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Re: [TowerTalk] Delorme data bases

Subject: Re: [TowerTalk] Delorme data bases
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Wed, 12 Sep 2007 08:40:27 +0100
List-post: <>
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 
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 

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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