[Towertalk] Terrain Analysis

[Pete Smith]

Use of DEM files to generate terrain profiles is a neat and quick way to do 
it without resorting to rulers and topo maps.  A couple of cautions, though:

1.  According to N6BV, "YT has a built-in limit to the number of terrain 
data points it will accept. The absolute maximum number of data points 
(distance from tower = x, elevation = y) it will read is 150. In other 
words, if you have 151 (or 1001) data points, YT will only read the first 
150 and discard the rest."  This made sense in the days when we were doing 
these profiles with a map and a ruler, but with some of these new tools it 
is possible to generate profiles that will overflow the program.

2.  I recently ran YT with a DEM-derived profile of my terrain in the 
direction of Europe.  The profile gives data points every 30 meters for 3 
KM (subsequently converted to feet).  The terrain has a "washboard" 
appearance in the direction of interest (frequent small changes in height, 
alternately up and down), and YT shows an unexpectedly large peak in 
antenna gain at VERY low angles.  With the same terrain file, TA shows the 
same kind of thing, but with somewhat lower magnitudes.  N6BV is aware of 
this phenomenon.  So far it is unclear, at least to me, whether it is a 
modeling artifact of some sort, or a real effect.


73, Pete N4ZR

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