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Digital Elevation Model (DEM) Data for Portugal

Digital Elevation Model (DEM) Data for Portugal

DEM PT 2

The links below contain digital elevation models for Portugal Continental, obtained from satellite missions: SRTM, ASTER, ALOS-Prism and TerraSAR-X. These data are open, but if used should be referenced to their origin (NASA, JAXA, DLR) and this web page regarding the preparation of data for Portugal Continental. See below details about the original data.

The cells of 1 degree of the area between (10W,36N) and (6W,43N) were combined into a continuous raster. Small groups of empty pixels were filled by interpolation using the GDAL_FILLNODATA command. The raster was projected to the national coordinate system in the PT-TM06 projection of the ETRS89 datum (EPSG:3763) using the GDALWARP command, with a 25 meter pixel (the 1" lat-long pixel has the dimension, at latitude 39d40'N, of 30.84 m by 23.84 m).

The pixel values were obtained by bilinear resampling. In the case of TerraSAR-X, with a resolution of 3" in geographic coordinates, the projected pixel was chosen with the dimension 80 m. All altitudes are above sea level. The boundary coordinates of the projected rasters were chosen as integer numbers in kilometers: (-120, 302) to (164, 278). At this resolution ETRS89 and WGS84 were considered coincident.

All rasters are in the GeoTiff format, as 16-bit integers, "deflate" compression, with internal "tiles" and "overviews", starting at level 4.

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DEMs comparison

The following figures show the 4 DEMs for a 15 km by 15 km area (Serra de Aire). The best quality is the ALOS AW3D. Its main limitation is the relatively high number of empty pixels in two rows (image at the beginning of the page).

ASTRALOSSRTMTSAR

Original datasets

SRTM-DEM was obtained by the Shuttle Radar Topography Mission by SAR Interferometry, in an approximate resolution of 30 m, between latitudes 60S and 60N. Data are available in 14297 granules of 1 by 1 degrees, from NASA Earth Data.

ASTER-GDEM was obtained by photogrammetric methods from the Japanese ASTER-VNIR sensor (infra-red nadir and backwards sensors with 15 m GSD). Data are available for most of the planet in 22912 granules of 1 by 1 degrees from the NASA Earth Data and from Japan Space Systems.

ALOS World 3D DEM was obtained by photogrametric methods from sensor PRISM onboard the Japanese ALOS satellite. PRISM is a triple linear sensor (forward, nadir and backward) which generates panchromatic images with a GSD of 2.5 m. Data was processed in order to obtain DEMs in a spatial resolution of 1 arc-second. Data are available from the Japanese Space Agency - JAXA.

The TerraSAR-X DEM was obtained by the Tandem TerraSAR-X mission, using SAR interferometry. Terrain elevation is obtained in a resolution of 10 meters, but is made available in 3 arc-seconds grid spacing. Data can be obtained from the DLR Geoservice.

This blogpost and its data were prepared by José Alberto Gonçalves ([jagoncal at fc.up.pt](mailto:jagoncal at fc.up.pt)) and André Pinhal ([apinhal at fc.up.pt](apinhal at fc.up.pt))