mapping impervious surface areas from gis ...

Survey Review, 40, 308 pp.108-115 (April 2008)

MAPPING IMPERVIOUS SURFACE AREAS FROM GIS PLANIMETRIC DATA W.T. Pang, H.S. Fok, and H. Bâki Iz Department of Land Surveying and Geo-Informatics The Hong Kong Polytechnic University, Hung Hom, Hong Kong, SAR China ABSTRACT Urban impervious surface is a characteristic of urban development. Knowledge about the impervious surface area is an essential component of planning decisions at metropolitan areas around the world. This paper demonstrates the use of available GIS planimetric information in the production of an impervious surface area map of Hong Kong. Hong Kong is one of the most populous cities in the world where there is a continuous need for new buildings, roads and other infrastructure. Hong Kong’s calculated impervious surface area constitutes 18 % of the total surface area of the Hong Kong territories, which makes the city one of the greenest of the world despite its high population. In this study potential challenges in the identification of impervious surfaces were investigated. KEYWORDS: Impervious surfaces. GIS. surfaces.

Planimetric information.

Hong Kong. Identification of

INTRODUCTION Hong Kong has undergone rapid urban development over the past three decades. Various vegetated lands in rural areas have been transformed into buildings and roads which are made of concrete as a result of the continuous growth of population and the expansion of its economy and transportation industry. Impervious surface, which is defined as any material, natural or man-made that prevents the infiltration of water into the soil, is commonly accepted as one of the reliable indicators of urbanization and its impact on natural resources. The impervious surface areas, (ISA) include rocks, roads, buildings, houses, sidewalks and parking lots [1],[4],[9]. Research indicates that the impervious surface has a significant impact on biodiversity [3],[10], [11], climate and air quality control [5], [10], air temperature [7], noise pollution, wildlife habitat and water resources [2], [14], natural energy and material cycles of ecosystems [8] and urban planning [6], [12]. Hence, there is a need to map and monitor the urban sprawl in any metropolis. The ISA map provides a synoptic view of the urban impact on the environment and can be calculated using different types of data. While the application of remote sensing data for ISA mapping is well documented, there is currently no systematic investigation on the accuracy of ISA maps derived from GIS planimetric data, which is frequently adopted as a baseline map for training remotely sensed images. In this study, we detail the process of the ISA map production using GIS planimetric data and identify potential error sources which can be reduced by the use of attribute information. The resultant ISA map and the percentage of the impervious surface coverage of Hong Kong are reported. GIS PLANIMETRIC DATA General Properties The digital map data for Hong Kong with a scale of 1:5000 published by the Survey Contact: H Bâki Iz e-mail: [email protected] © 2008 Survey Review Ltd.

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DOI 10. 1179/003962608X253565

W T PANG, H S FOK AND H BÂKI IZ

and Mapping Office of the Hong Kong Lands Department has been utilized in this investigation. The data are derived from the original 1:1000 scale Survey Sheets (HP1CSeries) or field survey data of equivalent accuracy with a careful checking using aerial imagery. The map is divided into 17 sheets and each sheet is further subdivided into 16 small sheets (Figure 1). There are 189 small sheets in total since the area of merely the sea and mainland China is not included. The large and small sheet covers an area of 15000×12000 m2 and 3750×3000 m2 respectively. Within the digital map, the data set is classified into different layers according to their nature. They are area feature, boundary, building, contour line, facility, general annotation, hydrographic line feature, railway, relief (it contains boulder, rock outcrop, cliff, quarry top and rocky area, which are all impermeable surfaces), road, spot height, utility and construction. Their description can be found in the Survey and Mapping Office of the Hong Kong Lands Department websites. Currently, the new dataset contains 18 layers [13]. In reference to the description, the building, railway and road layers are made of concrete and the relief layer is the natural impermeable surface while the area, boundary, facility, and hydrographic line feature layers simultaneously contain both pervious and impervious surfaces. The construction, contour line, general annotation, spot height, and utility layers require further information for identification as ISA. Therefore, building, railway, road layers, and relief are primarily considered as the impervious surface for ISA mapping. Those layers which contain both pervious and impervious surfaces are considered on a case-by-case basis.

Fig. 1. The map index of Hong Kong (Survey and Mapping Office of Hong Kong)

Considerations for rural areas In the case where buildings or houses are located in a less dense area, there are no symbols or layers in between the buildings or houses signifying whether the area is an impervious surface or not. Therefore, it is assumed that when the structures are 109

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clustered close to each other, they tend to form an impervious surface, which is usually the case in Hong Kong. The outermost boundary of these buildings or houses is then classified as the ISA. The boundary can take different shapes depending on the spatial arrangement of the buildings. It can be defined either by joining the nearest corner in between them or following the wall’s direction of the buildings to form a polygon. Figure 2 shows two different ways of digitizing the outmost boundary of a cluster of structures. However, the resultant digitized area using the above two approaches is similar and thus the impact of any potential difference is insignificant to the final result since such surface areas are quite small when compared to the whole land surface area of Hong Kong. In this study, the approach based on joining them using their nearest corner has been adopted.

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Fig. 2. The two approaches for a rural area (a) joining the nearest corner or (b) following the building wall’s direction.

METHODOLOGY Data manipulation ESRI ArcGIS software is used for this study. Since the digital map data of different layers are available in ArcInfo (.e00) format, they had to be converted into an appropriate format that could be viewable in ArcGIS. After the conversion, layers could be added to the viewer for further manipulation. Points, lines, and polygons are the basic spatial elements in GIS. Each layer represents different basic spatial elements. Since most layers, except area and building layers, are not in the form of polygons for area computations within the spatial database, they had to be digitized in the form of closed polygons in a new layer so that the area of each polygon could be computed in the GIS software (Figure 3). Operations are more tractable as it was done piece-by-piece in 189 small sheets for the whole area of Hong Kong. Attention had to be paid to the less dense or rural areas and to those layers that simultaneously contain both pervious and impervious surfaces so that details would not be overlooked or ignored. This topic will be elaborated in the following section. After all the impervious surfaces have been digitized to layers for different parts in Hong Kong, entire layers are merged together to form a single layer using the geo-processing tool provided by the GIS software. The merged layer of the impervious surfaces contains a large number of polygons and areas were computed as an attribute within the spatial database. Therefore, the total ISA can be determined simply by adding up all the area of each individual polygon (Figure 4).

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Fig. 3. Spatial elements for digitizing

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Attribute information table associated with the spatial element (e.g. polygon) Fig. 4. Operation process involved in layer merging and overall area calculation.

Assessing ISA on the layers displaying pervious and impervious surface characteristics As mentioned in the previous section, the area, boundary, facility, and hydrographic line feature layers contain both pervious and impervious surfaces. An intuitive decision is needed to clarify these elements in order to differentiate them. While the area feature can be identified easily as the layer is in the form of a polygon where the description in the attribute information table associated with the polygon is given, checking is required to verify that they overlap with clearly defined impervious layers. The hydrographic line feature layer, which is in the form of a line, indicates clearly the location and boundary of the features of impervious (e.g., fountain, filter bed, swimming pool, and pier) and pervious surfaces (e.g., river). Also, systematic check is also required to see that they overlap with impervious layers. The facility layer, represented by points, shows no indication of the spatial extent of the area of impervious surface, while the boundary feature layer, which is in the form of a line, shows no explicit description of the impervious boundary in the GIS environment. Nevertheless, the rich content of the description in the attribute information table associated with the facility layer is useful in calculating the ISA for 111

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the boundary feature layers. In some cases, it is difficult to determine whether it is an impervious surface when the boundary of an area contains two or more types of facilities as shown in Figure 5. In the figure, the point 1 within the circle represents the boundary as a sports ground, while the point 2 indicates it as a playground, and they are usually made of concrete in Hong Kong. However, during field check, it is identified as a sports playground covered with grass, which is a pervious surface as displayed in Figure 6.

Fig. 5. An example of a boundary feature containing two facility types

Fig. 6. A photo taken at the site.

This problem could not be resolved with the available information from the GIS alone. After the digitization, additional information about the sites using the city map guidebook for Hong Kong and an internet-based map published by different companies were used to cross-check the imperviousness of the areas on those layers that exhibit both pervious and impervious surface characteristics, since those information sources provide more detailed descriptions of the facilities to users. On those occasions, field checks were also conducted when there was no other information available for the GIS attribute information.

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FIELD VERIFICATION FOR THE ACCURACY OF THE ESTIMATED ISA The calculated total ISA of Hong Kong using GIS planimetric data is 206 km2. However, in order to quantify the variability in misidentification, 10 sites at different locations in Hong Kong territories have been selected (Figure 7) for field verification. All of the sites are small towns because they are the most likely locations subject to misidentification as a result of overlapping different layers, especially those layers with both pervious and impervious surface characteristics. The area of each site is set to be 400×400 m2. The ISA for each site before and after verification are shown in Table 1 in percentages.

Fig. 7. Site locations for result validation

Table 1. Percent of ISA before and after field verification. Site Name Happy Valley North Point Sai Kung Sha Tin Tai Po Tsim Sha Tsui Tsuen Wan Tung Chung Wan Chai Wong Tai Sin

ISA (%) Before and After Verification Before After 99 54 82 71 72 76 66 65 88 89 79 79 65 59 100 87 73 73 100 90

Percent Difference 45 11 -4 1 -1 0 6 13 0 10

One of the results displayed in Table 1 shows a 45% difference for Happy Valley (Hong Kong’s Horse Racing course) if the ISA is calculated directly from the boundary layer without using facility layer information or additional information. Overall, the differences for the selected areas have a tendency for overestimating the ISA up to 13% if the classification was not evaluated in the field.

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Figure 8 shows a typical misidentification within a site. Boundary layer information in all three cases is ISA but misidentified as a permeable one in the absence of facility layer data.

Fig. 8. The misidentification of the Wong Tai Sin site.

Since misidentification is reduced significantly by the use of facility layer information, the median value, 3.5%, of the sample sites from the Table 1 can be used to represent approximately the overall systematic error of overestimation committed in the total ISA computation. Therefore the total calculated ISA of Hong Kong (206 km2) needs to be corrected by 3.5%. Using this estimate, the corrected total ISA area for the whole territory is now 199 km2 When compared to the total land area of the whole territory of 1104 km2, i.e., land above the High Water Mark as reported by the Hong Kong Lands Department (2006), this result shows that 18% of the Hong Kong territories can be classified as ISA. The resulting ISA map of Hong Kong is displayed in Figure 9.

Fig. 9. The ISA map of Hong Kong

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CONCLUSION In the study, the use of GIS planimetric data was demonstrated in the computation of ISA maps. It was shown that in producing the ISA maps, the utilization of information in facility attribute layers reduced the misidentification errors considerably. Ironically once it is utilized it becomes the major source of error because it is the most influential source of information. With these results, ISA maps can be effectively used for monitoring urban development. The accuracy of the ISA maps is expected to improve in parallel with the completeness and reliability of the layer information, especially facility attribute layer information provided by the Hong Kong Lands Department. The ISA maps could also serve as references in the calibration and validation of ISA information estimated from other approaches such as using remote sensing images at regional scales. ACKNOWLEDGEMENTS This study was supported using funds from the Competitive Earmarked Research Grant (CERG) allocation project no. G-U170. We would like to thank the anonymous reviewers for their constructive comment. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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