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Generate Terrain Contours from Height Points

Application

This software feature enables to generate a new terrain model based on sparsely distributed height points by using the interpolation provided by the grid calculation in CadnaA.

Please note the following limitations when using this feature:

  • The subsequent generation of contour lines by grid interpolation generally reduces the accuracy of the local height information compared with the initial terrain model from height points.
  • The application of this feature is, therefore, useful only in cases where there are just few, spatially sparsely distributed height points across the project area available. In the triangulation, few height information results in relatively large, flat triangles of constant slope. This means that the height in these areas is approximative only.
  • For a given sufficiently dense - and perhaps equidistant - grid of height points it is not recommended to use this feature.

Note

Do not use this feature just because the resulting contour lines can be interpreted easier visually on the screen or because it may be easier to „imagine“ the terrain model.

Depending on the settings used for the grid appearance, option „Level Range“ (see Grid Appearance), a more or less fine grid of lines of equal height is generated. These lines are generated by linear interpolation from the heights of the existing height points and are displayed as grid contours („Lines of Equal Sound Level“) is displayed, which are afterwards converted into terrain contours.

Procedure

Initially, there is a file with height points.

Terrain model from non-uniformly distributed height points

Proceed as follows:

  • Specify the receiver spacing on the dialog Grid|Specification (in this example: 10 * 10 m).

At these grid points the terrain height is calculated using the height data of the height points.

Note

The receiver spacing affects the accuracy of the interpolation, since intermediate values ​​(between the height points) are interpolated linearly. A very fine spacing is not useful with respect to the resulting file size, on the other hand, it would fake a higher accuracy of the terrain model than had the original data.

  • Select from the list box of parameters on the symbol bar of the CadnaA main window the option „Ground“.

At present, no ground grid existent, the ground grid is empty.

  • Start the grid calculation via the command Calc Grid on the Grid menu.

The ground grid is displayed using the current appearance settings for the evaluation parameter „Ground“. In addition, in the right corner of the status bar the ground height is displayed at the location of the mouse pointer („G: xxx.xx“ m, with G for „Ground“) .

  • By moving the mouse across the grid area, study the value range of heights on the grid.
  • Open the dialog Grid|Appearance and enter in the dialog range „Level Range“ the lower and upper limits and the class width of the height range of the ground grid.
  • Select the display option „Lines of Equal Sound Level“.

In this case, the values ​​for lower and upper limit and for class width represent heights (in meters) and not sound levels. Thus, the isolines are lines of equal height.

Please note that the color palette selected for the evaluation parameter „Ground“ is not relevant when generating the contour lines. Always the values for „Level Range“ are used - even if disabled.

In order to display the interpolated ground heights at the grid points proceed as follows:

  • If necessary, reopen dialog Grid|Appearance.
  • Select for „Grid Points“ the option „Values.

After closing the dialog with OK, the ground heights at the grid points are displayed. If necessary, change the scale using the zoom tool to be able to read the values.

Ground grid with lines of equal height having a vertical difference of 1 m(display range: lower/upper limit: 80/120 m, class width: 1 m)

Height points with interpolated grid of the ground heightshowing the ground height in m at grid points (10x10 m spacing)

Now, the contour lines are then generated from the displayed grid lines:

  • To this end, press the key combination ALT+F12.

  • Confirm the prompt with OK.

The lines of equal height are converted to the object type „Contour Line“. With ALT+F12, the function „Simplify geometry“ with a value of 0.5 m is applied and contour lines with a length < 30 m are not created. With SHIFT+ALT+F12 these simplifications of the DTM are not performed.

  • Select on dialog Grid|Appearnce the option „No Grid“ to no longer display the grid.
  • Disable the height points to just display the contour lines.

New terrain model based on contour lines

When generating the lines of equal height the height at the corners of the limits (see Limits) is considered as well. This means that the resulting heights near the corners of the interpolated area are possibly not valid because there is no valid height at those points (e.g. too few height points). Therefore, if necessary, the contour lines generated in these areas need to be cut or deleted.

In the above example this applies to the north-west corner of the limits (upper left corner). In the corner area there are no height points. Consequently, the resulting height in this area is defined by the distance and the height of the corners of the limits (option „Standard Height“, see DTM Digital Terrain Model Tab). The triangulation ranges from the limit‘s corners to the nearest height defining object. This means that the height in this area is at random and therefore not relevant.

Enter a closed polygon that delimits the project area to get deleted from the one not to be deleted. Select from the context menu of that polygon the command Break Lines (see Break Lines in the manual „Introduction to CadnaA“) in order to cut the terrain contours. By selecting the command Modify Objects from the context menu of that polygon the contour lines outside can be deleted.