DEM from ALOS data and comparison to DEM from other sources. The case of Antiparos Island, Greece.
Panagiotis Tsombos(1) and Konstantinos Nikolakopoulos(1)
(1) Institute of Geology & Mineral Exploration (IGME), 20, Chrysoupoleos str., 15127 Melissia, Athens, Greece
One of the major projects of the Institute of Geology & Mineral Exploration (IGME) is called “Urban Geology”. In the frame of that project there is need for a high accuracy DEM covering the whole country. The DEM should be used for the orthorectification of high resolution images and other applications such as slope map creation, environmental planning et.c. Thus, there is a huge pressure for very accurate elevation data covering the entire country surface. Image stereopairs form satellite sensors seem to provide a quite accurate and cost affordable source of elevation data.
In 1986, SPOT was the first satellite to provide stereoscopic images that allowed extraction of DEMs over large areas of the Earth’s surface. For the first time, the scientific community was able to extract three-dimensional data over areas of interest that were still inaccessible before SPOT launch. Since this time, various analogue or digital sensors in the visible spectrum have been flown, providing users with spatial data for extracting and interpreting three-dimensional information on the Earth’s surface. During the early years the satellite stereo-pairs were acquired across track on different days (SPOT, ERS etc.). More recently, the same-date, along-track stereo-data acquisition was adopted on the ASTER radiometer onboard the Terra satellite and on the SPOT-5 satellite. It reduces the radiometric image variations (refractive effects, sun illumination, temporal changes) and thus increases the correlation success rate in any image matching. The automatic DEM generation has become an important part of international research in the last 10 years as a result of the existence of many satellite sensors that can provide stereo pairs.
One of the newest satellite sensors with stereo collection capability is ALOS. ALOS has a panchromatic radiometer with 2.5m spatial resolution at nadir. According to the specifications its extracted data will provide a highly accurate digital surface model (DSM). Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) has three independent optical systems for viewing nadir, forward and backward producing a stereoscopic image along the satellite's track. Each telescope consists of three mirrors and several CCD detectors for push-broom scanning. The nadir-viewing telescope covers a width of 70km; forward and backward telescopes cover 35km each.
In this study we compare a DEM created from an ALOS stereo-pair to DEM created from other elevation data sources: 1/50.000 topographic maps and airphotos stereo-pair. The area of study is Antiparos Island, Greece.
After a first control for random or systematic errors a statistical analysis was done. Points of known elevation have been used to estimate the accuracy of these three DEMs. The elevation difference between the different DEMs was calculated. 2D RMSE, correlation and the percentile value were also computed and the results are presented.