Geomorphometry of Cerro Sillajhuay (Andes, Chile/Bolivia): Comparison of Digital Elevation Models (Dems) from ASTER Remote Sensing Data and Contour Maps
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Geomorphometry of Cerro Sillajhuay (Andes, Chile/Bolivia): Comparison of Digital Elevation Models (DEMs) from ASTER Remote Sensing Data and Contour Maps Ulrich Kamp Department of Geography and Environmental Science Program, DePaul University 990 W Fullerton Ave, Chicago, IL 60614-2458, U.S.A. E-mail: [email protected] Tobias Bolch Department of Geography, Humboldt University Berlin Rudower Chausse 16, Unter den Linden 6, 10099 Berlin, Germany E-mail: [email protected] Jeffrey Olsenholler Department of Geography and Geology, University of Nebraska - Omaha 6001 Dodge Street, Omaha, NE 68182-0199, U.S.A. [email protected] Abstract Digital elevation models (DEMs) are increasingly used for visual and mathematical analysis of topography, landscapes and landforms, as well as modeling of surface processes. To accomplish this, the DEM must represent the terrain as accurately as possible, since the accuracy of the DEM determines the reliability of the geomorphometric analysis. For Cerro Sillajhuay in the Andes of Chile/Bolivia two DEMs are compared: one derived from contour maps, the other from a satellite stereo-pair from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). As both DEM procedures produce estimates of elevation, quantative analysis of each DEM was limited. The original ASTER DEM has a horizontal resolution of 30 m and was generated using tie points (TPs) and ground control points (GCPs). It was then resampled to 15 m resolution, the resolution of the VNIR bands. Five parameters were calculated for geomorphometric interpretation: elevation, slope angle, slope aspect, vertical curvature, and tangential curvature. Other calculations include flow lines and solar radiation. Although elevations are too low above 5000 m asl., the ASTER DEM offers reliable results when analyzing the macro- and mesorelief, and for mapping at medium scales (1:100,000 to 1:50,000). Introduction A DEM offers the most common method for extracting vital topographic information and even enables the modeling Digital elevation models (DEMs) are increasingly used of flow across topography, a controlling factor in distributed for visual and mathematical analysis of topography, models of landform processes (Dietrich et al., 1993; Desmet landscapes and landforms, as well as modeling of surface and Govers, 1995; Kirkby, 1990). To accomplish this, the processes (Bishop and Shroder, 2000; Dikau et al., 1995; DEM must represent the terrain as accurately as possible, Giles, 1998; Millaresis and Argialas, 2000; Tucker et al., since the accuracy of the DEM determines the reliability of 2001). DEMs play also an important tool for the analysis of the geomorphometric analysis. Currently, the automatic glaciers and glaciated terrains (Baral and Gupta, 1997; generation of a DEM from remotely sensed data with sub- Duncan et al., 1998; Etzelmüller and Sollid, 1997; Lodwick pixel accuracy is possible (Krzystek, 1995). A number of and Paine, 1985; Sidjak and Wheate, 1999; Theakstone and researchers have studied the effects of DEM resolution on Jacobsen, 1997). Bishop et al. (2002) used a DEM of Nanga the estimation of topography (Band et al., 1995; Chang and Parbat to map glaciers in the rough mountain terrain of the Tsai, 1991; Gao, 1997; Hodgson, 1995; Schoorl et al., 2000; western Himalayas. Gratton et al. (1993) calculated the net Stefanovic and Wiersema, 1985). radiation field of mountain glacier outlets in the Rocky DEMs can be generated from electro-optic scanners such Mountains from a DEM. as ASTER (Advanced Spaceborne Thermal Emission and Geocarto International, Vol. 20, No. 1, March 2005 E-mail: [email protected] 23 Published by Geocarto International Centre, G.P.O. Box 4122, Hong Kong. Website: http://www.geocarto.com Reflection Radiometer). ASTER offers simultaneous along- from a SPOT stereo-pair with a DEM derived from contour track stereo-pairs, which eliminate variations caused by multi- maps (1:50,000 and 1:250,000), and found a very good date stereo data acquisition. Only some results have been agreement between both DEMs. published in peer-reviewed literature about using ASTER Fieldwork at the Cerro Sillajhuay was conducted only on data yet, mostly on simulated ASTER data (Abrams and the Chilean side during March and April 1998 and focused Hook, 1995; Shi, 2001, Welch et al., 1998), or on the on geomorphological mapping after Kneisel et al. (1998) potential of using ASTER data in the future (Raup et al., and Schröder and Makki (1998) with special respect to 2000). Kääb (2002) and Kääb et al. (2002) used ASTER geomorphological processes as well as glacial and periglacial images for a monitoring of high-mountain terrain deformation forms above 4300 m asl. (Bolch and Schröder, 2001). Aerial and glaciers, respectively; Wessels et al. (2002) used ASTER photographs with a resolution of approximately 2.5 m from images for analyzing supraglacial lakes at Mt. Everest. Cheng 1961 were used for orientation and to monitor the and Bean (2002) published first results about the generation geomorphological mapping. As fieldwork was not possible of ASTER DEMs for Afghanistan. Eckert and Kellenberger on the Bolivian side, a detailed, realistic geomorphological (2002) compared two ASTER DEMs from mountainous mapping of the entire study area is only possible with the regions in Switzerland with DEMs from InSAR data and the help of DEM data. Swiss DEM25. For steep, forested terrain the calculated mean difference between the DEMs was only 15-23 m. Study Area This paper compares the quality of two DEMs of the Cerro Sillajhuay in the Andes of Chile/Bolivia. One DEM Cerro Sillajhuay (5982 m asl., 19º45' S / 68º42' W) is was derived from contour maps (here: Contour DEM) and located in the Andes of Chile/Bolivia, and represents the the other was derived from ASTER (here: ASTER DEM). highest peak of the Andes between 19 and 21 degrees south Similar work has been done by Al-Rousan et al. (1997) and (Fig. 1). The stratovolcano is in a horst running crosswise to Al-Rousan and Petrie (1998), who compared a DEM derived the main north-south orientation of the main Andes ridge as Figure 1 Location map of the study area in the Andes of Chile/Bolivia. For profiles A and B see Fig. 10. 24 a result of tectonic stress during uplift of the western Cordillera. It rises ~ 2000 m above the surrounding plain (Fig. 2), and is surrounded by several other volcanos. The median altitude of the massif is ca. 5030 m asl., and most of the terrain lies between 4750 and 5250 m asl. The ca. 3700- m high Salar de Coipasa and Salar de Uyuni on the Bolivian side represent the local erosion level. From here, the pediments smoothly slope up to the Cerro Sillajhuay massif, and only isolated hills break through the generally flat terrain. The massif itself is deeply incised by steeply sloping valleys. For example, the 2-km long Rio Blanco Valley ascends ca. 1100 m from mouth to peak. The ridge of the entire massif is more or less complete, and from here the relief plunges down to the valleys. In the southeast of the volcano lies Cancosa Basin at 3900 m asl., which was formerly covered by a lake (Schröder et al. 1999). The study area is part of the Atacama, which is the driest part of the Andes; precipitation occurs in summer. Bolch and Schröder (2001) assume a precipitation of ca. 200 mm in 4500 m asl., and 300-400 mm in 5000 m asl. Cerro Sillajhuay itself is characterized by a local mountain climate with high diurnal variations in temperature and frequent days with frost-thaw-cycles. Discharge is orientated to the interior in the east. Vegetation cover is extremely sparse; grasses and shrubs are dominating and only isolated trees (polylepis) occur. DEM from Contour Maps Figure 2 View across Cancosa Basin to Cerro Sillajhuay, a 5982-m high stratovolcano in the Andes of Chile and Bolivia: (A) photograph; DEM Generation and 3D-Visualization (B) ASTER DEM. (Photo: G. Kröber, March 1998). Converting contour maps is a common way to create DEMs. For the Cerro Sillajhuay, a DEM was developed by digitizing contour lines from rectified topographic maps Therefore, in a second step some contour lines were manually 1:50,000 with 50 m-equidistance (1945-6845 Pampa Lirima, added using information on elevation from stereoscopic 1945-6830 Cancosa, 1930-6845 Lagunas Chuncara, 1930- analysis of the aerial photographs from 1961 with a resolution 6830 Villa Blanca), which represent the best available maps of 2.5 m. In the resulting DEM, nearly all artifacts could be for the study area. Additionally, depth lines and some ridges eliminated. A disadvantage of the triangulation is the so- were digitized. Digitizing was carried out using a digitizing called diamond effect, i.e. each triangle has its own inclination tablet and the software ArcView GIS 3.1 from ESRI. The and exposition, which impedes differentiation of such DEM was developed in the Universal Transverse Mercator locations. Thus in a third step the TIN-DEM was converted (UTM) projection, zone 19S, WGS 84. It covers an area of into a grid-based DEM using the software ArcInfo 7.2 from 10 x 17 km with the peak of Cerro Sillajhuay in its center. ESRI. The grid-based DEM was smoothed two times using Three calculation methods were tested for data the ‘quintic’ filter, a nearest-neighbor algorithm of ArcInfo, interpolation: triangulated-irregular-networks interpolation and the ‘grid generation tool’, a nearest-neighbor filter of (TIN), inverse-distance-weighted interpolation (IDW), and ArcView. The horizontal DEM grid resolution was set to 50 spline interpolation. Both IDW and spline method produced m, matching the underlying topographic map. It is a common very good results for areas of high relief, but produced problem in DEM generation that smoothing operations lead artifacts such as unrealistic low slopes in generally flat to generalization, i.e.