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. 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. On the other hand, the topography surface has to be approximated on the different theories and assumptions. Different software as available in commercial market is based upon the approximate creation of topography surface. Thus the accuracy of the topography surface might be lost behind the acceptable tolerance value, if the points are not densely located to form reasonably good triangles. In fact, the density of the location of the points could also depend upon the scale of the map to be produced. In addition, the location density of the points can also be guided upon the actual slope change of terrain. So the survey points need to be measured by considering all necessary factors described here. Extra points essential to satisfy the basic requirements of TIN data structure can also e created in a linear change of slope surface within the limit of each surface that could be considered as a small piece of polygon. But some of the digital model has also used a curve surface (as presented below) to approximate the terrain surface on the basis of minimizing the quadratic deviation of sufficient number (minimum 10) points. Such a surface model can be presented by the mathematical equation which is: H = AX2+ 2BXY + CY2+ 2DX + 2EY + F. Where A, B, C, D, E, and F are the coefficients of the equations and these coefficients can be determined by solving minimum 6 number of equations that could be written as below: AX12+2BX1Y1+CY12+2DX1+2EY1+F=H4: AX22+2BX2Y2+CY22+2DX2+2EY2+F=H2: AX102+2BX10Y10+CY102+2DX10+2EY10+F=H10 However, any approximation of the surface should be based on the basic requirements of the density of the points that should be located within the minimum distance and suitable location as well. These are the various factors that can directly or indirectly affect the accuracy of the topography surface essential to create a real terrain model. Considering the factors just discussed above, a new computerized terrain model named Slope Surface Nepal is developed by the author of this paper with a view to minimize the errors on the formation of the different types of terrain surfaces such as natural surface, design surface, and other type of surfaces. The slope surfaces are represented by a reliable digital model that ensures the generation and creation of the surface points as per the need of different projects depending upon the nature of the slope surface created or selected by the user. TheSlope Surface Method has been classified as follows:
The Slope Surface of Points can be represented by a polygon of four points that could be a mathematical function for which the first derivative is true throughout the surface while the second derivative does not exist within the same surface. In practice, the earth surface can always be divided into small pieces of polygon represented by four or three points by forming the slope surface for all practical purposes. The computerized model of Slope Surface has also fully considered various aspects including the function that can create the slope surface of points in the form of small pieces of polygon. This has enhanced the basic facility of the Slope Surface Model developed to handle the different cases of surfaces in general. Thus this paper has considered the critical cases of the road corridor that could be represented by small pieces of polygon in which the ground slope can change in a linear manner. Once the roughness of surface points are collected in the form of small polygons, additional grid points (at given interval) as required by the TIN data structure can be created by the Slope Surface Method. The facility of generating good points at required interval can enhance the quality as well as the reliability of digital surface on which various engineering projects can be designed and constructed as per the various requirements of engineers, surveyors, and the planners. Number of experimental works have already justified the advantages of the Slope Surface Method compared to many terrain models for road design job. 2. Slope Surface of Lines The Slope Surface of Lines can be represented by the lines of points that could have same or different level values (Z), but each surface of lines should be represented by one code number (ID). Thus the different slope surfaces represented by lines should have different code number (IDs). The contour lines can also be considered as the line slope surface for which the Z value can be assigned as ID number. In this context, the instrument survey procedure and method are also dependent upon the method that shall be applied to process the survey data to generate the result as per the requirements of project tasks. In the course of field instrument survey work, it is also recommended to maintain the sketch diagram of all the survey points so that the connectivity of the points could be checked and corrected during the data processing stage through the computerized system. It is also equally essential to assign suitable code for all instrument survey points so that the computerized system could identify and join these points with the same code to meet the basic requirements of the Slope Surface Method as per the assumptions and concepts followed in the digital model of the Slope Surface Nepal. On the other hand, the instrument survey work of most of the hill road projects are generally conducted on the cross-section points with a view to avoid the risk of variation on the ground profile of L-section and cross-section. Such an approach cannot allow for searching a better location of the road alignment. On the other hand, many road design tasks conducted by different computerized systems as used by many users have also not ensured the extraction of the reliable ground surface generated by the TIN data structure of DTM. Although there are various reasons for such a substantial variation of the ground profile generated by processing the TIN data structure for survey points, the main reason behind the ground variation is that the surveyors can not spend a longer period to capture a large number of ground points that are essential to create the topography surface through an use of TIN data structure of the digital model as followed generally by most of the commercially available computerized systems. The Slope Surface of Lines is found to be very effective to build the terrain surface for linear road project. Thus this method can be used for planning and design of small corridor to extract the ground data with higher degree of accuracy compared to the data generated by using the TIN data structure of DTM surface. 3. Different Line Types of Slope Surfaces Generally speaking, there could be of different line types of slope surfaces which could be described as follows:
4. Creation of Extra Grid Points Extra grid points on the surface at a required interval are found to be useful before one can apply the TIN data structure with a view to have good triangles that can produce a reasonable topography surface. The random points as measured by field instrument survey method can not always ensure the full requirements of TIN data structure. Realizing such a draw-back of surface generation method, the Slope Surface Nepal (SSN) is developed to provide a facility of creating the extra grid points on the slope surface based on the procedures and approaches discussed above. Thus the option for creating extra points should be considered as a new approach for enhancing the use of TIN data structure for generating a reasonable topography surface. However, if the instrument survey data are considered to be sufficient and the points are located such that the good triangles are formed, then this option for creating the extra grid points may not be required at all. 1. Boundary Concepts and Procedures A review on the surface data processing systems suggests that there is a need to process the surface data on a boundary-wise basis. For an example, the task of hill road design might require a specific technical approach to carry the optimum plan and design of the alignments and structures for some critical areas due to land slide, geology, and other environmental factors. In addition, when some changes are made in the design in one part of the alignment, other parts should not be affected for design data generation and processing tasks. Such an independent design approach can be carried-out only when the entire alignment corridor is divided into several parts so that each project part can be handled separately or together as per the technicalities and norms to be followed. Experiences on the application of computerized systems suggest that the users should have a facility of preparing the design data on boundary-wise basis. Considering these aspects, the Slope Surface Nepal has also provided a user’s friendly facility to plan and design the project on boundary to boundary basis. On the other hand, the designer might need to adopt the special norms for some areas of the linear project. Many practical issues have been considered while preparing the boundaries of a linear project in the operating system of Slope Surface Nepal. 2. Quality of Slope Surface’s Output The evaluation on the output result of the slope surface method is considered to be a vital task. In order to judge the performance of the quality of the slope’s surface output, several practical examples had been considered to generate the topography surface as well as the ground profiles of the cross-sections and longitudinal sections. The GIS data as digitized by Department of Survey, Nepal (1:25, 000 scales) had also been processed through a DTM to generate new topography surface on the basis of using the original digitized points of topography contours. The GIS data had also been processed through the digital slope surface method to create new points that had been processed again by DTM to generate another topography surface. It had been observed that the topography surface processed through the new points created by slope surface model had been found to be closer with the original topography surface compared to the topography surface created directly by using the original topography digitized points. Similar experiments had also been carried-out by considering other examples on the use of slope surface model. Mass of the experimental results on both the extraction of ground profiles and generation of topography points confirm the high degree of reliability and accuracy of the slope surface method. Thus the Slope Surface Nepal is recommended for a wider use to extract the ground profiles through the slope surface of lines to be joined by code number. However, the changes in the alignment of linear element like a road project should be done by generating the contour surface that might be ignored while extracting the ground profiles. It is to be noted that the proposed digital model has fully utilized the advantage of the topography contour, but the creation of the reliable ground surfaces that might take place through an alternative surface in which the surface approximation never takes place. Thus the solution of ground surface takes place by utilizing the combined advantages of existing digital terrain models and other new approaches and methods as introduced by the slope surface Nepal as well. II. Advantages of Slope Surfaces The main advantages of the Slope Surface Method can be listed as follows:
Most of the digital terrain models as available commercially are based on TIN data structure, which has specific requirements on the locations and intervals of the points. Thus the new method known as the Slope Surface Nepal can be considered as a complementary system before one can use his own DTM to generate the topography surface. The successful application of DTM also depends upon the point intervals and the constraints to be imposed on the connectivity of the surface points. The DTM generates the contour surface, which provides useful idea about the terrain for planning and design of different development infrastructures. However, the contours generated by DTM without imposing the constraints on joining the points may not always represent the real ground features. As a result, there are many cases of over-designed or under-designed of the alignments and structures of the linear project like the road project. In these aspects, the existing systems have neither provided solutions, nor guidelines to avoid such critical errors on the formation of the ground profiles in g eneral. IV. Findings and Recommendations
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