DTM Digital Terrain Model Tab
Standard Height (m)
The standard height defines the absolute terrain height to be used in the calculation when no other information of the actual terrain height is available. This applies to areas in the model which are not covered by distinct contour lines or height points (for example, at the corners of the limits, see Limits). Based on this standard height and the terrain model all objects having a relative height get assigned an absolute height. The predefined standard height is 0.00 m.
Model of Terrain
The following options for the generation of terrain models are available:
Triangulation
By triangulation a terrain model consisting of triangles is generated from the entered height points (see Height Point) and the polygon points of terrain contours (see Contour Line) and lines of fault (see Line of Fault), if any. In this case, no ground grid needs to be calculated in order to display the terrain shape in the 3D-Special view (see 3D-Special View).
The triangulation is the contemporary used method to generate a digital terrain model.
Explicit Edges Only
When activating the option „Explicit Edges Only“, only the entered terrain contours will cause a screening effect and not the terrain edges resulting from triangulation. This option has no effect on the 3D-Special view.
Objects with ’Ground at every point’ influence DTM
Activating this option will cause the height of the base point of objects with a height using the option “absolute Height/Ground at every Point“ will determine the terrain height at that location upon triangulation.
Note
The options „Search Contour Lines (Average)“ and „Search Contour Lines (Local Inclined Plane)“ were available in former versions of CadnaA - before the triangulation method was introduced. These options are kept primarily for reasons of backward compatibility ans should not be used for new projects.
Search Contour Lines (Average)
This option generates a terrain model by assessing the contour lines regarding the squared distance and by averaging.
Search Contour Lines (Local Inclined Plane)
This option approximates a terrain model by inserting planes between the existing terrain contours and height points.
Search Radius for Contour Lines (m)
In order to determine the local terrain height, all terrain contours within this radius will be considered. The larger the search radius, the longer the calculation time will be.
Displaying triangulated terrain contours
The terrain contours resulting from triangulation can be displayed. Select the command Properties from the Options menu. Specify a line type for the object type „Triangulation“ on dialog Appearance (see Object Appearance).
By default, triangulated lines are not displayed.
General information on terrain models
Normally, the triangulation method generates a terrain model which leads to realistic approximations of the terrain at locations between the existing terrain contours and height points. The terrain contours resulting from the triangulation cause a screening effect in the calculation by default. With the option „Objects with ’Ground at every point’ influence DTM“ activated, also these objects determine the local terrain height and thus the position of the resulting edges of the triangles.
The closeness to reality of a numerically generated terrain model applying the triangulation method depends also on a sufficiently dense sequence of terrain contours and height points. With too large distances between points defining the height, cause a relatively coarse terrain model results with large triangles, each being a plane surface.
for more information see Chapter 4 - Topography
Lift ’Sources under Ground’ to Ground-Niveau
With this option enabled, the height of sources which are below ground level is lifted to a relative height of 0 m within the calculation. This holds for line and area sources as well, causing the polygon points to be raised to a relative height of 0 m. With traffic sources (road, railway and parking lot) the absolute height of the sub-sources is increased by the source height specified by the respective standard or guideline (for example, by the source height of 0.5 m for RLS-90 and 0.6 m for Schall 03). In any case, the (absolute) height used in the calculation is listed on the protocol of receivers (see Calculation Protocol). The specified object height and the terrain model itself will remain unchanged.
It is recommended to use this global option after importing digital data from external data sources only. In general, any inconsistencies between the object’s geometry data and the terrain model shall be avoided by checking the project thoroughly.
Area sources with constant relative height follow terrain
With this option enabled, the geometry of area sources and parking lots will be split into triangles according to the entered triangulated terrain contour lines. The new object geometry is displayed on the 3D-Special view.
Regardless of whether the sound power level PWL or the area related sound power level PWL“ is specified as emission data in the source dialog, the resulting value of the not specified emission level is based on the total area of the 3D-surfaces of all fitted triangles. This sum does not match the 2D-area displayed on dialog Polygon:Geometry of the source, as this is the projected area.
Note
If this option is activated, switching between variants may result in longer loading times, as CadnaA recalculates the DTM for each variant. This is necessary because 1. the DTM may differs between variants, 2. the terrain-following area source and its area may change accordingly and 3. can lead to varying sound power levels when entering Lw''.
Example
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| option deactivated: The parking lot is covered by the terrain as the parking lot’s polygon is outside the terrain contours. | option activated: The parking lot is fitted to the terrain’s shape. |
In the example above, the command „Fit Object to DTM“ from the parking lot’s context menu (see Fit Object to DTM in the manual „Introduction to CadnaA“) would not cause any effect, because the parking lot’s polygon does not intersect with any of the entered or triangulated terrain contour lines.