Validation of SRTM elevation data in Norway

The Shuttle Radar Topography Mission (SRTM) was carried out in February 2000. The goal was to produce a global and consistent digital elevation model with a grid posting of 30 m or 90 m, and with an absolute vertical accuracy better than 16 m (90 % confidence level). FFI used data from the Vestfold and Bykle regions in Norway to validate the SRTM elevation data. Results show that the SRTM DEMs may have a small vertical offset from 0-3 meters. After correcting for vertical offsets in Vestfold, the SRTM X-band and C-band DEMs gave an absolute vertical accuracy of 5.2 m and 6.5 m (90 % confidence level) respectively for agricultural fields. This is much better than specifications. The SRTM interferometric SAR system will normally refer its elevations to the reflective surface. This causes dense Norwegian forest stands to introduce an SRTM elevation that is 15-17 m above the ground. This means that the SRTM system underestimates the true tree height by 6- 8 m. On the contrary, the height of some large buildings is estimated within 1-2 m accuracy. It is also shown that hydroelectric power dams can be used to calibrate the SRTM DEMs to sub-meter accuracy. Poorer SRTM elevation accuracies are noted in areas of particularly low SAR backscatter (e.g. lakes or runways), or in terrain with steep slopes facing the SRTM radar beam direction. The high quality SRTM DEMs can be recommended for many land mapping applications: substituting the 1:50 000 DEMs at many places, correcting/updating old maps, geocorrecting satellite images, or line-of-sight analysis.