Slope Surface Nepal (SSN) and GIS Application
This paper is prepared on the basis of actual research that has been carried-out by the author since more than 20 years in the field of computerized road design methods within and outside Nepal. This research experiment is expected to establish scientific procedures for building the terrain surface based on which engineers need to plan, design, and prepare construction drawings that match with the actual field situations for a linear road surface corridor.
There are various ways to represent the terrain relief information either in the analog form such as different surfaces, contour lines and the digital terrain model (DTM). The digital terrain model can also be based on various concepts and assumptions based on the location of different points and the lines that could be linked depending upon actual locations and the levels of the points governed by field situations. In this context, Triangular Irregular Network (TIN) data structure is one way to store the terrain relief in the form of the digital model. However, the terrain relief can also be built in a number of ways that could vary from the procedures and methods to be applied in building the terrain model. One of the drawbacks of TIN theory is that there are many possibilities of forming triangles from one set of points. As a result, the topography surface might vary from one set of triangles with another set of triangles. For an example, one set of triangles that satisfy the basic requirements of
contour generation might produce good surface that can have high degree of accuracy and reliability; while other set of triangles (formed by same set of points) may also be developed under different constraints and connectivity of points which do not fulfill the basic requirements essential to produce a good topography surface. The first- constraint might be such that the choices of selecting good triangles might be quite restricted due to limited points. On the other hand, this situation might create a demand to conduct the instrument survey work for a large number of geographical points that should be densely located, but such points might also have the same area of roughness (levels). In fact, the instrument survey job for extra mass points that have the same area of roughness or levels could be not only expensive but also quite difficult and time consuming as well. Theoretically, the task of conducting the instrument survey work for extra field points required due to specific need to triangulations of
TIN data structure is not only expensive, but also unnecessary.
This is the serious gap between the technical requirements of TIN data structure and the instruments survey points, which has been ignored by the existing digital terrain model. As a result, there are many examples of variation of ground profile generated through TIN data structure of digital terrain model. Many cases of variation of the ground profiles are found while comparing the profiles prepared during the design and construction stages by using different methods to produce the project drawings including profiles and cross-sections. Thus the data structure required for using the TIN model cannot always ensure the generation of ground points with the permissible tolerance of accuracy.
