Integration of Aerial Remote Sensing, Photogrammetry, and GIs Technologies in Seagrass Mapping Vanina Pasqualinl, Chistine Pergent-Martini, Philippe Clabaut, Hew6 Marteel, and G6rard Petgent Abstract surface effects, depth of penetration; Belsher et al., 1988). Con- With a view to implementing a sound management of littoral versely, image processing of normal color aerial photographs plant formations, and more particularly that of seagrass beds, has been successfully used in the littoral environment and, it would appear of interest to test the potentiality of image more particularly, within marine phanerogam seagrass beds processing for various types of photographs (color, infmred, (Kelly, 1980; Walker, 1989; Green et al., 1996; Pasqualini et al., and black and white). The present study was carried out within 1997). Indeed, the littoral environment harbors a vast array of the Gulf of St-Florent (Corsica, France) on a Posidonia complex ecological systems which are extremely diversified oceanica reef platform. Over the last 40 years, no major and productive, such as seagrass beds (Orth et al., 1984; Fortes, diachronic evolution has been observed within this plant 1989). Distributed throughout the planet, these seagrass beds formation. An erosion of the coastline has been noted inland occupy large surface areas (Hartog, 1970) and play a major role of these seagrass beds, however, with erosion values of up to in terms of sedimentology (McRoy and Helfferich, 1980; Fon- 40 m. The use of photogrammetric techniques, which are new seca and Fisher, 1986). In the Mediterranean, the endemic spe- to the marine environment, allows the possibility of obtaining cies Posidonia oceanica (L.) Delile covers a large portion of a numeric model of a given site. The comparison of carto- littoral bottoms, and this from the surface to depths of about 40 graphic and bathymetric data, brought together in a geo- m (Molinier and Picard, 1952).It forms vast underwater mead- graphic information system, allows the spatial distribution of ows but can also, in sheltered areas and at shallow depths, form seagrasses to be evaluated for the first time. homologous structures such as barrier reefs and reef platforms (Molinier and Picard, 1952), thus creating- spectacular- under- water landscapes (UNEP, 1990). Introduction In order to increase our understanding of these seagrass To successfully manage and protect a given natural resource, an beds, a number of techniques, which are usually applied to the exhaustive inventory must be made of the species involved terrestrial environment, were used in the present study to and its geographical distribution. To this end, the techniques examine these shallow-water marine environments. To this incorporating satellite andlor aerial remote sensing, image end, image processing was applied to (1)false-color infrared processing, and photogrammetry were quickly perceived as positive transparencies in order to more easily distinguish being the most appropriate tools (Stretta et d.,1990; Lachow- between the marine plant formations present, and (2) black- ski et al., 1994;French et al., 1995; Cohen et al., 1996; Estreguil and-white photographs in order to follow the temporal evo- and Lambin, 1996; Green et al., 1996; Long and Skewes, 1996), lution of the seagrass beds over several decades. Finally, photo- because they allow the characterization of geographic regions grammetric techniques were used to determine site topogra- which are either very large or not easily accessible (e.g., man- phy, because bathymetric data for such sites are generally groves, mountainous regions, deserts). Thus, the use of infrared lacking, and to evaluate the spatial distribution of the seagrass radiance, obtained from satellite data, allows the accurate beds. mapping of plant formations (Aschbacher et al., 1995),whereas the use of photogrammetry allows the detailed topography of a given site to be determined based solely on the use of aerial Materials and Methods photographs (Fox, 1995). The present study was carried out on the Posidonia oceanica reef formation present in the Gulf of St-Florent (Upper Corsica, The numerous attempts to use image processing of satellite data in the management of benthic marine resources quickly France; Figure 1).This formation is located at the far end of the revealed the limits of these techniques (e.g., pixel size, water gulf, between the Roya rock formation and an artificial dike. This site is subject to moderate hydrodynamic conditions and receives a high level of sediment input due to its proximity to the mouth of a coastal river (the Aliso; Figure 1). The different techniques (image processing, photogram- V. Pasqualini, C. Pergent-Martini, and G. Pergent are with the metry) used were applied to aerial photographs. The different Universit6 de Corse, Equipe c(Ecosyst&mesLittoraux~, B.P. 52,20250 Corte, France ([email protected]). V. Pasqualini is also with the Universit6 de Provence - Centre de St. Jbrome, IMEP, UPRESA CNRS 6116, LBEM, Case 421 bis, Av., Escadrille Normandie Niemen, 13397 Marseille Cedex Photograrnmetric Engineering & Remote Sensing 20, France. VoI. 67, No. 1, January 2001, pp. 99-105. P. Clabaut is at 14 rue P. Doumer, 59 110 La Madeleine, France. 0099-1112/01/6701-99$3.00/0 H. Marteel is at 291 Bd. Clhmenceau, 59700 Marcq en 0 2001 American Society for Photogrammetry Baroeul, France. and Remote Sensing PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING formed of reflectances for each color of the recorded spectra (Blue-Green Plane, Green-Red Plane, and Red-Infrared Plane] for the false-color infrared positive transparencies, and formed of reflectances for 256 shades of grey (Plane in shades of grey] for the black-and-white negatives. Data analysis was then carried out using the Multiscope soft- ware (Version 2.4, Matra Cap System@).A geometrical rectifica- tion and the elimination of terrestrial domains was performed for all of the photographs (Pasqualini et al., 1997). For the normal-color aerial photographs as well as the false-color infrared positive transparencies, a principal com- ponent analysis (PCA) was applied to two planes. A classifica- tion supervised by hypercube was applied to the colored composition, and involved positioning polygons assumed to contain pixels belonging to the same assemblage and bottom type (Courboules et al., 1988). The positioning of the polygons was therefore based on in situ field observations. Homogeniza- tion was carried out to eliminate isolated or wrongly classified points of the image. Occasional corrections were made to the final images on the basis of field data (e.g., litter). Following the above pre-processing, the images generated from the black-and-white photographs were subjected to pixel thresholding. This thresholding consists in determining the thresholds or limits for each of the assemblages or bottom types to be examined based on field data. This method allowed a the- matic map to be drawn. Figure 1. Location of the study site. Photogrammetry Photogrammetry consists in using the common region of two photographs taken from different angles to rectify those photo- aerial photographs used were normal-color photographs, false- graphs and to determine the altitude fiom which the photo- color infrared photographs, and black-and-white photographs graphs were taken. The technical details involved in the taking (Table 1).False-color infrared positive transparencies were of the photographs (usable surface area, bottom cover, scale, obtained using a film for which the sensitivity to light encom- verticality) in addition to the meteorological conditions passed the entire visible range (400 to 750 nm) as well as wave- (amount of light, visibility) were determined by the Conseil lengths approaching the infrared (750 to 900 nm; Kodak National de l'lnformation Gtiographique (CMG, 1993). The Aerochrome Infrared 50-231). photogrammetric techniques were applied to the normal-color Regardless of the nature of the photograph used, four photographs taken in 1994. The "stereoscopic pairs" were digi- assemblages and bottom types were taken into account: (I) soft tized and processed by computer. This processing involves geo- sediment (mud and sand), (2) photophilous algae on rock metric rectification or transformation on the basis of accurately (including scree and pebbles), (3) continuous seagrass beds of located landmarks. The photogrammetric techniques allow an Posidonia oceanica (presenting coverage greater than 50 per- object or area to be represented in three dimensions. In this cent), and (4) dead "matte" (assemblage of intermingled dead way, a bathymetric survey of the reef formation was produced rhizomes with sediment) or mosaic seagrass beds of Posidonia at a scale of 1:5,000,with a minimum vertical distance of 0.5 m oceanica (on matte, rock, or sand). The collection of field data between the contour lines. was performed by a scuba diver using the transect method (Meinesz et nl., 1981). Depth was recorded along each transect Geographic Information Systems using an electronic depth gauge (Suntom with an accuracy of 10 A numeric field model was generated using the Arc Info soft- cm). ware (Version 7, ESRIa). This model was based on the coverage of the arcs corresponding to thebathyrnetric data and allowed a image Processing slope coverage to be determined. The totality of the carto- Aerial photographs were digitized by means of a scanner graphic data obtained by image processing was vectorized and (Canon CLC 10, driven by a Pentium 100 microcomputer)