Abstract
Difficulties in acquiring a complete aerial photography coverage on a regular basis in the Brazilian Amazon due to adverse environmental conditions affect the quality of the national topographic database. As a consequence, topographic information is still poor, and when available needs to be up-dated or re-mapped. In this research, altimetric information derived from RADARSAT-1 (Fine and Standard modes), SRTM3 (3 arc-seconds) and ASTER (band 3N-3B) was evaluated for topographic mapping in two sites located in the region: Serra dos Carajás (mountainous relief) and Tapajós National Forest (flat terrain). The quality of the information produced from Digital Elevation Models (DEMs) was evaluated regarding field altimetric measurements. Precise topographic field information acquired from Differential Global Positioning System (DGPS) was used as Ground Control Points (GCPs) for the modeling of the stereoscopic DEMs (RADARSAT-1, ASTER) and as Independent Check Points (ICPs) for the calculation of accuracies of the products. The accuracies were estimated by comparison of the DEMs values and real elevation values given by ICPs. The analysis was performed following two approaches: (1) the use of Root Mean Square Error (RMSE) for the overall classification of the DEMs considering the Brazilian Map Accuracy Standards (PEC) limits and, (2) calculations of trend analysis and accuracy based on a methodology that takes into account computed discrepancies and standard deviations. The investigation has shown that for flat relief, the altimetric accuracy of SRTM3 and Fine RADARSAT-1 DEMs fulfilled the PEC requirements for 1:100,000 A Class Map. However, for mountainous terrain, only the altimetry of SRTM3 and ASTER fulfilled these requirements. In addition, the performance of ASTER was slightly superior to SRTM3. However it is important to consider the difficulties in the acquisition of good stereo-pairs with optical data in the Amazon and the additional cost (GCPs) to produce ASTER DEMs. Despite showing systematic errors, the findings justify the usage of SRTM3 as a primary elevation source for semi-detailed topographic mapping in the region. It is suggested a combination of altimetry derived for SRTM3 and planimetry extracted from high-resolution SAR (ALOS/PALSAR, TerraSAR-X, RADARSAT-2) or if available optical data for semi-detailed topographic mapping programs in the Brazilian Amazon, where terrain information is seldom available or presents low quality.
Highlights
Topographic mapping in large areas of the Brazilian Amazon region has always been a challenge, even under the most suitable field infrastructure
SRTM3 Digital Elevation Models (DEMs) and Fine RADARSAT-1 fulfilled the PEC requirements for 1:100,000 A Class Map; (2) for mountainous terrain (Carajás), the altimetry of SRTM3 and ASTER fulfilled the requirements for 1:100,000 A Class, while Fine and Standard RADARSAT-1 did not; (3) the performance of ASTER was slightly superior to SRTM3
It is important to consider the difficulties of acquisition of good stereo-pairs with optical data in the Amazon and the additional cost with Ground Control Points (GCPs) to produce ASTER DEMs
Summary
Topographic mapping in large areas of the Brazilian Amazon region has always been a challenge, even under the most suitable field infrastructure. The topographic knowledge is still poorly known, with almost 25% of the area covered by maps at reconnaissance scale (1: 250,000 up to 1:1,000,000). The integration of the region to the economy of the remainder of the country is in a fast pace for the last four decades driven by a road network established in the sixties and by internal migration of agricultural and mining workers in search for better opportunities. Such changes in the demographic and land use patterns require thorough planning by State and federal levels administrations. The primary inputs supporting all geographic information are topographic maps
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