Bullington Topo Data Step
Q: What value should I use for the Topo Data Step with the Bullington model?
A: The Bullington propagation model uses the terrain elevation data along a path to compute knife-edge diffraction loss values.
The topo data step is the increment between topographic elevation points along a path.
Setting the topo data step for a link budget or area coverage study using the Bullington model will affect the predicted field strength computed by the model. This is especially likely if you set a very small data increment, since the model will inflate the predicted losses because it finds many knife-edge points and computes a loss for each one.
One TAP user who has extensive experience with the Bullington model suggests a topo data step of 0.5 miles with a remote antenna height of 40 feet AGL or higher, and 0.25 miles as a minimum step value for lower mobile antennas (to better account for terrain closer to the lower mobile antenna). This approach used primarily the “Inflection” method in the Bullington model templates.
Having computed literally thousands of coverage studies with the Bullington model demonstrates the value of this approach, with many measured points within 1dB of the predicted field strength value. These studies were over a wide variety of terrain types, from relatively flat to remote mountains. In these cases, using a topo data step of 0.5 miles yielded very accurate results from the Bullington model.
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