SRTM DEM vs one created from 1/50.000 topographic maps. The case of Kos Island

摘要

The Shuttle Radar Topography Mission (SRTM), used an Interferometric Synthetic Aperture Radar (IFSAR) instrument to produce a near-global digital elevation map of the earth's land surface with 16 m absolute vertical height accuracy at 30 meter postings. SRTM achieved the required interferometric baseline by extending a receive-only radar antenna on a 60 meter deployable mast from the shuttle payload bay. Continuous measurement of the interferometric baseline length, attitude, and position was required at the 2 mm, 9 arcsec, and 1 m (1.6 sigma) levels, respectively, in order to obtain the desired height accuracy. The collected data were used to generate a digital topographic map of 80 percent of Earth's land surface (everything between 60 degrees north and 56 degrees south latitude), with data points spaced every 1 arcsecond of latitude and longitude (approximately 30 meters). An SRTM 3-arc-second product (90m resolution) is available for the entire world. In this paper we compare a DTM created from SRTM data to a DTM created from 1/50.000 topographic maps. The area of study is Kos Island in the Aegean Sea. Since the SRTM elevation data are unedited, they contain occasional voids, or gaps, where the terrain lay in the radar beam's shadow or in areas of extremely low radar backscatter, such as sea, dams, lakes and virtually any water covered surface that are flat but they don't look so flat on SRTM tiles. We used different filters and masks in order to ameliorate the quality of the DEM. The first filter detected and removed the voids; a second one interpolated the missing values and then a mask was used in order to separate sea from land. We also created a DTM from digitized contours of 1/50.000 scale topographic maps and we used more than 1800 extra points in order to ameliorate the quality of this DTM around the coastline. We compared the two DTMs. All the results demonstrated that the SRTM DTM presents a very good accuracy.