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These maps display classified slopes on digital ortho image and topographic data. The upper map confirms steep slopes and warns that a field survey is required before updating. The map on the right indicates gentle slopes, permitting an update using the digital ortho image alone. |
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Information about the earth's terrain is of utmost importance for any land planning application. Whether laying a road section, constructing a dam, selecting a site for heavy industry, or planning a resort, the terrain slope characteristics of an area are a primary consideration. In fact, terrain slope characteristics affect the entire building process, from selecting a facility's location, to planning its layout, and ultimately constructing the facility. When populating a GIS, terrain information may be collected from various sources, including photogrammetric compilation, ground survey data, and existing contour maps. In the Qatar GIS, terrain information comes from photogrammetric compilation. Terrain data in its original form is stored as a set of ASCII files containing 3-D positional information. 3-D digital data sets depicting the terrain of Qatar were used to generate slope maps, which are used in the maintenance of the CGIS topographic database. While digital ortho images are only one among several different national data sets maintained in the Qatar GIS, using them to update topographic information on ground level features was found to be an economic alternative compared to the more costly ground surveying and photogrammetric compilation methods. Much care had to be exercised in updating the topographic database because the digital ortho image is a 2-D representation of a 3-D world and because digital ortho images are not corrected for the radial distortion of the input aerial photograph. In fact, the digital ortho images could not be used for updating features vertically away from the digital elevation model (DEM) surface. For example, a ground level feature may pass through a steeply sloping area. Such a feature, if updated using the digital ortho image, would have doubtful positional accuracy. Hence, the need for slope data to be used during the update process, to indicate slope characteristics of the area being examined. The slope map overlaid on the digital ortho image gives a warning to the person carrying out the update that the feature picked up within a steeply sloping area is positionally inaccurate and should be flagged for a ground survey update. Using this process, the topographic database is kept current and at the same time, positional accuracy is not sacrificed. To perform the update process in this manner, the slope maps were an essential information source. |
Slope maps can be created using a DEM, which in turn is generated using the terrain data maintained in the Qatar GIS. In order to employ any of the surface analysis capabilities provided by GIS, in this case ESRI's ARC/INFO software, it was necessary to first build a DEM, or more specifically a triangulated irregular network (TIN) using the original digital elevation data set. Once the TIN was constructed, it was possible to use any surface analysis or representation capabilities to depict terrain characteristics or to use the data set for further GIS analysis. ARC/INFO's TIN software, offers several surface modeling and representation capabilities such as contour generation, calculation of slope and aspect, generation of profiles, volumetric and cut-fill analysis, inter-visibility and visibility analysis, surface views, surface drapes, and so forth. This application concentrated on the calculation of "degree of slope" that would later be used to generate a classified slope coverage and ultimately to make a map showing the classified slopes with a digital ortho image as a backdrop. The 1:1,000 scale topographic database of the Qatar GIS is maintained in tiles, each covering half a square kilometer. Therefore, it was necessary to perform the application and related update process on a tile by tile basis. The main program requires the sheet number as its argument and the entire application is executed non-interactively. For this application the TIN and breaklines coverage had to be generated using the terrain data. Once the TIN had been generated, the degree of slope for each face of the DEM (represented by a TIN triangle) could be calculated. Since there are tens of thousands of triangles generated for each map sheet, it is very difficult to visually perceive the slope characteristics of the sheet. So, the calculated slopes were classified and neighbouring triangles falling within the same class were dissolved. The generated slope coverage and the breaklines coverage were overlaid onto the digital ortho image of the same map sheet to assist the update process. During the update, this map was displayed in a separate window and if specifically required, the slope coverage was used as a backcover. Slope coverages can be used for a wide variety of other applications. For example, they can be used to decide the location of a playing field or race track, sites where the general terrain must be less than 20 degrees in slope. It is clear that slope information, as well as other 3-D data, can be very useful in planning many different sorts of projects in the development of our cultural landscape. |
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