French Guiana mapped using ERS radar imagery

With near permanent cloud cover over most of the country, inaccessible terrain, and vast tracts of dense tropical forest, it has proved impossible to produce reliable topographic maps of central and southern French Guiana even at small scales, using either Spot satellite imagery or aerial photography as the main data source.Under conditions such as these - few existing maps, relatively uniform terrain, constant cloud cover - synthetic aperture radar (SAR) stands out as the ideal mapping tool. IGN, France's national survey organization, was thus asked to produce a complete coverage of French Guiana using SAR image maps acquired by ERS spacecraft.

Advantages of mapping using SAR imagery

Among the land-based applications of SAR imagery gathered by the European Space Agency's ERS spacecraft, topographic mapping remained one of the least significant until very recently indeed. With their natural penchant for uniform coverage and exhaustive data, professional cartographers felt that ERS SAR imagery left much to be desired. In particular, they felt it was less rich than Spot optical imagery for the extraction of such planimetric objects as roads, urban areas, land use, and so forth.

Then, studies by IGN Espace found that an absolute location accuracy of 30 m (rms error) could be achieved, without local ground control points (GCPs), using physical modelling by block triangulation and precision orbit data. This result plus the conclusions reached by a pilot project encouraged a digital geography group (Cellule d'Étude en Géographie Numérique, CEGN) within the French Ministry of Defence to investigate further the mapping potential of ERS imagery. In particular, the CEGN asked IGN Espace to produce a complete coverage of French Guiana in the form of ten ERS satellite image maps at 1:200 000.

Available data

The coastal fringe being the only developed part of the territory, it is also the only part covered by existing IGN maps at 1:25 000 or 1:50 000. Mapping of the rest of the territory only began in late 1995. The only imagery available at the time consisted of two complete descending-mode ERS coverages acquired in 1993 and 1994 and a single ascending-mode segment intersecting the other two coverages near the centre of the territory.

Just under 40 images in precision image (PRI) format were ordered from ESA through Spot Image. Existing maps of the coastal fringe provided ample GCPs throughout the northern part of the territory. For the south, an expedition along the Maroni and Oyapock rivers by groups from the French Army's geography department resulted in further GCPs using the GPS system in differential mode.

The pronounced distortion of radar imagery caused by relief can only be corrected using a digital terrain model (DTM). However, with the exception of the IGN's elevation database (BD Alti) for the coastal fringe, no elevation data were available. The only solution was the labour-intensive selection and compilation of existing maps and early survey data followed by the digitization of contours and spot elevation until a satisfactory overall DTM had been built up. Although the accuracy varied somewhat from north to south, the DTM proved sufficient to orthorectify and mosaic all available ERS scenes. Stereo setting of image landmarks within the ascending-mode segment added to the list of points of known elevation derived from existing maps.

Extract of a radar spacemap (image © ESA 92-93) *1

Production methodology

Adopting the same basic methodology as that developed since 1993 for the physical modelling of Spot scenes by block triangulation, IGN Espace began work on the physical modelling of ERS scenes in 1994. The method is based on a physical description of the imaging geometry and the general, block-by-block compensation of the main sources of error, each block consisting of several adjacent or overlapping segments. This ensures geometric consistency across the entire block while at the same time reducing the number of GCPs required for scene adjustment. Given the difficulty associated with GCP setting in radar imagery, this is very important indeed. The GCPs used at this stage were taken from geometrically reliable maps at 1:25 000, which is to say maps of the coastal fringe. Modelling by block triangulation effectively "propagates" this geometric control along the segments, hence from the north to the south. Landmarks identified and located during the GPS survey were then used to check the accuracy of the geometric control achieved by block triangulation. This quality control procedure demonstrated an absolute location accuracy for the southern portion of these segments of 15 m (rms error) using block triangulation to propagate geometric control, which is to say, without the use of local GCPs.

While work proceeded on the orthorectification and mosaicking of the coverage, efforts were also undertaken to improve image readability by reducing the amount of speckle (a form of noise inherent in radar imagery). This involved the multidate merging of the two complete ERS coverages using a new algorithm based on work by Mr Stroobants of the CEGN.

Used as a map backdrop, orthorectified radar imagery offers an estimated planimetric location accuracy of 25 m along the main rivers flowing from the interior, but less in other areas. Although considerably degraded in mountainous areas with few landmarks of known elevation (spot heights), the overall accuracy is nevertheless considerably better than that of rough maps produced in the 1950s on the basis of land surveys. Radar returns being highly sensitive to relief in the presence of uniform landcover, the images contained a wealth of textural detail resulting in the first representation of French Guiana's general topography.

Image interpretation and map compilation are proceeding. While teams at IGN Espace add roads, rivers and streams, and details of inhabited areas to the radar imagery backdrop, thematic specialists are also contributing. A team from the Paris-based Université Pierre et Marie Curie has drawn up a legend for landcover interpretation allowing specialists to identify urban areas, mangroves, swamps, savanna, and forest.

Example of 1:200 000 radar spacemap (image © ESA 92-93) *1


Example of radar image interpretation (image © ESA 92-93) *1

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Complementary References (images ref *1)

Customer References:
Mr. Claude PENICAND - Responsible for Radar Applications
Company: DGA/CEGN
16 bis avenue Prieur de la Côte d'Or, 94 114 Arcueil Cedex - France
Tel: 33 (0) 1 42 31 99 37. Fax: 33 (0) 1 42 31 99 77.

Provider References:
Mr. Gérard LUTROT - Head of Commercial Department
IGN / Commercial Department
2 Avenue Pasteur, 94160 Saint Mandé - France
Tel: 33 (0) 1 43 98 80 00. Fax: 33 (0) 1 43 98 80 31. E-Mail :

Scientific References:
Mr. Mickaël TONON - Project Manager
IGN Espace
Parc Technologique du Canal, 24 rue Hermès, 31527 Ramonville Cedex - France
Tel: 33 (0) 5 62 19 18 18. Fax: 33 (0) 5 61 75 03 17. E-Mail : ignespace@sicoval.fr

Mr. Jean-Paul RUDANT
Université Pierre et Marie Curie
Boîte 129, tour 2616, 4 place Jussieu, 75252 Paris Cedex 05 - France
Tel: 33 (0) 1 44 27 38 70. Fax: 33 (0) 1 44 27 50 85. E-Mail : rudant@dedale.univ-mlv.fr