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Application of Digital Elevation Models to Geological and Geomorphological Studies — Some Examples

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Application of Digital Elevation Models to Geological and Geomorphological Studies — Some Examples Przegl¹d Geologiczny, vol. 53, nr 10/2, 2005 Application of digital elevation models to geological and geomorphological studies — some examples Janusz Badura*, Bogus³aw Przybylski* Abstract. Analysis of the Earth’ssurface using three-dimensional models provides a wealth of new interpretation opportunities to geologists and geomorphologists. Linear elements, not visi- ble on classical maps, become distinct features; it is also possible to interpret both small-scale glacial landforms and entire complexes of postglacial landscapes at the regional scale. Geomorphic features are frequently difficult to recognise in the field, either due to their scale or field obstacles. Three-dimensional visualization of the Earth’s surface and its examination at different angles and differently orientated source of light is extremely helpful in geological and geomorphological studies. This tool is, however, relatively seldom used due to either limited access to digital data bases or time-consuming procedures of individual construction of such bases from the existing cartographic data. For instance, analysis of small-scale glacial, fluvial J. Badura B. Przybylski or aeolian landforms in lowland areas requires cartographic data of resolution compatible with 1 : 10,000 scale. Nevertheless, less detailed digital elevation models, constructed at the scale of 1 : 50,000, are also extremely helpful, since they allow for regional interpretations of those morphostructures which are associated not only with neotectonics, but also with ice-flow directions, block disintegration of an ice-sheet, subglacial drainage, stages of fluvial ero- sion, or location of dune belts. A possibility of superposition of geological or geomorphological maps onto 3-D models is equally important, enhancing readability of the maps and providing clues to verification of the origin of landforms and proper cross-cutting relationships drawn on the map. Key words: digital elevation model, geological cartography, SW Poland Rapid technological progress in electronics during the 2004). Linear elements of the Earth’s surface of potentially last years has resulted in increasingly more widespread tectonic origin, called topolineaments, used to be analysed application of digital elevation models (DEM) in geomor- by methods based on topographic maps and aerial and phological and geological research. Methods of obtaining satellite images (Ostaficzuk, 1977; Graniczny, 1994; aerial and satellite radar images have become more and Badura & Przybylski, 1995; Migoñ, 1996). Application of more sophisticated, and significant progress in digital data DEMs enables for more detailed, in-depth and quick analy- processing is observed. Topographic data, whose pro- sis of such structures. cessing required large data centers ten to twenty years ago, DEM images are more and more frequently used in an can now easily be transformed into DEMs and analysed on analysis of areas covered by Pleistocene ice-sheets (Clark, a regular PC. Digital elevation models more and more 1997; Munro & Shaw, 1997; Clark & Meehan, 2001; Fisher frequently replace traditional topographic maps in earth & Taylor, 2002; Munro-Stasiuk & Shaw, 2002; Sjorgen et sciences. al., 2002; Jorgensen & Piotrowski, 2003, Raunholm et al., Shaded relief maps, portraying pseudo-3D image on a 2003; Clark et al., 2004). Depending on DEM resolution, plane, were already introduced in the 17th century. At the one can analyse either single landforms or regional distri- end of the 18th century, a Saxon cartographer, Johann Leh- bution of assemblages of glacial landforms, like: eskers, mann, formulated mathematical rules necessary for the subglacial channels, drumlins, terminal moraines, margi- construction of maps showing relief shaded at a different nal streamways (“pradolinas”), or melt-out water valleys. angle. Shaded relief maps produced with the help of digital These studies help to reconstruct the peculiarities of elevation models are based on more or less the same princi- ice-sheet advances, dynamicity of ice streams, and glacier ples. Computer-stimulated progress consists first of all in extent. shortening the time necessary for processing of such maps, We have used digital elevation models in neotectonic from many months or even years to seconds or fractions of studies and analyses of glacial landforms of the Lower seconds. Suitable DEM software enables one to quickly Silesia and Gniezno Pomerania regions. Selected examples generate shaded relief maps using differently orientated of such studies are presented below. sources of light, as well as to construct different blockdia- grams. Further processing of a virtual 3D model makes it Methodology — elevation data bases and software possible to generate images at such speed that they produce an impression of virtual reality. Such an effect is used, i.a. Each digital evelation model requires a spatial data in computer games and flight simulators. base wherein points on the Earth’s surface are assigned As far as geological sciences are concerned, the studies coordinates XY on the plane, and the vertical coordinate Z. of neotectonic structures are one of those disciplines which In order to construct digital maps and blockdiagrams, it is widely use digital elevation models (Badura, 1996, 1999; necessary to transform XYZ data into the so-called grid, Przybylski, 1998; Grygar & Jelinek, 2003; Sherrod et al., i.e. 3D network which is recognised by the computer as a model of the Earth’s surface. Such grid can be generated by different programs. One of popular tools of such type is the *Polish Geological Institute, Lower Silesian Branch, Surfer program, produced by Golden Software. Reliability Jaworowa 19, 53-122 Wroc³aw; [email protected]; of projection of a surface depends indirectly on grid densi- [email protected] ty, but in fact it is limited by the available input of XYZ 977 Przegl¹d Geologiczny, vol. 53, nr 10/2, 2005 data. If points used for grid construction are widely-spaced, As far as lowland areas are concerned, however, their appli- their artificial digital condensing does not influence projec- cability is limited due to obscuring effects of vegetation tion quality. Some interpolation methods can lead to signi- cover. For instance, complexes of high trees obliterate visi- ficant distortions of the genuine surface, instead of its bility of most of glacial landforms, and limit image resolu- generalization, particularly when the grid density is too tion of fluvial valleys. high in respect to that of point distribution. More detailed data are not easily accessible. The DTED In geological and geomorphological studies, the requ- level 2 data base of Poland is operated by the Management ired density of spatial data and grids is dependent on scale of Military Cartography. This base provides resolution of 30 and expected resolution. The higher resolution, the lower m by 40 m, that of altitude being 2.5 m, i.e., like in 1 : 50,000 availability of data or grids of the required density. In regio- topographic maps. Still more detailed digital elevation nal analyses, one can use Internet elevation data bases. For models can be purchased from satellite data distributors. The instance, the GTOPO data base (http://edcdaac.usgs.gov/gto- available models of 20 m grid density are relatively expen- po30/gtopo30.asp) provides elevation data for all continents sive; data covering an area of a few thousand square kilo- in the DEM format at a density of 30", i.e., one point per 1 sq. metres may cost a few thousand euros. km. These data enable continent-scale analyses. Another An allegedly cheaper method of acquiring DEM is con- Internet-accessible data base, Digital Terrain Elevation Data tour digitalization from topographic maps. This technique (DTED level 0 and 1), acquired from the space shuttle Ende- is, however, extremely time-consuming. Average time of avour (SRTM — Shuttle Radar Topography Mission) mis- manual digitalization of a single sheet of 1 : 10,000 topo- sion, makes it possible to construct Earth’s surface models graphic map, system 65, is between 60 and 100 hours, that are compatible with 1 : 250,000 maps, at grid density 90 depending on relief type and elevation differences. m (ftp://e0mss21u.ecs.nasa.gov/srtm/; http://net- Semi-automatic digitalization cuts this interval into half; gis.geo.uw.edu.pl). These data are perfectly suitable for, i.a,. although semi-automatic data vectorization is slowed analysis of mountain ranges of high elevation differences. down by contours drawn by dashed lines. In every case, all 0 10km Fig. 1. An example of plastic visualization of a geological map. Geological map 1 : 200,000 (sheets K³odzko and Nachod) superposed upon a shaded relief map, constructed on the basis of a digital elevation model DTED level 2 978 Przegl¹d Geologiczny, vol. 53, nr 10/2, 2005 contours need to be digitized and points situated within a geological map. Raster calibration is sometimes difficult river valleys need to be condensed. Scarps and undermined to perform due to either the lack of coordinates or their slopes of large rivers require special care, and a great deal intentional bias, a procedure frequently used in the former of cartographic and geomorphological experience. Digiti- Warsaw Pact countries. In such cases, the map should be zed 1 : 10,000 maps provide detailed models for an analy- calibrated basing on characteristic points in the field. sis of small-scale glacial or fluvial landforms. Building of individual digital elevation data bases and Digital models can also be used in analysis and verifi- constructing 3D models, as well as superposition of geolo- cation of the existing geological and geomorphological gical maps on the former do not require sophisticated and maps, provided that their scanned, raster images are availa- expensive GIS programs. One can easily use a few simple ble. Proper software enables such raster image to be cali- programs, the total price of which is much more lower than brated according to the coordinates of a digital elevation that of elaborated, specialized software provided by Micro- model, and then be superposed on a 3D shaded relief map Station or ArcInfo.
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