Shadow Ratio Levels

Shadow Ratio Levels

Q: When I run a shadowing study, how do I determine areas of clear line of sight, shadowing, etc.?

A: The shadow study under TAP 4 computes a "shadow ratio" for every point in the study (along radials or in a tile area). The shadow ratio is the lowest value along the path of the ratio between clearance at each point along the path and the first Fresnel zone at the specified operating frequency.

The clearance at each point is defined as the distance between the elevation of the topography at that point (adjusted for earth curvature) and the elevation of the line of sight path at the same location. A positive value indicates that the line of sight path is above the terrain at that point. A zero or negative value indicates that the terrain is higher and the line of sight is blocked. A negative clearance value will also result in a negative shadow ratio.

The shadow ratio defines the "worst case" shadowing for the path. For example, it only takes one point to block the line of sight along a path. If several points of the terrain block the line of sight, the shadow ratio for the path will be based on the point with the worst "clearance-to-Fresnel" ratio. This gives some quantitative information about blocked paths: the lower the negative value, the worse the blockage is. In the same way, positive shadow ratios indicate the relative amount of clearance on a clear line of sight path.

The values for traditional shadow studies can be based on the shadow ratios.

Any path with a zero or negative ratio (i.e., less than or equal to zero) is blocked.
Any path with a positive ratio (i.e., greater than zero) has clear line of sight.
Grazing paths can be defined using half the first Fresnel zone, so any path with a ratio less than .5 (but greater than zero) can be considered grazing. (Some engineers prefer to use .6 of the first Fresnel zone.)

In the TAP plotting software you have the opportunity to set shadow levels to plot, similar to field strength threshold levels, with different colors and/or fill patterns for different levels.

For example, suppose you are plotting (using the File|New Map function from the TAP menu) a shadowing tile study. When the study is selected it is displayed on the Map Window Task Files form. When one or more shadowing studies are included, the "Shadow Levels" button is displayed. Click this button to set the shadow ratio values:
The Shadow Plot Options form is displayed. You can set the shadow levels as desired. For example, to show the traditional areas of blocked, grazing, and clear paths, you could set the values as shown:
When the shadow study is plotted, the grazing and blocked areas are shown. (Note in this example there are no areas of clear line of sight.)
Using the shadow ratio values you could also show information about the degree of shadowing. For example, you could set another level in the Shadow Plot Options form to show the areas where the clearance to Fresnel ratio was -.05 or greater, indicating a small amount of blockage. (Note that this value is for illustration only.) You would probably also want to specify a different color or fill pattern:
Now when you plot the study, you can differentiate between areas with "minimal blockage" and the more severe blocked line of sight. Both are blocked, but with different levels of severity.