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## Re: [TowerTalk] terrain profile algorithm

 To: towertalk@contesting.com Re: [TowerTalk] terrain profile algorithm Jim Lux Sat, 16 Jun 2012 10:14:29 -0700 mailto:towertalk@contesting.com>
 ```On 6/16/12 9:55 AM, Ian White GM3SEK wrote: > Jim Lux wrote: >> I'm looking for an algorithm for my "generate all profiles for HFTA" app.. >> I've taken a couple trivial shots at this, but I'm pretty sure someone >> out there knows of a "better way". >> >> here's what I want.. >> >> I have an array of heights vs distance at fairly close post spacings (on >> the order of 10 meters) maybe 1000 points or so, representing the >> profile along a particular radial. What I want to do is "compress" that >> into about 100 or so segments that are connected (e.g. HFTA input). >> That is, I want to convert that to a series of x,y coordinates connected >> by straight lines that "adequately" represents the terrain. >> >> I started with a simple "if the adjacent point is within X meters in >> altitude, then collapse the two into one", but that produces profiles >> that somehow don't look right (and I can't say much about their HFTA >> fidelity). If the underlying terrain were a ziggurat or mesas with >> valleys, it would probably work, but it turns a gentle slope into a >> staircase. >> >> This is a classic problem in cartographic generalization, and, as it >> happens, this kind of profile is well represented by a fractal, but not >> that this helps me much. I was hoping that someone out there knew of a >> clever way to do it. >> >> Preferably in Python, since that's what I've written everything else in, >> but I can convert anything.. so if you have one written in COBOL, that's >> fine. >> _______________________________________________ > > If may be easier than you think, because HFTA already makes a linear > extrapolation between data points, regardless of horizontal separation. > > Faced with 360 sets of computer-generated radial data at 100m intervals > going out to 200km, my manual algorithm was: > > 1. Cut off the data somewhat beyond the visual horizon. You can do this > the old-fashioned way by looking at a map, by writing your own routine > or by plotting an approximate horizon profile using www.heywhatsthat.com > > 2. Wherever there are>=3 points at the same height, remove all but the > first and last. Large areas of water are obvious targets [insert other > flatlander jokes here]. > > 3. If the number of points is still too large for HFTA, consider > removing redundant data anywhere that the slope can be approximated by a > straight line between the first and last points. The further away from > the antenna, the less any errors will matter, so work inward from the > greatest distance. > > 4. Repeat 2 and 3 until the data set is small enough for HFTA to handle. > > 5. And then - most important - go outside and LOOK AT WHAT'S REALLY > THERE in the immediate foreground of the antenna. > > Don't hesitate to modify the satellite data wherever you know better. > You are much closer to the reality on the ground! For example, dense > areas of trees would have given a good radar reflection from the > tree-tops, but at HF the true ground level will be a better > approximation. Yes.. that's the sort of algorithm, but I'm looking at something automated.. Step 2 is basically what I have now.. It's the step 3 part that is tricky to automate.. You want straight line segments that approximate a section, but that are continuous with the segments on either side. Before I spent significant time designing and coding such an algorithm, I thought I'd ask. _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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