Summary
Geographic information systems
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Extract
Creating buffers on surfaces.
Introduction
A GIS is often utilized in conservation and planning applications because of its power, ease of use, and rigorous and repeatable analysis of data. Buffering is a common spatial analysis process performed on GIS data, particularly for conservation and planning purposes (Chrisman 1997). For researchers working in these fields, accurate delineation of buffers around sensitive areas such as riparian habitats (Holder 1992; Phillips 1989; Xiang and Stratton 1996) or setbacks in zoning and planning applications (Xiang 1996; Chrisman 1997) is necessary to make informed decisions. While governments and regulatory agencies are creating and using planimetric buffers in mountainous areas because of the lack of appropriate GIS tools, landowners and property rights activists desire accurate buffers on surfaces to prevent "unreasonable" setbacks or development restrictions (Barnes 2002; Phillips 1989). Delineating buffers in mountainous areas is problematic. A typical vector buffer function in GIS is based on 2D Euclidean distance instead of surface (or slope) distance, resulting in an inaccurate representation of buffers when they are verified in the field. To illustrate the problem, 50-meter buffers for two lines are generated on a 2D plane and then projected onto the slant planes in Figure 1. The line in Figure 1a runs in the slope direction of the slant plane, while the line in Figure 1b is perpendicular to the slope direction. The slant plane has a 30[degrees] angle with the horizontal plane. The 50-meter 2D buffer in Figure 1a, when projected on the slant plane, will have the same buffer width on the surface. On the other hand, the 50-meter 2D buffer in Figure 1b, when projected on the slant plane, will have a buffer width of 57.7 meters (50/cos (30[degrees])), which is 15.4 percent more than the true surface buffer width. As the angle of the slant plane increases, so does the buffer width when projected on the surface. The projected buffer (dark grey region on the slant plane in Figure 1b) delineates a larger area than the desired buffer on the surface (light grey region on the slant plane in Figure 1b). This example shows how 2D buffers, when treated as surface buffers, may give false results. [FIGURE 1 OMITTED] In areas that are either relatively flat or where the buffered features are parallel to slope direction, the traditional buffering method that relies upon 2D Euclidean distance is acceptable. Over steep and complex terrain, however, buffers generated based on 2D Euclidean distance will be larger when projected on the terrain surface. In this research, we investigate the problem of generating accurate buffers on complex surfaces. We p...See the full content of this document
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