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The feasibility of topographic mapping through orbital remote sensing was investigated for the Brazilian Amazon. The study area is in a region of low topographic terrain within the Tapajós National Forest. Two kinds of radargrammetric digital elevation models (DEMs), one based solely on satellite ancillary data and one calibrated with ground control points (GCPs), were produced based on a fine RADARSAT-1 stereopair (F2/F5) and evaluated regarding accurate field planialtimetric measurements. The geometric modeling for the DEM extractions was based on the RADARSAT-1 specific model from OESE software (PCI Geomatics Inc.). The planimetric features were extracted from integrated fine and Landsat thematic mapper (TM) products. Precise topographic field information from a differential global positioning system (DGPS) was used as GCPs for the modeling of the DEMs and for the orthorectification of the synthetic aperture radar (SAR) and optical data and as independent check points (ICPs) for the calculation of planialtimetric accuracies of the products. The investigation has shown that the accuracy of the topographic map met the requirements for a 1 : 100 000 scale map (class A) as requested by the Brazilian Standard for Cartographic Accuracy. The approach is a realistic alternative for topographic mapping at a semi-detailed scale in similar environments of the Amazon, where terrain information is seldom available or is of low quality. Résumé. Dans cet article, on étudie la faisabilité de la cartographie topographique à l’aide de la télédétection satellitaire dans la région de l’Amazone, au Brésil. La zone d’étude, caractérisée par un terrain à faible topographie, est située dans la Forêt nationale de Tapajós. Deux types de modèles numériques d’altitude radargrammétriques, un MNA basé uniquement sur des données satellitaires auxiliaires et un MNA calibré à l’aide de points d’appui (GCP), ont été produits basés sur des couples stéréoscopiques (F2/F5) d’images RADARSAT-1 en mode fin et évalués par rapport à des mesures planialtimétriques précises sur le terrain. La modélisation géométrique pour l’extraction des MNA était basée sur le modèle « RADARSAT-1 specific model » du logiciel OESE (PCI Geomatics Inc.). Les caractéristiques planimétriques ont été extraites des produits intégrés mode fin/TM de Landsat. L’information topographique précise acquise sur le terrain à l’aide d’un système DGPS (positionnement par la méthode spatiale différentielle) a été utilisée comme points d’appui pour la modélisation des MNA et pour l’orthorectification des données RSO et optiques, et comme points de contrôle indépendants (ICP) pour le calcul des précisions plani-altimétriques des produits. L’étude a démontré que la précision de la carte topographique rencontrait les exigences pour la carte au 1 : 100 000 (classe A) tel que recommandé par le Standard brésilien pour la précision cartographique. L’approche est une alternative valable pour la cartographie topographique à l’échelle semidétaillée dans des zones similaires de l’Amazone, où l’information sur le terrain est rarement disponible ou de basse qualité. [Traduit par la Rédaction] 148 Introduction and context The Brazilian Amazon, with an area of almost 5 500 000 km2, presents, under an apparently homogeneous forest physiognomy, enormous spatial variability in climate, geology, soils, river systems, and plant and animal communities. Integration of the region into the economy of Brazil has been occurring at a fast pace for the last four decades, driven by a road network established in the 1960s and by intense internal migration of agricultural and mining workers in search of new and better opportunities. Such changes in the demographic and land-use patterns of the region require thorough planning by state and federal administrations. Government infrastructure planning and resources assessment, monitoring, and management are based on geospatial information. The primary inputs supporting all geographic information are topographic maps. Owing to the adverse environmental conditions (rain, cloud, smoke, dense vegetation, and poor access), the use of optical data for regular coverage is expensive and often not possible. As a consequence, the Brazilian topography is poorly known, with © 2005 CASI 139 Can. J. Remote Sensing, Vol. 31, No. 2, pp. 139–148, 2005 Received 15 September 2004. Accepted 31 January 2005. W.R. Paradella.1 National Institute for Space Research (INPE), CP 515, CEP 12227-010, São José dos Campos, São Paulo, Brazil. C.G. Oliveira, S. Luiz, I.C. F. Cecarelli, C.P. Cottini, and R. Okida. Geoambiente Sensoriamento Remoto S/C Ltda, CEP 12242-280, São José dos Campos, São Paulo, Brazil. 1Corresponding author (e-mail: waldir@ltid.inpe.br). almost 25% of the area covered by maps at the reconnaissance scale (1 : 250 000 up to 1 : 1 000 000). For the remainder of the region, the cartography at a semidetailed scale (1 : 100 000) needs to be updated or redone (Figure 1). With the advent of RADARSAT-1 in 1995, providing data with day or night, all-weather imaging capability and multiple modes (distinct look azimuth, incidence angle, spatial resolution, and imaged area), digital elevation models (DEMs) could be for the first time systematically generated using orbital stereoscopy. Several results have been published in the literature with a general consensus of the elevation accuracy with this technology: around 20 m for standard and 12 m for fine stereopairs (Toutin, 1998; 1999; 2000; Sylvander et al., 1998; Dupont et al., 1998; Raggam and Gutjahr, 1998; among others). The extension of these results for operational use in the moist tropics should be taken with caution, however. First, these investigations were concentrated at sites with very favourable environmental (low to moderate vegetation cover, well-defined ground features within the synthetic aperture radar (SAR) images, etc.) and experimental conditions (ancillary planialtimetric data with high quality and quantity to derive DEMs and test accuracies) compared to the overall absence of information in the remote regions of the tropics. Second, selecting ground features with which to collect an adequate number of well-distributed and good-quality ground control points (GCPs) is critical for the operational use of the technology in the tropics, an issue that has not been thoroughly addressed in the literature. Natural ground features (clearings, outcrops, ridges, minor drainages, etc.) or anthropogenic targets (buildings, bridges, road intersections, agricultural field boundaries, etc.) are generally not available or poorly expressed in SAR images in this type of environment. In addition, topographic maps are rare, outdated, and of poor altimetric quality. In this research, the production of a topographic map based on two sources of orbital remote sensing (RS) data was investigated for a region located in the central Amazon. Using digital global positioning system (DGPS) measurements and planialtimetric information derived from RADARSAT-1 fine stereoscopy integrated with Landsat5 thematic mapper (TM) data, it was possible to produce a topographic map that fulfilled the requirements of the Brazilian cartographic standards for the 1 : 100 000 scale. The research was funded by the State of São Paulo Research Foundation (FAPESP) and had support from the Diretoria de Serviço 140 © 2005 CASI Vol. 31, No. 2, April/avril 2005 Figure 1. Status of Brazilian cartography: 1 : 50 000 scale (blue); 1 : 100 000 scale (red); 1 : 250 000 up to 1 : 1 000 000 scale (green) (from IBGE, 2001). Geográfico (DSG), an agency of the Brazilian army with the national mandate for cartographic production.