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Bambang Trisakti, Ita Carolita Research Officer of Indonesian National Institute of Aeronautic and Space (LAPAN) btris01@yahoo.com ABSTRACT This paper explains a method to generate DEM from ASTER (Advance Space borne Thermal Emission and Reflection Radiometer) stereo data and evaluates the generation of ASTER DEM and SRTM DEM (Space Shuttle Radar Topography Mission) with 90 m spatial resolution. ASTER DEM is generated from 3n (nadir) and 3b (backward) level 1b, with 10 ground control points (XYZ coordinate) derived from ASTER RGB 321 geometric-corrected and SRTM DEM. Almost all tie points are collected automatically and several tie points are added manually. The triangulation and DEM extraction process are done automatically by using ERDAS Imagine Software. DEM Evaluation is carried out by comparing between ASTER DEM and SRTM DEM for the height distribution of vertical and horizontal transect lines and the height value of the whole DEM images. The process is continued by analyzing the height differences between ASTER DEM and SRTM DEM. The results show that ASTER DEM has 15 m spatial resolution with height differences less than 30 m for about 67% of total area, and absolute mean error is 27 m (compared with SRTM DEM). This absolute mean error is large enough, because the GCP's used in this study are only in a small amount and most of study area is in the high terrain area (mountainous area) with dense vegetation coverage. 1. INTRODUCTION ASTER (Advance Space borne Thermal Emission and Reflection Radiometer) on board of Terra spacecraft is multi spectral optical sensor that is launched on December 1999. ASTER sensor has 14 spectral bands that range from visible to thermal infrared band. All spectral bands of ASTER are divided into three radiometers: VNIR (Visible Near Infrared Radiometer), SWIR (Short Wave Infrared Radiometer) and TIR (Thermal Infrared Radiometer) (Ersdac, 2003). VNIR has a high performance, high resolution optical instrument (15 m spatial resolution) which is able to detect reflection from the ground surface ranging from visible to near infra red (0.52 - 0.86 mm). VNIR has 2 (two) near infra red bands which have similar wavelengths, those are 3n (nadir looking) and 3b (backward looking). The 3b band is used to achieve the backward looking, with setting angle between the backward looking and the nadir looking is design to be 27,60° (Ersdac, 2002). The objection of band 3b addition is to obtain stereoscopic image that will be processed to generate the height of land surface or DEM (Digital Elevation Model). DEM of land surface provides significant information for many research activities and important data as the input of image processing and image analysis, such as image correction due to height of land surface (Ortho rectification), contour mapping, 3D images generation, disaster management (determination of vulnerable area due to landslide, flood and tsunami disaster), monitoring land subsidence phenomenon and many others. Therefore, the capabilities of ASTER stereo image that provides DEM with high spatial resolution (15 m) is very important for remote sensing and GIS (Geographic Information System) users to enhance the accuracy of desired height information. The method of DEM generation from satellite stereo image (Such as ASTER, SPOT, etc) and the accuracy of generated DEM have been published in many research reports, for examples: (Goncalves and Oliveira, 2004; Tsakiri-Strati et al, 2004; Pantelis et al, 2004; Ulrich et al, 2003). Several study results show that the vertical accuracy of ASTER DEM approaches to 25 m, but in area with less vegetation coverage, the accuracy can rise approximately to 9-11 m (Goncalves and Oliveira, 2004; Richard Selby, PCI Geo). Furthermore, Ulrich et al (2003) have reported that ASTER DEM has better accuracy for medium scale mapping (1:100.000 and 1:50.000). This paper describes DEM generation method from ASTER stereo image, and evaluates the accuracy of generated ASTER DEM by comparing it to SRTM DEM (Shuttle Radar Topography Mission) with 90 m spatial resolution. DEM is generated from 3n (nadir looking) and 3b (backward looking) band of ASTER data level 1b by using Ground Control Points (XYZ coordinate) from ASTER RGB 321 geometric-corrected image and SRTM DEM. Furthermore, evaluation of DEM accuracy is done by analyzing the height different between ASTER DEM and SRTM DEM. 2. METHOD This study uses level 1b ASTER data (Source: ERSDAC and Indonesia RS-GIS Forum) with 15 m spatial resolution, SRTM DEM (Source: obtaining from USGS website) with 90 m spatial resolution and ASTER RGB 321 geometric-corrected image based on IKONOS image. The Study area is located in Lhok Nga (Nanggroe Aceh Darussalam Province, Indonesia) as shown in Picture 1. The coastal region of this area is affected by tsunami that happened on 26th December 2004. Data pre-processing is started by cropping the interest area for 3n (nadir looking) and 3n (backward looking) band. Then both images are rotated 270 degrees clockwise. Destriping process should be done for level 1a of ASTER data, but it is no need to be done for level 1b data, since this level has been corrected from striping distortion. The next processes are done by using Erdas Imagine Software (Orthobase-Pro module). The processes are started by pyramid layer making, and then insert appropriate sensor model and orientation parameters (such as: side incidence, sensor column, pixel size, etc) which some of them can be obtained from ancillary data. Ground Control Points (GCPs) collecting (XY coordinate point and height (Z) point) is carried out by using ASTER RGB 321 geometric-corrected image based on IKONOS for XY references and SRTM DEM for Z references. This study uses 10 GCPs, and those GCPs will be used as the reference for tie point making. Almost all tie points are collected automatically and several tie points are added manually. The next step is triangulation process which aims to relate the XY points on image, GCPs and sensor specification information, so that the formulation of these 3 (three) related parameters can be developed. The last step is generating DEM from the overlapping area between 3n and 3b bands.  Picture 1 Study area and image of band 3n and 3b of ASTER data The Accuracy of the generated ASTER DEM is evaluated by comparing height value of ASTER DEM and SRTM DEM. The vertical and horizontal transect lines are drawn along the both DEM images, and then the height distribution of each transect lines are compared. Finally, the height differences between ASTER DEM and SRTM DEM are extracted and classified to investigate mean absolute error between the both DEM images. Classification is done in land area only (water area is not included).
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