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Géomorphologie : relief, processus, environnement, 2002, n° 2, p. 119-126

Photogrammetric analysis of the coastal erosion in the Algarve (Portugal)

Analyse photogrammétrique de l'érosion côtière en Algarve (Portugal) João Catalão*, Cristina Catita*, Jorge Miranda*, João Dias**

Abstract A pilot study was conducted to investigate the applicability of photogrammetric techniques to the monitoring of coastal erosion in the south of Portugal. A set of six different aerial vertical surveys, made between 1938 and 1995, was used to compute robust evaluations of the erosion parameters. We show that in the past 57 years, the Olhos-de-Água/Quarteira cliff segment of the Algarve coast (South Portugal) has been receding at an average rate of 17 cm/year. A 3D model was constructed for each survey, allowing the computation of the volume rate change during each interval between surveys. We verified that in the period between 1938 and 1969 there was a significant loss of volume whereas after this period and until 1995, there was a volume gain in the area above the 2-m level. Over the full 57 year time span, the rate of cliff retreat was stable. Key words: photogrammetry, cliff retreat, erosion rate, digital elevation model, differential GPS, Algarve, Portugal.

Résumé Une étude préliminaire a été conduite pour vérifier l'intérêt de l'utilisation des techniques photogrammétriques appliquées à la surveillance de l'érosion côtière dans le sud du Portugal. Un ensemble de six relevés aériens verticaux, effectués entre 1938 à 1995, a été employé pour le calcul d'estimations fiables des paramètres d'érosion. Nous prouvons que dans les cinquante-sept dernières années, le segment de falaise Olhos-de-Água/Quarteira de la côte de l'Algarve (Sud Portugal) a reculé à une vitesse moyenne de 17 cm/an. Un modèle 3D a été construit pour chaque relevé, permettant l'étude du taux de changement volumique entre deux relevés. Durant la période 1938-1969 il y a eu une perte significative de volume tandis qu'après cette période, et jusqu'en 1995, on observe un gain de volume dans la zone au-dessus du niveau de 2 m. Pour toute la période étudiée, on observe une grande régularité dans la vitesse de recul de la falaise. Mots clés : photogrammetrie, recul de falaise, vitesse d'érosion, modèle numérique de terrain, GPS différentiel, Algarve, Portugal.

Introduction The coastal zone is an area of continuous morphological changes due to its dynamic behaviour (climate variations, relative sea level changes, storm events, and coastal tectonic processes) and its quick response to natural changes and human interventions. Coastal zone monitoring provides essential information for the understanding of the coastline dynamics. It can be used by decision-makers for the implementation or improvement of coastal protective strategies. The integration of aerial remote sensing with GPS (Global Positioning System) kinematic surveys may be considered as an optimal approach for deriving high resolution mapping products to be used in support studies on coastal erosion

(Marden and Clearly, 1999). The advantage of photogrammetry is that of providing a full coverage of areas of interest. By contrast, the classical geodetic approach, though usually characterised by higher accuracy, describes the deformation pattern from a limited number of points. Furthermore, recent developments in digital photogrammetry processing and in the extensive use of aerial and terrestrial GPS control points have resulted in operational and processing time reduction without loss of accuracy in the derived products. In some parts of the Algarve region, in southern Portugal (fig. 1), a systematic cliff retreat has been observed (Dias and Neal, 1992; Correia et al., 1996; Marques, pers. comm., 1997), which puts at risk the existing ecosystems and deeply

* Science Faculty of Lisbon, Campo Grande, Ed C8, Gab. 8147, 1749-016 Lisbon, Portugal. E-mail: [email protected] ** University of Algarve, Faro, Portugal

João Catalão, Cristina Catita, Jorge Miranda, João Dias

affects the constructions near the shore. Several studies have been carried out in this area. These have focussed on coastal mapping and erosion monitoring (Bettencourt, 1991; Dias and Neal, 1992) based on simplified treatment of aerial photographs. F. Marques and C. Romariz (1991) presented the results of a study with aerial photographs from 1947, 1958, 1980 and 1985, later complemented (Marques, 1998) by photographs from 1974 and 1992. Erosion rates in this area range from 0.2-0.3 m west of Quarteira to 2 m east of this locality. P. Bettencourt (1991) based his study on aerial photographs taken in 1972, 1980 and 1986, at a scale of 1:5000, and concluded that the cliff retreat rate was always less than 1 m/year, while according to F. Marques and C. Romariz (1991) it reached a maximum value of 3m/year in the period 1980-1986. Later, J.M.A. Dias and W.J. Neal (1992) published new results on the rate of cliff retreat based on ground survey, which substantially differed from F. Marques and C. Romariz (1991) and P. Bettencourt (1991). These studies were based on manual comparison of aerial photographs with archive cartography; retreat rates obtained in this way show significant discrepancies, not only in terms of the lateral variations (Ciavola et al., 1997) but also the associated time regimes. Although there has been a systematic monitoring of this area by different research teams, the physical processes responsible for coastal erosion and cliff retreat are still not well characterised. It is thought that the differences observed in erosion rates are due to the use of inadequate strategies for the quantification of the coastal changes, based on simplified techniques used for measurements on aerial photographs and also the systematic use of old small scale maps. There has been considerable progress in aerial remote sensing with most of the classical procedures of photograph manipulation (manual and analogic stereo restitution) being replaced by digital processing of photos and automatic procedures (Ackerman, 1984; Welch, 1992) that enable the extraction of digital terrain models and specific features. Several methods have been proposed for coastal monitoring 120

Fig. 1 – Aerial photo of Olhos de Água–Quarteira beach, in the Algarve, southern Portugal. The studied sectors are delimited by the rectangles.

Fig. 1 – Photo aérienne de la plage de Olhos de Água–Quarteira, en Algarve, au sud du Portugal. Les secteurs étudiés correspondent aux rectangles.

based on aerial photographs and maps (Thieler and Danforth, 1994a, 1994b; Marques, 1998). Most of these methods are based on the digitisation of photos using a digitising table and on the acquisition of control points from old maps. Following this digitisation, software programmes are used to compute the external and absolute orientation of the models. Though very handy, this system is not accurate enough for the monitoring of coastal areas with small retreat rates in a short period of time. Its weakness resides basically in the digitising process and in the absence of any stereoscopic measurement. This limits the precision of the final maps to tens of metres (Thieler and Danforth, 1994b). Nowadays, the direct application of photogrammetric stereorestitution, combined with aerial GPS, inertial systems data and ground GPS data, is an area still being developed with an enormous potential for high precision monitoring and management of coastal areas. With the most recent technological development in digital photogrammetric techniques, there has been an overall increase in published works with good results (e.g., Brown and Arbogast, 1999; Cheng and Stohr, 1996). However, the cost associated with the specific acquisition of aerial photographs, involving dedicated pre-planned flight missions, the photogrammetric equipment and particular technical specifications, has discouraged geologists and geophysicists from using these techniques. On the Portuguese coast a few attempts have been made (Correia et al., 1996; Catalão et al., 2000) wherein the photogrammetric techniques were properly applied for coastal dynamic studies, particularly in the quantitative assessment of cliff changes. The applications of photogrammetric stereorestitution for shoreline delineation or volume



determination, and its multitemporal variations in different time scales, have not yet been analysed. The present study aims at overcoming this lack of knowledge and it corresponds to a technological challenge to get higher precision measurements for coastal areas for longterm and short-term studies. The focus of the present study will be the exploitation of high precision photogrammetric techniques for the monitoring of long-term coastline retreat rates due to erosion. Therefore, high hazard areas can be targeted and integrated in Geographic Information Systems, which can later be used by coastal managers. This will allow the analysis of the impact of coastline changes based on coastline change/status and other datasets, provided by previous studies, and the subsequent definition of policies to prevent further severe erosion of the coastal area.

Methods The Algarve region is in the south of Portugal and has more than 200 km of beaches largely dominated by sea cliffs. The study area corresponds to a segment of the Algarve coast between the Olhos de Água beach and the Quarteira river (fig. 1), dominated by a cliff several tens of meters high. The cliff retreat can be studied in several time and space scales. In this work, we considered a total time coverage of 57 years, and we used high precision photogrammetric techniques. The accuracy level can be estimated at about 20 cm. For this work, a set of six different periods was analysed, covering the years 1938-1995. The major difficulty arose from the need to render compatible the different cameras used. Technology has changed dramatically during this coverage and the oldest photographs do not have fiducial points. Another difficulty arose from the need to identify natural points good enough to allow a robust computation of the external orientation parameters for all the surveys. Due to the irregularity of the erosion processes near the natural water lines, we selected two zones (1 and 2 in fig. 1). Zone 1, close to Olhos de Água, is about 480 m in width, and zone 2, close to the Quarteira river, has an extent of about 1000 m. The main source of information for this study comes from the Portuguese Air Force. The data were provided by the Portuguese Instituto Geográfico do Exército. The survey periods and the characteristics of the flights are presented in table 1. The 1938 survey was carried out with a photograph format of 189 mm by 192 mm, with 4 internal triangular orientation marks in the perimetre of the photograph. This standard is no longer used and we do not have any calibration parameters for either the lens or the orientation marks. The 1958 flight did already make use of the 230 mm by 230 mm format, although the calibration information is lacking. All the more recent flights were made with WILD cameras, for which all the calibration data are available. The first step in the photo processing consists in determining the internal and external orientation parameters. The internal orientation for the 1938 flight, with unknown camera parameters, was made in an interactive way until a

Year

Camera

Focus

Scale

1938 1958 1969 1972 1976 1995

----------Wild Wild RC 10 Wild RC 10 Wild RC 30

204.40 152.04 152.19 152.05 153.36 152.73

1: 1: 1: 1: 1: 1:

18000 25000 25000 25000 25000 40000

N. Photos 4 4 3 5 4 3

Table 1 – Aerial surveys used in this study.

Tableau 1 – Missions utilisées dans cette étude.

predefined minimum acceptance accuracy of 20 µm was achieved. As for the other cases, the orientation was achieved with an accuracy higher than 4 µm. The greatest challenge in this study was the computation of the external orientation parameters for the six surveys within a coherent geometrical framework. The identification of common elements to all the surveys being impossible, we decided to make a two-step multitemporal aerotriangulation. In the first step, an aerotriangulation involving the 1995, 1976 and 1972 surveys was performed, considering the natural points and the photogrammetric points correctly identified in these photographs. The photogrammetric points were measured by differential GPS in the static mode with two dual frequency receivers. The base station logged continuously while the rover receiver logged for 20 minutes in each photogrammetric point. The logging interval for both receivers was 15 s. Following this procedure, 18 photogrammetric points were measured with an overall estimated precision of 3.7 cm. In the aerotriangulation computation, an a priori 5 cm error was considered for the photogrammetric points. The photogrammetric points are distributed throughout the photographs of the 1995 survey with a slightly larger concentration of points in the onshore area. The natural points are very well defined elements that could be identified and measured in at least two surveys – the pass points. A large number of pass points (118) were measured forcing a high degree of redundancy and, as a consequence, a higher precision in the aerotriangulation results. In the second step, another multitemporal aerotriangulation was performed on the remaining flights (1969, 1958 and 1938), including the 1972 flight. The 1972 flight was considered as a reference survey for this second-step aerotriangulation. This means that the previously determined triangulation points identified in this flight were considered as tie points in this second step, thus assuring temporal and spatial continuity in the entire photographic data set. The aerotriangulation measurements were made in a first order stereo equipment (Zeiss Planicomp P1) and the aerotriangulation computation was made by least squares bundle adjustment of all intervening surveys. This methodology, while enssuring the coordination of all the photographs, led to a decrease in the absolute accuracy for the older survey external orientation parameters. This can be seen in table 2, where we present the final results for all six surveys. From this table we can confirm the temporal improvement of the internal orientation precision, as a result of the tech-


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Table 2 – Mean square-root of error on the internal and absolute orientation parameters.

restitution of the cliff top line, base line and zero level line, and the second the photogrammetric restitution of vertical Tableau 2 – Racine carrée de 1938 19 0.59 0.30 cross profiles along the beach. These prol'erreur sur les paramètres 1958 8 0.49 0.26 files were measured with a 25 m horizond'orientation interne et d'orien1969 10 0.59 0.31 tal separation along the shore, and with a tation absolue pour les coor1972 2 0.36 0.21 variable spacing along the profile, dependonnées (x,y,z). 1976 7 0.42 0.34 1995 1 0.35 0.31 ding on the height variation, this spacing being proportional to the slope. nological improvements in the most recent cameras, and The photogrammetric stereorestitution and the interior also the lack of knowledge of the camera parameters for the and absolute orientations were carried out in a digital steoldest flights. Concerning absolute orientation precision, reoplotter equipped with crystal glasses and a high resoluthis temporal dependence is not verified, assuming a relati- tion monitor of 66' x 55'. The photos were scanned in a phovely small improvement with time in planimetry and having togrammetric scanner (Zeiss PhotoScan) with a 14 µm resoa very similar precision in height. These two results may be lution and an internal geometric precision of 4 µm. partly due to the aerotriangulation procedure applied to all Figure 2 shows the results obtained for the cliff top line, the surveys, resulting in a global adjustment of all data sets, for the 1995 and 1938 surveys, as well as the measured proimproving considerably the precision of the oldest photo- files. We can conclude that there is a clear cliff retreat, of graphs. On the other hand, they may also be due to the dif- several metres, for the time span 1938/1995, photogrammeferent photograph scales used, the most recent ones having trically measured with a 0.70 m estimated precision. Along a smaller scale (1:40,000). This combined effect results in a the coastline, the cliff retreat is very irregular, with values of similar absolute orientation precision for all photos (sur- between 1.5 m and 15.2 m between these two surveys. veys). This effect is particularly important for the height With the photogrammetric profiles, we were able to build precision, which, together with the low altitude of the study a 3D terrain model, in the form of a 10 m x 10 m grid. Thus, area (0-40 m), results in an estimated altimetric precision of one Digital Elevation Model (DEM) was constructed for 0.30 m for all surveys. each survey allowing the visualisation of the terrain details To check the final accuracy of the six surveys, direct mea- and the visual comparison between surveys. The DEM has surement of common elements between all the surveys also enabled some relevant calculations concerning the (houses or old trees) were made. We concluded that the posi- volume quantification and the spatial determination of multioning error between the 1995 and the 1972 survey was titemporal changes in the cliff during the entire study period. about 0.23 m, while the discrepancy between the 1972 and the 1938 flights was about 0.35 m. Results After the orientation phase, we proceeded to delimit the The computation of the morphological changes was cartop and base of the cliff for all surveys, as well as the zero level line, using an analytical stereoplotter. The base of the ried out following three different approaches: individual cliff is out of reach of wave influence except in winter. analysis of the profiles, comparison between the top cliff Because of this, the base line and the shoreline were delimi- lines and volume analysis. Figure 3 shows a sequence of profiles, measured phototed only for representative purposes and for visual interpretation. The stereorestitution was made by means of two dis- grammetrically, corresponding to the 1938 and 1995 surtinct processes, the first being the photogrammetric stereo- veys. The 1938 profile is represented in light grey and the Year

Internal (micron)

Absolute XY (m)

Absolute Z (m)

Fig. 2 – Top cliff line for the 1938 and 1995 periods. Location of the photogrammetric profiles.

Fig. 2 – Position du sommet de la falaise en 1938 et en 1995. Localisation des profils photogrammétriques.

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profile for 1995 is represented in dark grey. The overlapping of these two 3D models results in a bicolour image showing the land movements between these two surveys as a result of the erosive process. Taking into account the fact that clifftop line retreat occurred between these surveys, one should expect that, in this picture, the 1938 profiles would entirely overlap the 1995 3D model. However, we can see that, in some areas, the 1995 surface extends further seawards than the 1938 surface. This effect may be seen along the entire beach, pointing out cliff-top retreat associated with an increase in the area of sandy beach. Other parts in which the volume in 1995 exceeds the 1938 surface are seen to occur in the intermediate cliff heights, suggesting cliff-top retreat by means of rotational mass movement processes. The erosion rate was determined from the area integral between the 1938 top line and all the others, divided by the width of each zone (480 m for zone 1 and 980 m for zone 2). Thus, we were able to discard small local discrepancies that are generated by the distribution of the horizontal location error. The results of all the computations performed for the cliff-top line, determined for each survey, are shown in table 3. A very similar behaviour of the retreat rate in the two zones is observed, showing a high magnitude in the first 20 years, with apparent oscillations between 1969 and 1976, due to the positional uncertainties of the delimitation of the cliff top line. This process is particularly clear for the 1972 survey, which shows a cliff retreat that is not consistent with the two closest surveys (see fig. 4). The graphs in fig. 4 show time evolution of the absolute cliff retreat value for both zones. We can deduce that there is a similar behaviour in both zones, with an average value of about 0.17 m/year, revealing a certain degree of regularity in the coastal erosion

Fig. 3 – Superposition of two digital terrain models from the 1938 (represented in light grey) and the 1995 (represented in dark grey) epochs showing the spatial distribution of the eroded areas along the cliff.

Fig. 3 – Superposition de deux modèles numériques de terrain correspondant aux années 1938 (en gris clair) et 1995 (en gris sombre) montrant la distribution spatiale des secteurs érodés le long de la falaise.

process in this region, during the 57-year analysis period. This value was determined including all surveys, though there is some suspicion regarding the period 1969-1972 for both zones and, probably, a too high rate in the first survey (1938-1958), for zone 1. The aforementioned measurements and results correspond to a linear approach of the retreat rate of the cliff top line that must be complemented by volume computations for each survey. The previously determined 3D-terrain model allows the determination of the volume for each survey, and the volume integral between any height plane and the terrain surface. Thus, it is possible to numerically evaluate the mass movements between two surveys. In zone 1, with a maximum height of 43 m, the terrain was segmented in five height planes; the volume between these planes and the 3D topographic surface was computed for each survey. For this zone, the five planes were 1 m, 2 m, 4 m, 10 m and 20 m (fig. 5). In this picture the computed volumes for each height plane are presented in different colours and for the different surveys, allowing the temporal analysis of the volume evolution for each plane. The top line of this graph, corresponding to the height 1 m, reveals an unexpected irregular temporal evolution of the volume, with a constant "decrease" until 1969, consistent with the high Zone 1

Table 3 – Values of cliff retreat for the two selected zones, taking the 1938 survey as a reference.

Tableau 3 – Valeurs du recul de la falaise dans les deux secteurs étudiés pour différentes périodes depuis 1938.

Zone 2

Period

Duration (Years)

Area (m2)

Retreat (m)

Rate (m/year)

Area (m2)

Retreat (m)

Rate (m/year)

1938-1958 1938-1969 1938-1972 1938-1976 1938-1995

20 31 34 38 57

3663 3532 4018 3684 5201

7.66 7.31 8.35 7.69 10.83

0.38 0.23 0.24 0.20 0.19

4430 6659 8107 7173 9150

4.5 6.7 8.2 7.2 9.7

0.22 0.21 0.24 0.18 0.16


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Fig. 4 – Cliff-top line retreat in metres/year assuming as reference the 1938 period.

Fig. 4 – Recul du sommet de la falaise en mètres par an depuis 1938.

cliff retreat rate over this period. This systematic behaviour became reversed in 1969, generating a new process of volume growth, which is not consistent with the cliff retreat rate. However, this process becomes weaker in the higher elevations and we can see that in the planes higher than 20 m there is a real loss of material between 1938 and 1995. The temporal evolution of the volume for the five segmentation planes and the continuous cliff-top line retreat suggest a mass movement from the cliff top line to the intermediate planes or even to the beach. A similar process may also be observed in zone 2, in which the volume between the topographic surface and the 1 m plane decreases until 1972 and increases from 1972 to 1995, although less than in 1938. In this area, with a maximum height value of 10 m, one can see that the 4-m height plane does still correspond to a turn-around point, which concerns the volume evolution in time. The temporal evolution of the volume computed over the 4-m height plane presents a continuous loss between 1938 and 1995. In both zones, the continuous loss of volume and the cliff-top line retreat were interrupted in 1969 or probably in 1972, dates that correspond to the construction of the Vilamoura piers and marina. It is not possible, due to the observation sequence of dates, to determine whether there is as a direct relationship between the construction of the Vilamoura marina and the volume increase in the planes lower than 1 m. However, the coincidence of dates as far as the two phenomena are concerned may suggest an increase in the average beach level as a consequence of the construction of the

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eastern pier. In order to clarify this phenomenon and to corroborate these results, other flights, which took place between 1976 and 1995, should also be analysed.

Discussion All previous computed values (cliff retreat rates and volume changes) based on the photogrammetric measurements are affected by positional errors of the photograph perspective centres. This introduces a positional uncertainty in all linear measured elements. Having in mind that this is a multitemporal prospective study in which some of the analysed photographs are more than 50 years old, the estimation of precision ensures the reliability of the results. In this study there are two types of uncertainty that must be taken into account: the temporal uncertainty concerning the 1938 photographs, and the positional uncertainty. Temporal uncertainty is easy to deal with, since it affects only the computation of the cliff retreat rate. The uncertainty of the 1938 flight results from the fact that this flight corresponds to a mission performed by the British Royal Air Force, in the period 1938–1948, covering the whole country, and there are no records of the exact date for the Algarve flight. It is believed that the Algarve flight was the first one Fig. 5 – Computed volumes for different height planes in zone 1 and zone 2.

Fig. 5 – Estimation des volumes érodés le long de différents transects dans les zones 1 et 2.



Fig. 6 – Geometric principle adopted for the introduction of the planimetric positional uncertainty in the computation of the cliff-top line retreat.

Fig. 6 – Principe géométrique utilisé pour prendre en compte l'incertitude de la position planimétrique dans l'estimation du recul du sommet de la falaise.

and that is the reason why 1938 was adopted as the most probable date. If this date is changed to 1948, then the cliff top line rate would be 0.19 m/year for zone 1 and 0.18 m/year for zone 2. The positional uncertainty of the cliff top line was represented by an uncertainty region or envelop around that line with a maximum distance equal to the lowest estimated positional precision obtained from two surveys (fig. 6). Based on the results presented in table 2, in which the absolute orientation results are presented, we concluded that the lowest precision values reached 0.7 m and were obtained from the 1938 and 1958 surveys. In addition to the absolute error of the photogrammetric models, the interpretation error must be considered. This error results from the misinterpretation of the delimitation of the cliff top line and it was considered to be 1 pixel in the scanned photo. For the highest photo (1:40,000), scanned at 14 µm, this error is 0.56 m. Thus, for the comparison of two surveys, this error is 0.79 m, and taking into account the positional absolute error, an error value of 1.1 m was considered for the precision of the cliff top delimitation. An external envelope, Fig. 7 – Cliff-top line retreat in metres/year assuming as reference the 1938 period in zones 1 and 2. In these two graphs the measured values are represented by a square. The bounded values for each period determined by the introduction of the 1.1 m positional uncertainty are represented for the upper bound with a circle and for the lower bound by a triangle.

Fig. 7 – Recul du sommet de la falaise en mètres/an depuis 1938 dans les zones 1 et 2. Les valeurs mesurées sont représentées par un carré. Les valeurs limites, correspondant à l'introduction d'une incertitude de 1,1 m sur la position du sommet sont indiquées par un triangle pour la limite inférieure et un cercle pour la limite supérieure.

parallel to the cliff top line of each survey, with a distance of 1.1 m was constructed, and the surface integral for this new delimitation was recomputed for each survey. Fig. 6 shows that there are two situations that must be considered: the first concerns the region between the two closest lines, i.e., the situation with a lower cliff retreat, and the second the region delimited with the exterior lines corresponding to the higher cliff retreat. The results of the inclusion of the positional uncertainty in the cliff-top line retreat rate are presented in fig. 7. In this graph, the maximum value of the cliff movement for each survey is represented by a red circle, and the minimum value by a blue triangle. The squared figure represents the measured value that was presented in fig. 5 and in table 2. Two new regression lines were plotted corresponding to the maximum and minimum cliff rates. The cliff rate assumes values between 0.14 m/year to 0.20 m/year in zone 1, and 0.14 m/year and 0.20 m/year in zone 2. In this graph, the 95% confidence limits of the measured solution are also plotted. There are only one observation in zone 1, and two observations in zone 2, that lie out of the confidence limits. In both cases these observations correspond to simulated data resulting from the introduction of the positional uncertainty into the observed data. The combined result of the temporal and positional uncertainty gives the opportunity to derive the maximum and the minimum estimated values for the cliff retreat rate. Taking 1938 as the reference survey, the cliff top line rate is 0.17 m/year with a possible variation from 0.14 m/year to 0.20 m/year for zone 1 and 2. As for the borderline case, in which an error in the date of the photographs is assumed, and the 1938 flight was changed to 1948, the cliff rate is 0.19 m/year with a variation of 0.16 m/year to 0.23 m/year, for zone 1 and zone 2.

Conclusions The overall objective of this study was to enhance our knowledge on aerial remote sensing techniques for fundamental coastal erosion studies in order to reduce the increasing costs of


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problems and losses on the Algarve coast. The multitemporal study was based on archive photogrammetric surveys that covered a total time coverage of 57 years. The cliff retreat was determined by photogrammetric stereorestitution resulting in a value of 0.17m/year with a variation of ± 0.03 m/year for the study area of 1500 m of cliff length between Olhos de Água and Quarteira. A 3D-terrain model was constructed and the volume for each zone and each period was computed allowing the determination of mass movements. A constant loss of volume in the period 19381969 was observed, as well as a constant gain of material in the lower heights (less than 2 m) in the period 1972-1995. In this study we showed that photogrammetric methods could be useful in enabling a robust quantification of coastal erosion parameters, such as cliff retreat. The values obtained for the cliff retreat seem to be well established from the geometrical point of view, but only relate to the "regular" segments of the Algarve cliff, away from local erosion maxima. The results obtained, which indicate a steady reatreat over such a long time span, must be taken into account as far as the near future evolution of this fragile system is concerned.

Acknowledgements We acknowledge the collaboration of the Portuguese Air Force and the Instituto Geográfico do Exército for the use of the historical aerial surveys of the Algarve. We also wish to thank Edward Anthony for his constructive comments in his reviews of the manuscript. This project was supported by RIMAR PRAXIS XXI nº 2/2.1/MAR/1743/95. References Ackerman F. (1984) – Digital Image Correlation: Performance and Potential Application in Photogrammetry. Photogrammetric Record, 11(64), 429-439. Bettencourt P. (1991) – Erosão em quatro sectores da costa algarvia: avaliação das causas, processos e consequências. Seminário A Zona Costeira e os Problemas Ambientais (Eurocoast). Universidade de Aveiro. Brown D. and Arbogast A. (1999) – Digital Photogrammetric Change Analysis as Applied to Active Coastal Dunes in Michi-

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