Slope Surface Nepal (SSN) and GIS Application
Mohan Kumar Dangal
Managing Director
Integrated Research Application and Development, IRAD,
Kathmandu, Nepal
Email: irad@ntc.net.np
Tel: 977-1-525-278,: 977-1-524492,: 977-1-528-897
I. Background and Need of Research
Although the GIS data representing the terrain surface has become a need of many organizations around the world including Nepal, the accuracy and reliability of data processing system for acquiring the results of different terrain (ground) models with varied geographical situations and locations have not yet been satisfactorily defined. There are various ways to represent the terrain relief information either in analog form such as contour lines or digital terrain model (DTM). The computerized approaches and methods for generation of terrain surfaces are becoming quite popular among surveyors, engineers, and planners. However, the accuracy and reliability of the computerized digital terrain surface do not always lie within the acceptable tolerance of the design drawings, maps, and quantity estimation of different projects including hill road projects.
Observations and assessments on the results of many digital terrain models used by different organizations especially in the hilly terrain of Nepal suggest that there is an urgent need for selection of an appropriate digital model to be applied which would accurately represent both the earth ground and the design surfaces of different types of engineering projects. As an example, the actual representation of ground surface for a linear road project is dependent on not only the connectivity of different ground points, but also in the formation of digital model. Thus it is necessary that the ground representing the exact relief of the feature should be extracted to from any digital model that could be used to represent the terrain surface. Experience on the use of the digital model in road projects of different countries including Nepal has shown that there are many cases of deviation of the ground surface (as generated by using DTM) from the actual surface due to wrong use of the TIN data structure. The
deviation is also due to improper selection of a suitable digital model that could have ensured both the reliability and accuracy of earth ground surface formation or creation. On the other hand, when the instrument survey data are not sufficient to represent the actual terrain, the contour surface generated by TIN data structure cannot represent the exact relief. As a result, there are many cases when the actual ground surface found during the construction phase is different than that represented in the map generated by survey and design works. These are the reasons for a wide variation of the ground profiles of the design drawings and the construction drawings prepared in the field after actual measurements.
