Surface Feature Files: Obstructions

TAP Surface Feature Files: Obstructions

Q: How do I include obstructions (such as buildings or vegetation) on a profile or in a coverage study?

A: Obstruction information is saved in Surface Feature files. These are data base files that can be named and organized in whatever way is most useful for you. Typically the files are located in a directory named SURFACE below your TAP directory, such as C:\SWTAP\SURFACE.

(This article describes the function in TAP5 and earlier. If you are using TAP6, see the article on TAP6 Surface Feature Files.)

The terrain elevation data base contains topographic elevation values that can be used to compute radio propagation, including attenuation due to terrain shielding. The surface feature data base files supplement the topographic data with elevation information for manmade obstructions.

File Organization

While it is possible to enter information about all obstructions in a single data base file, this approach is not recommended. While every effort has been made to minimize increased execution time resulting from obstruction checking, careful organization of surface feature files is critical.

The more obstructions there are in a file, the greater the impact on TAP's performance, especially with the coverage studies. The recommended approach is to organize the files by geographic areas. For example, if you use TAP for studies of topography or propagation in the Denver area and in the Dallas area, separate files (e.g., SURFACE\DENVER.DBF and SURFACE\DALLAS.DBF could be used. Putting all obstructions for the two areas in a single file would unnecessarily degrade program performance when checking obstructions, since buildings in Dallas rarely affect paths in Denver. If all of your uses of TAP are in a single geographical region (such as a metropolitan area) a single surface feature file usually is acceptable.

To edit or create a surface feature file with obstructions, click on the TAP Editors menu and select Surface Features:

You will be prompted for the file to open. For this tutorial, select the SURFACE\TUTORIAL.DBF file:

The Surface Features File Editor form will open:

The Surface Features data bases include several items for each record. You can use the horizontal scroll bar to move left and right in the editor

Description is an identifier of what the obstruction is. You can enter a description of up to 40 characters.
Lat/Lon/Datum specify the center of the obstruction.
Type identifies how the obstruction is handled by TAP. The type is selected with a pull-down list:

Base Elevation is the elevation above mean sea level (AMSL) of the base of the obstruction, such as a building. If a value is entered, the height of the obstruction will be added to that base elevation during profile plotting, coverage studies, etc. If the value is left blank, the base of the elevation will be interpolated from the terrain data when needed, and the height of the obstruction will be added to that value.
Height is the height of the top of the obstruction above ground level (AGL).
Radius is the radius from the center coordinates of the obstruction. All obstructions are treated as cylinders (with the given radius and height) for the purpose of defining the area affected by the obstruction.

For this tutorial, be sure the following two obstructions are included in the file:

Description	Test Building	Test Vegetation
Latitude	39 02 08.00 N	39 10 50.57 N
Longitude	104 51 47.81 W	104 51 54.28 W
Datum	NAD 27	No Adjustment
Type	Obstruction	Vegetation
Base Elevation	(blank)	(blank)
Height	300 ft	100 ft
Radius	100 ft	5 mi

Three types of obstructions are currently supported:

Obstructions (buildings, water towers, etc.) are treated as knife-edge obstructions. If the cylinder centered on the obstruction coordinates with the given radius overlaps a path, the obstruction elevation is added to the closest point on the path within the radius. Only one point of the path is affected by this type of surface feature.
Vegetation and Clutter are "blanket" features. If the cylinder centered on the obstruction coordinates with the given radius overlaps a path, the obstruction elevation is added to every point on the path within the radius. (Receiver location elevations are not affected by obstructions. The receiver is assumed to be based on the elevation of the terrain, plus the receiver antenna height, and not at the elevation of any surface feature.) In the current version of TAP, Vegetation and Clutter are treated the same. Vegetation represents features such as foliage, forests, etc. Clutter represents buildings, etc.

As an example, draw the profile specified in the TAP 4 Tutorial for path profiles (click the TAP Path menu and select Path Profile to display the Point to Point Study form):

Click the Surface tab:

Click the "Add" button to select a surface feature file:

Select the SURFACE\TUTORIAL.DBF file:

Click the "Continue" button to draw the profile with the obstructions included:

The profile is displayed showing the obstructions from the file that affect this path:

You can use the "magnifying glass" icon on the graphic window to zoom in to see the obstructions on the path:

The zoom will show the obstructions in more detail:

Use the "R" (Refresh) button to clarify the drawing: