Primary Data

There are many methods for obtaining primary data for the terrain model/terrain modelling. Field measurements using total station instrument/electronic field book, logging mapping data directly from autographs, digital maps, digital mapping data from stereoscopes, and manual keying in individual points, are the most commonly used methods today.

It is important to note that all of the logging methods can be used within one and the same database.

The number of points required to attain a certain level of accuracy greatly depends on the terrain. On a flat surface, a terrain line around the edge might suffice, while in steeply undulating terrain it may be necessary to have multiple terrain lines. The requirements for accuracy within an area will often vary. This ought to be exploited by taking fewer readings in less critical areas, or by using simpler and cheaper logging methods. It is also possible to use different logging methods for different types of terrain lines.

It will be a great advantage, if surveyors make themselves familiar with the terrain model method of operation before carrying out surveys using total station instruments/electronic field book. For e.g. by surveying terrain lines such that the sequential points describe a characteristic line, a more accurate terrain model can be established than if the points were logged as an arbitrary point grid. One of the main features of terrain model is that it interpolates heights by the linear method, between two successive points on a straight-line. Few useful details while surveying are explained in the topic 'Survey methodology'.

A total station instrument is a tachometer or theodolite with built-in distance meter, which automatically logs horizontal and vertical angles, and stores this information as required. The survey data is then transferred to a computer for further processing.

Surveying with a total station instrument is often an expensive method, although highly accurate. It is primarily used in areas, which require great accuracy, or in areas, which have not been mapped before.

Surveys may either be done in the form of “point grid” or in the form of lines. If lines are surveyed, this means that the surveyor books a code in the measurement record, stating that the heights should be interpolated along a surveyed line (a series of points). By surveying lines, the model will contain more information than if points alone were logged. Also see the topic 'Survey methodology'.

The KOF Format: KOF format has been developed to provide a common format for total station instrument data.

Extracting details from the measurement book requires a program, which can extract and print data into the correct format (Nova Field). The KOF format allows logging while using several staff carriers, saving both feature code and point number and logging characteristics such as depth of topsoil and drilling depth.

Aerial photography with subsequent digitizing in stereoscopic instruments is a relatively inexpensive logging method. The newer types of instrument can supply digitized contours in addition to map at a modest extra cost. To obtain the best possible model, it is essential that the mapping company is given instructions about digitizing any marked breaklines in the terrain - beds of streams, slope markings, etc.

The height accuracy of the points from such a photograph is dependent on the altitude of the aircraft, the type of instrument used, and the degree of complexity of the terrain.

Digital maps should be supplied in accordance with the guidelines on digital maps for use in terrain modelling. It is important to include lines, which describe characteristic breaklines within the terrain, for example road edges, lines of ditches, etc.

If the terrain data are in digital format from a supplier of mapping data, group numbers ought to be used with a certain logical relationship to the mapping sheet number. When surveying with a total station instrument, it may be suitable to assign each TP a group number, or the terrain surface one group number and sub-surface layers other group numbers.

A summary of feature codes used by the supplier of the mapping data should always be included with the terrain data diskette. In addition, the drawing of the supplied mapping data should always be included so as to clearly indicate which areas and which features the various files cover.

Scanning is done by way of optical/electronic instruments registering grey-tones in a drawing. In this process, photosensitive cells, or lasers, make a sweep over the drawing. Based on dots with equivalent grey-tones, the lines making up the drawing can be reconstructed on a computer, which has a scanner link. With the aid of a graphics screen, the individual lines can be coded and assigned heights.

A scanner requires a good quality original map, preferably without intrusive elements such as houses, roads, etc. Scanned results will be available in the same way as results from digitizers. Accuracy is good if the lines on the map are clean and clear.

Quadri can accept data from traditional land survey instruments. Data can also be calculated in other computer systems, but will have to be in a compatible format, which can be imported into the model, preferably SOSI or KOF. Both 2D and 3D data can be imported from AutoCAD.

Functions have been developed for transferring data from AutoCAD to database and from Database to AutoCAD. Individual points or continuous lines in 2D or 3D can be transferred. These routines are explained in the topics 'Import from Current Drawing', 'Draw Map from Terrain Model', and 'Draw Map from File'.