The ease with which total stations produce horizontal distance, vertical height and horizontal direction makes them ideal instruments for rapid and accurate contouring in virtually any type of terrain. The data recorded may be transformed from direction, distance and elevation of a point, to its position and elevation in terms of three-dimensional coordinates. These points thus comprise a digital terrain or ground model (DTM/DGM) from which the contours are interpolated and plotted.
The total station and a vertical rod that carries a single reflector are used to locate the ground points (Figure). A careful reconnaissance of the area is necessary, in order to plan the survey and define the necessary ground points that are required to represent the characteristic shape of the terrain. Break lines, the tops and bottoms of hills or depressions, the necessary features of water courses, etc., and enough points to permit accurate interpolation of contour lines at the interval required, comprise the field data. As the observation distances are relatively short, curvature and refraction might be ignored. However, in most total stations corrections for curvature and refraction may be applied.
From Figure 3.42, it can be seen that if the reduced level of point A (RLA) is known, then the reduced level of ground point B is:
RLB = RLA hi h − ht
When contouring, the height of the reflector is set to the same height as the instrument, i.e. ht = hi, and cancels out in the previous equation. Thus the height displayed by the instrument is the height of the ground point above A:
RLB = RLA h
In this way the reduced levels of all the ground points are rapidly acquired and all that is needed are their positions. One method of carrying out the process is by radiation.
As shown in Figure , the instrument is set up on a control point A, whose reduced level is known, and sighted to a second control point (RO). The horizontal circle is set to the direction computed from the coordinates of A and the RO. The instrument is then turned through a chosen horizontal angle (θ) defining the direction of the first ray. Terrain points along this ray are then located by measured horizontal distance and height difference. This process is repeated along further rays until the area is covered.
Unless a very experienced person is used to locate the ground points, there will obviously be a greater density of points near the instrument station. The method, however, is quite easy to organize in the field. The angle between successive rays may vary from 20◦ to 60◦ depending on the terrain. Many ground-modelling software packages interpolate and plot contours from strings of linked terrain points. Computer processing is aided if the ground points are located in continuous strings throughout the area, approximately following the line of the contour. They may also follow the line of existing watercourses, roads, hedges, kerbs, etc. (Figure).
Depending on the software package used, the string points may be transformed into a triangular or gridded structure. Heights can then be determined by linear interpolation and the terrain represented by simple planar triangular facets. Alternatively, high-order polynomials may be used to define three dimensional surfaces fitted to the terrain points. From these data, contours are interpolated and a contour model of the terrain produced.