Digital Terrain Modeling in a Gis Environment

Digital terrain models (DTMs) in recent times have become an integral part of National Spatial Data Infrastructure (NSDI) of many countries world-wide due to their invaluable importance. DTMs play a major role in hydrologic modeling, sediment transport, soil erosion estimation, drainage basin morphology, vegetation, and ecology. DTM generation however has a setback of high cost of primary data input acquisition (i.e. elevation) and complexities in processing. Most DTM data are derived from photogrammetric data, satellite image data, laser scanning data, cartographic data sources, and ground surveys though accurate is expensive, timeconsuming and require skilled personnel. This paper describes a low cost means to generate DTMs where elevation data is not readily available in digital form and when the accuracy of the output is not a dominant factor. The existence of large volumes of hardcopy topographic maps provides a source of primary input in the form of contours which can be digitised and used for DTM generation purposes. This research therefore aims at generating DTMs within a GIS environment using elevations digitized from topographical maps. That is, a geographic information system (GIS) based approach is presented for the development of a terrain model. After digitizing contours and points along contours from a topographic map, various interpolation algorithms were used to generate DTMs and their accuracies in terms of root mean square errors were assessed. The resulting surface model provides a good representation of the general landscape and contains additional detail information (such as slope, aspect, cross-sections, volumes, hillslope) to describe, evaluate and quantify terrain properties. Also, if the digital contours are geo-referenced, they can be readily integrated into existing GIS for 3D surface visualization and spatial analysis. Results of surface analysis (intervisibility analysis) are presented and among other applications, the DTMs proved useful in surface analysis.