William B. Stacy wrote:
>
>In Topo, you can set your QTH and then draw a radial toward a point of
>interest such as EU. Then you can get a plot of the terrain in that
>direction. The resolution is really, really fine. This sure beats the
>daylights out of using a USGS topo map and a ruler to work out the
>contours.
>
Whichever method you use, always add your own local knowledge about the
first few hundred feet from the tower.
The closer the reflection point is to the antenna, the more fine detail
is needed. A typical database output will contain spot heights at equal
distance intervals, so it will probably contain unnecessary amounts of
detail about the terrain at long distances, and maybe not enough detail
about the terrain immediately in front of the antenna.
HFTA draws the terrain profile by simply 'joining the dots' from the
data points in the .pro file. If your local terrain is complex and
bumpy, you will need to add extra lines of data to make the profile
follow the true shape of the land. The data points do not need to be at
equal intervals in distance, so you can insert extra lines wherever
needed.
Don't hesitate to edit the downloaded data if you can see out the window
that it's wrong. For example, in one direction from here, the Shuttle
Radar data shows a small hill that simply doesn't exist.
By comparing the vertical patterns from HFTA 'before' and 'after' these
changes, you can see where the added detail is important.
(A couple of details:
First, remember that only relative heights are important to HFTA -
everything is computed relative to the height at the base of the tower.
This can help when you're eyeballing the local terrain, and judging the
rise and fall relative to the tower height.
Also, there is a limit on the total number of data points that HFTA will
plot. If you add extra lines of data at short distances, you may find
that the terrain is not being plotted out to the full maximum distance.
If this happens, delete some unnecessary lines from the data file.
Wherever you see three lines together with the same height, you can
always delete the middle one, because HFTA will still draw exactly the
same profile through that point.)
>
>Beware: HFTA is addictive. ;-)
And how! But it's also very, very informative.
[I'll be off-list for a couple of weeks now... will be sorry to miss the
rest of this discussion.]
--
73 from Ian GM3SEK
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