Digital Terrain Model (DTM) Concepts

A digital terrain model DTM is a mathematical representation of the spatial distribution of a specific feature linked to a real surface. The surface is generally continuous and the represented phenomena might be varied. Check below some examples of DTM usage:

  • Storage of altimetry data for the generation of topographic maps;
  • Landfill and cut-off analysis for road and dam projects;
  • Preparation of slope and aspect maps to support geomorphology and erosion analysis;
  • Analysis of geophysical and geochemical variables;
  • Three-dimensional presentation (in combination with other variables).

To represent a real surface on the computer, it is necessary to elaborate and create a digital model that might be represented by analytical equations or network of points (grid), communicating to the user the terrain spatial features. In SPRING, a DTM is generated as a grid of regular and irregular points.

The creation of a digital terrain model is new way of facing the problem of project elaboration and implementation. By means of models (grids) you can directlycalculate volume, areas, draw profiles and transversal sections, generate gray level or shaded images, slope and aspect maps, slicing and three-dimensional visualization.

There are three different phases on the digital terrain modeling process:

  • Data acquisition through data import and data edition
  • Grid generation
  • Product generation representing the information obtained

Check next each one of them:

     

Check also:

About Image Processing
Vector Data Manipulation
Edition of Numeric Maps


Data Acquisition

The DTM data are represented by the xyz coordinates, where z is the parameter to be modeled: z=f(x,y). These data are usually acquired following an irregular distribution on the xy plane and there are no topological relations defined between the positions of the sample points, or along the lines with the same value as z, or with the same regular spacing.

These data acquisition is done by field surveys, map digitalization, photogrammetry measures from stereoscopic models and altimetry data obtained from GPS, airplanes or satellites.

However, the DTM applications or products are not elaborated over sample data, but from models generated in the format of regular or irregular grid. These formats make the implementation of application algorithms easier and faster.

The data acquisition can be done through sample points, with regular or irregular spacing, or through contour lines maps.


Sample Points

According to the type of acquisition, the samples are distributed as shown of the following figures:

    a) Totally Irregular

    b) Aerial survey

    c) From the Drainage

    d) Regular

The quality of the final product of an application over a model is directly related with the care when choosing the points and amount of sample data. The amount of sample points and the care when choosing them is decisive for more realistic applications. The more the number of representative points on the real surface the greatest the computational effort for storing, retrieving and processing these data until the final product is obtained.


 

Sample Contour Lines

A contour lines map is the representation of a surface using isovalue curves. A very common example would be the altimetry contour lines existent on topographic maps, printed using equipment such as stereoplotters and obtained on aerial surveys. Notice that there are on these maps some points irregularly sampled that were acquired on field work. The Figure below shows an example of a topographic map with contour lines and spot heights:


Example of a topographic map

The contour lines acquisition can be manually obtained using a digitizing table or automatically using a scanner.

When manually, the contour line is identified with a value of height (z value), using the digitizing table of cursor, directly on the screen.

When a scanner is used, a matrix with points where we find the contour lines and z values is obtained. A vector process goes through a contour line and transform it in a sequence of points with XY coordinates with same Z value.

See also :

Grid Generation
Edition of Numeric Maps

Modeling Processes