What is Digital Ground Model in Surveying ( DGM )?
A Digital Ground Model is a three-dimensional, mathematical representation of the landform and all its features, stored in a computer database. Such a model is extremely useful in the design and construction process, as it permits quick and accurate determination of the coordinates and elevation of any point.
The Digital Ground Model is formed by sampling points over the land surface and using appropriate algorithms to process these points to represent the surface being modelled. The methods in common use are modelling by ‘strings’, ‘regular grids’ or ‘triangulated irregular networks’. Regardless of the methods used, they will all reflect the quality of the field data.
A ‘string’ comprises a series of points along a feature and so such a system stores the position of features surveyed. The system is widely used for mapping purposes due to its flexibility, accuracy along the string and the ability to process large amounts of data very quickly. However, as the system does not store the relationship between strings, a searching process is essential when the levels of points not included in a string are required. Thus the system’s weakness lies in the generation of accurate contours and volumes.
The ‘regular grid’ method uses appropriate algorithms to convert the sampled data to a regular grid of levels. If the field data permits, the smaller the grid interval, the more representative of landform it becomes. Although a simple technique, it only provides a very general shape of the landform, due to its tendency to ignore vertical breaks of slope. Volumes generated also tend to be rather inaccurate.
In the ‘triangulated irregular networks’ (TIN) method, ‘best fit’ triangles are formed between the points surveyed. The ground surface therefore comprises a network of triangular planes at various inclinations (Figure 1.14(a)). Computer shading of the model (Figure 1.14(b)) provides an excellent indication of the landform.
In this method vertical breaks are forced to form the sides of triangles, thereby maintaining correct ground shape. Contours, sections and levels may be obtained by linear interpolation through the triangles. It is thus ideal for contour generation (Figure 1.15) and computing highly accurate volumes. The volumes may be obtained by treating each triangle as a prism to the depth required; hence the smaller the triangle, the more accurate the final result.