There are several necessary definitions and other preliminaries. The lines-of-sight (LOSs) radiate away from the observer, who is some fixed distance above ground level such as 5 meters. The observer is trying to see the target, also at a fixed distance above the ground, such as 25 m. If the observer height is different from the target height, then visibility is not reflexive. That an observer above point A on the ground can see a target above point B does not necessarily mean that an observer above B can see a target above A. We are interested only in targets within a certain fixed range, or radius of interest, of the observer, such as 40 km.
We have no reason to suspect that the particular values of the above parameters, which are useful in aircraft radar tracking, affect the nature of the results, at least within wide ranges, so we generally use the stated values. Note that our range, about 500 pixels, is larger than often reported in the literature, about 100--200 pixels.
The curvature of the earth can be most easily compensated for by systematically changing elevations before processing, at least for regions much smaller than the earth's radius.
We work with raster elevation databases, such as DTED or DEM, which has 1201x1201 elevations for a 1 degree times 1 degree region, such as the following datasets.