Use of digital terrain modeles for the infrastructure development in sri lanka
Nihal Wijesekera
Senior Lecture, Superintendent of Surveys
Institute of Surveying & Mapping, Diyatalawa
Tel: +94-57-31735 Fax: +94-57-22004
E-mail: ism_surv@sri.lanka.net
SRI LANKA
Keywords : Terrain, Infrastructure
ABSTRACT :
Digital Terrain Models (DTMs) have become a major constituent of geographical information processing. DTMs facilitate to model, analyse and display phenomena related to topography or similar surfaces. This paper presents an approach to use those capabilities in DTMs for the infrastructure development in Sri Lanka. The different techniques of data capturing methods are given, with a comparison, providing opportunity to select the best method that appropriate for the requirement. The theoretical basis and a number of DTM packages are described and an attempt is made to determine how best such system can cover many of the practical requirements. Some applications are mentioned where they have particular features that relate to the theme of the paper.
Introduction
A digital terrain model (DTM) can be described as a three - dimensional representation of a terrain surface consisting of X, Y, Z coordinates stored in digital form. It includes not only heights and elevations but other geographical elements and natural features such as rivers, ridge lines, etc. With the increasing use of computers in engineering and the development of fast three-dimensional computer graphics the DTM is becoming a powerful tool for a great number of applications in the earth and the engineering sciences.
Data Collection For Digital Terrain Modelling
The acquisition of accurate three-dimensional coordinates which represent the surface of the Earth is a vital stage in the process of terrain modelling. The raw data of the DTM can be captured by a number of different techniques. The particular technique used will depend on factors such as:
- The size of the area to be surveyed
- Data accuracy
- Type of information which will eventually be extracted from the model
- Availability of resources
At present, most DTM data are derived from three alternative methods.
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Ground survey methods
- Photogrammetric methods
- Graphics digitizing methods
Ground Survey Methods
Ground surveys could be done with computer aided systems which may be coupled to field instruments, available from all the major manufactures, such as Kern, Zeiss Wild, Sokkia etc. Total station instruments such as Elta-2, Wild, Sokkia Set 2, Leica are common instruments used for data capturing in ground surveying method.
Since ground survey data tend to be very accurate, the accuracy of the resulting DTM is very high. Hence, it is most suitable for the small area site planning and designing. However, as this particular data collection technique is relatively time consuming and expensive its use is limited to small areas.
Photogrammetric Methods
Photogrammetric data capturing is widely used for large area projects, especially in rough terrain. Photogrammetric methods based on the use of stereoscopic interpretation of aerial photographs or satellite imagery (e.g. from the French SPOT Satellites), using suitable photogrammetric equipment such as manual or analytical stereo plotters. The accuracy of the resulting DTM depending on the sampling method and imagery that are used. The resulting DTM accuracy will be in the range of medium to high. Photogrammetric data capture is used in large engineering projects such as dams, and roads as well as nation wide data collection.
Graphics Digitizing Methods
The third alternative is the use of a very large number of topographic maps already existed in printed form. These maps are converted into a digital format by digitization. When the topographic maps are in digital format, digital map data generally occur as string or series of strings of (X,Y) coordinates with an associated Z value for the contour level.
An example of a DTM (Figure 2) constructed from an existing contour map (Figure 1) is shown below.
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| Figure 1 Contour map |
Figure 2 DTM of the area
shown in Figure 1
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The existing small-scale topographic maps e.g.1 inch sheets (1:63360) and 1:50000 scale maps produced by the Survey Department of Sri Lanka can be used to create DTMs with low accuracy. However, as long as users are aware of this limitation, this lower-quality data is quite acceptable in regional planning, landscaping, visualization, simulation, civil engineering projects and reconnaissance studies.
