Fit Objects to DTM

The basic functions of this command are described in chapter Fit Object to DTM of the manual „Introduction to CadnaA“.

Application

The data on object heights (e.g. roads, buildings) and terrain height (i.e. height points, contour lines) often originate from different sources. In practice it occurs quite often that the given object heights (e.g. the road‘s height) does not correspond with the terrain heights.

In these situations, CadnaA can fit the object height (e.g of a road or roads or other line- or area-like objects) to the terrain height by means of the command Fit Object to DTM. With this, new vertex points will be generated of the object‘s geometry, the height (z coordinate) of which will be adjusted to the local ground level.

When using this command, it is assumed that the terrain model and, thus, the local height (z coordinate, i.e. the absolute height) is correct and relevant. This implies that the actual object heights are less correct and less relevant than the local terrain height.

The decision whether the object heights or the terrain model is/are relevant or not, cannot be decided by the software, it is up to the user.

Consequences

With regard to the effects caused by this command we will distinguish line-like and area-like objects in the following.

Line-like Objects

Examples of this type of object are line sources, roads or railways.

For these objects, the entered polygon line represents the central axis of the object (especially with roads and railways). This means that even with a varying road width, the geometry (and hence the height) is known for the center line only. Thus, fitting to the terrain can, therefore, just occur for this center line, since the heights at the road‘s curbs (or on the edge of the railway track) are unknown. This holds - regarding roads - especially when a lateral slope has been entered for the road (see Common Input Data). Even in this case, fitting to the terrain just occurs for the road‘s center line.

The consequence is - regardless whether a lateral slope has been entered or not - after using the command Fit Object to DTM the both road curbs may either be covered by terrain or are floating above the ground. When no lateral slope has been entered the road surface is assumed to be flat. Furthermore, this may have the additional consequence that two emission lines of the road may also be covered by terrain or may float above the ground. In case a lateral slope has been entered it will be kept when using the command Fit Object to DTM. In both cases - with or without a lateral slope - no adaptation of the road curbs to the local terrain height occur.

Example

Road with two contour lines The height is known at the polygon points of the road. Here the two points are located in flat terrain (the area between contour line at 0 m and the corners of the limits).	3D-Special view The road is covered by the terrain because there is no height defining polygon points of the road within the hill area.
When executing the command Fit Object to DTM additional polygon points are inserted to the object‘s geometry at all intersections with explicit or triangulated terrain contours. Their local ground height results from the terrain model.	3D-Special view The road is no longer covered the terrain. The fitting of the object‘s geometry is based on the center line of he road.

Nevertheless, the road may still be overlapped by terrain or be hovering above the ground at the road‘s curbs.

After applying the command Fit Object to DTM the road‘s curbs may still be covered by terrain or float above the terrain.

With a road having a lateral slope (in this case 20%) the fitting to the DTM just occurs for the road‘s center line.

Area-like Objects

Examples of this type of objects are area sources and parking lots.

With area-like objects, explicit polygon points just exist for the perimeter line of the source. Inside of the area object, however, there are no polygon points the height of which could be fitted to an existing terrain model.

Consequently, with an area-like object just a defective fitting to a complex terrain may result when using the command Fit Object to DTM.

Example

	An area source in a complex terrain before applying the command Fit Object to DTM
	Situation after applying the command Fit Object to DTM: The polygon points of the area source‘s outline have been fit to the local terrain height. However, the areas inside the source‘s outline are still below terrain.

In order to properly fit area sources to the terrain do not use the command Fit Object to DTM, but enable the option „Area sources with constant relative height follow terrain“ from the „DTM“ tab, on the Calculation| Configuration menu (see DTM Digital Terrain Model Tab).

Fitting an area source to the terrain with option „Area sources with constant relative height follow terrain“ enabled