Quality assessment and validation of SRTM digital surface models in Lower Saxony, Germany

Cooperation with surveying authority of Lower Saxony Landesvermessung und Geobasisinformation Niedersachsen Hannover (LGN)

Research Group: Thematische Bildauswertung und Geo-Information

Contact Persons: Andreas Koch, Peter Lohmann

Background and aim
All over the world there is an advancing demand for Digital Height Models. Traditionally terrestrical survey and / or photogrammetry was the basic to create this product.New technologies like active remote sensing systems will be very important today. These systems are optical laser scanner systems and data of interferometric synthetic aperture radar (SAR) from plane or satellite. At the past missions like ERS-1 / ERS-2 the problem was the bad coherence, i.e. no identical surveying configurations, changes of object and different atmospheric effects. At the XSAR / SRTM (Shuttle Radar Topographic Mission) there is an optimal surveying configuration ("single-path" interferometry).The Digital Surface Models of this mission have to be calibrated and validated (proof of accuracy and quality of the data).

Problem and technique
The ITED-2 data (Interferometric Terrain Elevation Data) of the SRTM mission are compared to the Height and Landcape Models (ATKIS) of the surveying authority in a well known test area. The quality of the Digital Surface Models has to be obtained. Factors influencing the quality like vegetation, land use, terrain slope etc. have to be considered and there effects on the data quality are investigated.
Because of the time difference between the acquisition of the reference and the SRTM data, the reference data were completed with in-situ measurements, analytical photogrammetric measurements, and classifications of satellite imagery.
The project will result in a documentation yielding the geometric accuracy and quality of the ITED-2 products in the complete test site and with special regard to the mentioned parameters. Then it could be a criterium to estimate ITED-2 Surface Models in other regions of the world.

Results
The Shuttle Radar Topography Mission (SRTM) was successfully realized in February 2000. The result of the mission is a threedimensional Digital Surface Model, including objects like vegetation and buildings.
In autumn 2001 the data sets were made available for intensive investigations.
The strategy of investigating the data can be devided into two groups. The first group contains the investigation of the accuracy of the whole test site. The second group contains the investigation of sub regions, which describe the mentioned parameters (e.g. vegetation, land cover, terrain slope, ...).
For analysing the data of the whole test site an algorithms was developed. This method tries to match the ITED-2 data onto the reference data of the surveying authority. As a result we get 7 parameters of a spatial transformation. These parameters serve as measures for existing systematic errors. The method was implemented in C++, the user interface was created by use of the script language TclTk (Manual of the program ELEMA - in German).