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Surface area processing in GIS
Sandeep N Kundu IFGI, University of Münster, 48149 Münster, Germany Phone: 0049 251 8330058 Fax: 0049 251 8339763 Email: skundu@ifgi.uni-muenster.de  Biswajeet Pradhan Asian Institute of Medicine, Science & Technology, 08000, Sungai Petani, Kedah Darul Aman, MALAYSIA Phone: 00604 445 2233 Ext. (222) Fax: 00604 442 2887 Email: biswajeet@aimst.edu.my (presenting author)
Introduction
Area is a fundamental parameter derived from terrain analysis which is necessary for numerous decision-making processes. There is a need to obtain an accurate approximation of distance and area which are the fundamental terrain parameters associated with spatial data content since these have a bearing on cost (Navigation, cadastral applications). This problem has been addressed within the framework of a given data scale and a suitable projection. In most elementary GIS models, distance and area calculations are based on the vector data model which is a planar approximation of data layers. This does not approximate distance or area on an undulating surface as in reality. Given the pre-conditions that the data layers are in the desired scale and projection, a novel way to integrate a raster elevation model to retrieve the near true area and distance (involving elevation data & slopes) is proposed. Methodology The technique utilises a Digital Elevation Model (DEM) layer in addition to the vector polygon layers and experiments on some algorithms for surface area calculations for the polygon parcels. The technique can also be extended to distance calculations. A DEM is essentially, “a regular gridded matrix representation of the continuous variation of relief over space” (Burrough & McDonnell, 1987). This in fact means that a DEM is a continuous grid containing the elevation at its spatial location in each grid cell. Before the 80s, topographic contour maps and aerial photographs were the primary sources of geomorphological information. The accuracy of these maps was highly questionable and there were also discrepancies in information content for two maps for the same region but from different sources. After the advent of DEM this discrepancy was reduced to a large extent. Nevertheless, a DEM is also subject to error depending of the method of its acquisition as each method has its own advantages and limitations. Usually a DEM is interpolated from digitising spot heights as recorded on field or from an existing contour map. But a better DEM is extracted from stereo aerial photographs or stereo digital remote sensing imagery. A DEM was preferred to a TIN because the latter is based on slope inflection points and fail to capture the subtle difference in slope along a particular aspect. |