Digital Elevation Models and Relief Shading Bernd Eversmann Royal University of AgricultureFaculty of Land Management and Land Administration
ContentsWhat is a DEM? How are DEMs generated? Accuracies you can expect What can DEMs be used for? Tools for relief shading
What is a DEM?A digital elevation model is a digital model or 3D representationof a terrains surface (wikipedia.org)
How is a DEM generated? RADAR Interferometry with 2 receivers, 60m apart A) From airborne or satellite remote sensing: Stereo images, RADAR or LIDAR data are used in a semi-automated process to create a DEM.Airborne Laserscanning or LIDAR
How is a DEM generated?B) From terrestrial measurements with precise land surveyinstruments. These spot measurements are then interpolated intoan elevation surface. With this method, it is costly and time-consuming to cover bigger areas.
DEM, DTM and DSM...Digital Surface Model Digital Terrain Model Digital hillshade models generated from a Lidar scan.
What are DEMs used for?Orthophoto production and topographic mapping A DEM is needed for orthorectification of an image, the process of removing the effects of sensor tilt and relief to obtain a photographic map with uniform scale.
A comparison of DEMs ASTER GDEM v2 1) SRTM v3 1) Geoeye 2)Data source ASTER Satellite Space shuttle radar Geoeye SatelliteGeneration and METI/NASA NASA/USGS DigitalglobedistributionRelease year 2011 2003 ~ n/a 2000 - 2010Data acquisition period 11 days (in 2000) n/a (1.514 million scenes)Posting interval 30m 90m 2mVertical accuracy 7 - 14m 5 - 10m (~4m f. 1-arcsecond data) <= 1.5m(RMSE)DEM coverage 83 degrees N ~ 83 degrees S 60 degrees N ~ 56 degrees S n/a Areas with no ASTER data due to Topographically steep areaArea of missing data constant cloud cover (supplied n/a (due to radar characteristics) by other DEM)Vertical accuracies depend on land cover, slope angle and aspect (mainlySRTM), number of stacked stereo pairs (Aster), availability of precise GCPand other factors.1) figures taken from: http://www.jspacesystems.or.jp/ersdac/GDEM/E/2.html and A. K. Karwel, I. Ewiak (2008): ESTIMATION OF THE ACCURACY OF THE SRTM TERRAIN MODEL ON THE AREA OF POLAND2) Paradella, W.; Cheng, P.: Automatic DEM Generation – Using Geoeye-1 Stereo Data in Mining Application; in Geoinformatics, Vol. 16, 01-2013, p. 10-12
Vertical Accuracy NED 1) vs. SRTM Differences1) The National Elevation Dataset (NED) is the primary elevation data product of the USGS. It is a seamless dataset with the best available raster elevation data of the United States.Source: http://topochange.cr.usgs.gov/assessment.php
Absolute Vertical Accuracy SRTM and NED vs. GPS Benchmarks Source: http://topochange.cr.usgs.gov/assessment.php
Hill Shaded Maps Eduard Imhof (1895 – 1986) , Prof. of Cartography at ETH Zurich.
Hand-drawn vs. digital Relief Problems in smaller scales, example CambodiaHand-drawn relief from CIA archives SRTM digital shaded relief
Relief Shading – digital vs. manualShaded relief at 1:1,000,000. Left: Federal Office of Topography swisstopo(DV033492.2). Middle: from raw SRTM data. Right: from SRTM datageneralized with Terrain Sculptor. (Cartographic Perspectives, No. 67, p.52)
Relief Shading – digital vs. manual (2)Shaded relief at 1:15,000,000. Left: Swiss World Atlas 2008. Middle: from rawGTOPO30 data. Right: from GTOPO30 data generalized with Terrain Sculptor.(Cartographic Perspectives, No. 67, p.52)
Filtered digital relief shading Adjustment of visible RidgesMore information: http://www.terraincartography.com/terrainsculptor
Filtered digital relief shading Increasing Valley DepthsMore information: http://www.terraincartography.com/terrainsculptor
LinksEverything on shaded reliefs (techniques, software, ...)http://reliefshading.com/http://shadedrelief.com/Software tools and publications on digital relief shading:http://cartography.oregonstate.eduAster GDEM v2:http://asterweb.jpl.nasa.gov/gdem.aspSRTM v4:http://srtm.csi.cgiar.org/