analysis’’, the premise behind this approach is that if such patterns are detected, then they can be evaluated against spatial patterns of different demographic indicators to identify any similar spatial patterns. If similar patterns are found with respect to a specific demographic indicator, then that indicator can serve as a potential driver of VGI quality. Such analysis can also lead to possible insights on how demographics may affect different types of quality measures. For example, in the case of completeness, if there is a statistically significant difference in the spatial pattern of demographic indicators associated with matched features compared to the spatial pattern of the same demographic indicators associated with unmatched features, then those demographic properties could potentially drive VGI completeness rates. Testing whether the VGI quality measures exhibit any patterns is therefore the first step in developing an understanding of possible relationships between VGI quality and demographics. Table 4 summarizes the results of the Nearest Neighbor analysis. In this analysis, features that could be matched in the OSM and OSMCP data sets to the
ORNL data set were analyzed in order to determine if they are clustered, dispersed, or randomly distributed. In addition, a NN analysis was applied to the ORNL data set in order to determine if the reference features themselves exhibit a spatial pattern. For each of these analyses a z-score was computed in order to establish statistical significance. Z-score values above 2.58 indicate a 99 % confidence level that the data is not randomly distributed while values above 2.33 indicate a 98 % confidence level and values above 1.96 indicate a 95 % confidence level. As can be expected, when the spatial distribution of all features (schools) is analyzed a statistically significant pattern emerges even at 99 % confidence level. This is expected, as generally schools are not randomly distributed across a given region. For the matched VGI features, both the OSM and the OSMCP features did not exhibit a statistically significant pattern at a 99 % confidence level. When reduced to 98 %, the matched features in the OSM data set do exhibit a statistically significant pattern, while further reduction to 95 % leads to a statistically significant pattern in both VGI data sets. Moran’s I and local indicator of spatial association (LISA) Following the NN analysis, spatial autocorrelation analysis was carried out at the global and local scales. As described in section ‘‘Spatial analysis’’, Moran’s I is used to in this study to determine whether the demographics indicators and the positional accuracy measures in the two VGI data sets are spatially autocorrelated at the global scale. Table 5 summarizes the results of the Moran’s I analyses of the demographic indicators. As can be seen, the z-scores of two of the demographic properties, namely total population, and percent with HS diploma, failed to reach the 99 % confidence level, while the remaining indicators did show a statistically significant spatial autocorrelation. The fact that these two demographic properties
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GeoJournal Table 5 Scores of spatial autocorrelation in the demographic indicators (non-significant results at a 99 % confidence level are marked in bold) Demographic property Total population
2.462 23.827
Median age
10.898
Percent male
4.509
Percent female
3.414 5.918
Median home value
16.494
Percent below poverty line
16.033
Percent of homes receiving food stamps
16.398
Percent without HS diploma
21.322
Percent with HS diploma Percent with BA degree
1.779 23.360
Percent white
11.517
Percent white—not Hispanic
22.163
Percent African American
26.173
Percent American Indian
25.294
Percent Asian
12.127
Percent Hispanic
32.354
are most likely random greatly reduces their utility as a property to be used for pattern analysis. Since the accuracy of the OSM data was shown to have a nonrandom distribution in the nearest neighbor analysis described above, the spatial autocorrelation of the positional accuracy distance of both OSM and OSMCP was also computed. Table 6 provides the results of these analyses. The low z-scores for both datasets indicate that the accuracy is randomly distributed throughout the study area. In summary, these analyses show that sixteen of the demographic properties are non-randomly distributed while positional accuracy on the OSM and OSMCP data sets is randomly distributed. This challenges the premise that spatial patterns in demographic indicators could provide a possible explanation to the spatial distribution of positional accuracy and completeness in our study area. Following the global autocorrelation analysis, LISA was applied to identify possible local autocorrelation relationships in the positional accuracy measures of OSM and OSMCP. Once a dataset is shown to exhibit spatial autocorrelation, LISA can identify areas within the overall dataset that generate the
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Data set
Z-score
OSM positional accuracy
0.23
OSMCP positional accuracy
0.32
Z-score
Population density
Median income
Table 6 Z-scores of positional accuracy calculated from Moran’s I
spatial autocorrelation. Within this analysis, LISA was used to determine which, if any, features in the OSM and OSMCP data drive the spatial autocorrelation values that are observed. Although both the OSM and OSMCP datasets yielded low z-scores indicating no significant spatial autocorrelation at the global scale, LISA was applied on the OSM and OSMCP data sets to explore whether evidence of spatial autocorrelation could be found at the local scale. Figure 4 shows the LISA analysis results overlaid on a population density map of the study area. The darker red circles are classified as High–High Moran’s I values, and dark blue circles are classified as Low–Low Moran’s I values. The lighter red and blue circles are locations where the values were mixed, high and low indication the presence of an outlier. The white circles, which include 356 of the 401 records (89 %), are locations where LISA did not identify a local spatial autocorrelation pattern. The LISA analysis results show for the OSM dataset a Low–Low cluster located towards the southeast of the study area, although this does not appear to correlate with the population density values. The OSMCP data had a Low–Low cluster to the north of the center of the study area and one further north than that. However, in each case, the insignificant records greatly outnumber the others and are interspersed within them, suggesting that overall local clustering occurs very sporadically in the study area. This observation is in contrast to the spatial autocorrelation that was observed for the demographic indicators (as shown in Table 4), suggesting that the studied demographic indicators do not support the behavior of the VGI quality measures in the study area. The LISA analysis results therefore indicate that while no global spatial autocorrelation appeared to exist in VGI quality, some local clusters do emerge within the study area (Fig. 5). Global exploratory regression Exploratory regression was used to assess at a global study area scale both completeness and positional
GeoJournal
(a)
(b)
Fig. 5 LISA analysis of positional accuracy for the OSM a OSMCP, b VGI data sets
accuracy with respect to demographic indicators presented in Table 1 above. The analysis of the OSM and OSMCP dataset indicated several demographic indicators presented in Table 1 demonstrated multicolinearity issues (see section ‘‘Linear regression’’). These included: percent female, percent HS diploma, percent white and percent African American. The exploratory regression results indicated that three demographic indicators: Percent white not Hispanic, percent black, and percent Hispanic also exhibited multicolinearlity. These indicators were then removed from further analysis and the exploratory regression was run again. The results of the second exploratory regression of positional accuracy data for both the OSM and OSMCP datasets were consistent with the results of the spatial autocorrelation reported above and yielded no discernable model with respect to demographic indicators that would explain the spatial distribution of positional accuracy. Similarly, results of the exploratory regression for completeness, using presence (1) and absence (0) as the determinant values was conducted for both the OSM and OSMCP datasets
respectively. Similarly to positional accuracy, the results did not yield a discernible model of completeness with respect to demographic indicators. Geographically weighted regression Following the results of the exploratory linear regression, GWR was applied in order to explore possible relationships between demographic indicators and positional accuracy and completeness at the local scale. However, this analysis did not yield any statistically significant results (at a 95 % confidence level) or discernible regression models. These results are consistent with the regression results presented earlier.
Discussion The analyses carried out in this study addressed the commonly suggested notion that VGI quality is generally associated demographic indicators of the
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area covered by the VGI (e.g. Haklay 2010; Girres and Touya 2010; Zielstra and Zipf 2010; Cipeluch et al. 2010; Elwood 2009; Zook and Graham 2007a, b; Crutcher and Zook 2009). Using 18 demographic indicators from four separate categories (general population; economic status; educational attainment; and race/ethnicity), and focusing on point features (schools), we examined whether such associations could be detected, and if so whether they are statistically significant. In order to accomplish this, we used two VGI sources (OSM and OSMCP) and compared them to a reference (ORNL) using the feature matching methodology developed by Jackson et al. (2013). This enabled us to evaluate two quality measures of each VGI source, namely positional accuracy and completeness, and compare and contrast them with the various demographic indicators. Our analysis included both non-spatial and spatial methods, which ranged from the comparison of the distributions of the quality measure and simple regression to exploratory and geographically weighed regressions. The results of these analyses however, failed to identify a clear and consistent association (or statistically significant correlation) between either positional accuracy or completeness with any of the demographic properties. While some associations did emerge (e.g. in the NN or LISA analysis), they were found to be localized and sporadic. As a result, we were not able to identify any patterns or combinations of demographic indicators that are associated with improved VGI quality. These results suggest that, at least in some cases, the underlying mechanisms that control VGI quality are more involved, and that modeling VGI quality through a direct relation with demographic indicators may not be able to account for the intricate nature of VGI quality. One potential explanation for the emerging complex nature of VGI quality is that in general, VGI contributions are typically not restricted to contributors from the mapped area, thus enabling virtually any user across the globe to contribute information. Consequently, the VGI quality characteristics resulting from such process are driven by a potentially heterogeneous mixture of demographic indicators, rendering the process of identifying specific demographic drivers of VGI quality difficult. To some extent, this issue has been addressed in this study through the use of the OSMCP data set for which the contributors were indeed local to the area covered by
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the dataset. However, clear associations with demographic indicators were not found in this dataset as well. Recent research related to our study by Li et al. (2013), which focused on social media usage (Twitter and Flickr) did find a relationship between Twitter usage percentage of well-educated people with an advanced degree and high income. In addition, high Flickr activity was found to be correlated with a high percentage of highly educated white and Asian people. Similarly, Kent and Capello (2013), who studied the use of social media during a crisis situation (a wildfire) indicated that demographic characteristics of the area impacted by the emergency situation could be used to reveal the propensity of its population to contribute information in social media during such a crisis. While our findings appear to be in contrast to these findings, it is important to note that their focus is on social media rather than VGI. Arguably, social media offers its users an environment that is substantially different from that of VGI, which may lead to differences in the motivation of users to contribute information as well as differences in usage patterns. Consequently, the question whether the findings of these recent studies could be generalized to a VGI setting as well remains open. It is important to recognize that there are several noteworthy limitations to our study. First, our study focused on a single type of features, namely schools. While this was beneficial for the construction of our analysis workflow, further work is required in order to explore whether our findings are consistent across different feature types (e.g. stores or hospitals). In addition, our study did not explore other types of VGI quality measures, such as attribute accuracy or logical consistency. Lastly, additional analysis methods— both non-spatial and spatial—and other demographic indicators should be further explored in an attempt to model the relationship between demographic indicators and VGI quality. For instance, while our analysis focused on a linear model, non-linear models should also be explored.
Summary and outlook The enabling of citizens without formal training to produce geographical products for mass consumption through the use of web-based tools and technologies is
GeoJournal
introducing opportunities and challenges for our field. Opportunities arise from the availability of additional data that may extend the coverage of authoritative datasets, or even better represent particular types of events (e.g. capturing rapidly evolving events). The challenges are associated with the integration of such contributions with authoritative content. This integration is impeded by the lack of an understanding of the accuracy of VGI datasets, and potential patterns behind the variations of such accuracy. The study presented within this paper has explored the relationship between demographics and VGI quality, in order to assess the often-repeated argument that demographics may relate to corresponding accuracy variations. We conducted a quantitative study to address this issue, using data from a major metropolitan area, and focused on point-represented areal features. The Denver metro area that was selected as our study area offers a unique advantage as we have available for it not only authoritative (ORNL) and traditional crowdsourced content (OSM) but also content derived through a hybrid, partially supervised crowdsourcing process (OSMCP). For this area we selected schools as a representative feature type, for a variety of reasons. First, schools tend to be more easily identifiable, as they are large facilities, with well-defined components. Secondly, in a demographically segregated city such as Denver (James 1986; Aske et al. 2011) school locations can be viewed as representative samples of demographically diverse neighborhoods. The results of our analysis do not support the arguments that a correlation may exist between VGI error and local demographic properties, as they show no statistically significant such association. VGI on a massive scale is a very complex process and we need to gain a better understanding of the mechanics that drive participation and content quality. This study addressed one potential avenue, assessing the role of demographics. While the findings of this first quantitative study do not support earlier arguments for a correlation between demographics and accuracy, a natural future extension of this work would be to extend the analysis to additional areas, and to consider additional areal feature types (e.g. hospitals, fire and police stations). Given the findings of this research, one could argue that another logical next step would be to study the process characteristics (rather than its spatial and demographic indicators) and their relation to data quality, because the process may be
more complex than can be described by work done so far. Understanding who and why contributes VGI data still remains an open research question. As Steinmann et al. (2013) noted, a small percentage of contributors are responsible for the majority of mapping. Determining which ‘small percent’ of the local population is actually contributing poses a significant challenge to future research relating demographics of place with VGI contribution quality. It is through such studies that we will be able to gain an understanding of patterns behind the volunteering of geographical information, and the corresponding accuracy variations. This will improve our capabilities to integrate such datasets with authoritative collections, thus allowing us to harvest the full potential of VGI.
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