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Re: [TowerTalk] HFTA Disc....

To: "'StellarCAT'" <rxdesign@ssvecnet.com>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] HFTA Disc....
From: "Ian White" <gm3sek@ifwtech.co.uk>
Date: Sun, 17 Jul 2016 13:43:35 +0100
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>

HFTA does not require evenly spaced data points, so the 149-point limit in the 
data file is not the problem it might seem. If you are prepared to remove some 
redundant data from the computer-generated file, you can often reclaim space to 
insert important local detail that the satellite surveys had missed.

Redundant data includes:

* All but the first and last points from a string of data showing the same 
height (so a large body of water can be represented by just two points near the 
opposite shores, and HFTA will interpolate as needed).

* All intermediate data points on a uniform slope (these opportunities are 
harder to notice in the file, but they may well be there)

* Excessively dense data at great distances (it makes no sense to use the same 
point spacing at the far horizon that you're using close in). 

* Everything beyond the horizon (it is advisable to leave a few points in just 
beyond the ridge line to allow for diffraction, but distant locations in 'deep 
shadow' can be safely ignored).

Always keep back-copies, of course, and always check for the effects of your 
editing on the computed results.

And having done that, you can then insert some new data points close-in where 
it matters, based on your own local knowledge. For example, I deleted a 'hill' 
which was actually a clump of tall trees (replacing that data point with the 
true ground level) and inserted new data points to represent a sharp drop-off 
which the satellite data had missed.


73 from Ian GM3SEK


>-----Original Message-----
>From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
>StellarCAT
>Sent: 17 July 2016 12:51
>To: towertalk@contesting.com
>Subject: Re: [TowerTalk] HFTA Disc....
>
>Yes but to Shawn's point there is an arbitrary limit to the number of points
>allowed. I had a  friend in CO ask me to do a plot for him using MicroDEM
>and the 10 meter points (1/3 arcsec) ... he had distant mountains that
>didn't show in the normal 14,000' range ... but when I tried to go out
>further, using this with HFTA it told me there was a limit to 149 points. I
>had never seen that before in my use - using 10 meter and 5 deg
>increments... but to Shawn's point in a reply today, something I hadn't
>thought about with my runs, 4400M (14K' roughly) would be 440 points if it
>was 10 meters and 147 if 30 meters... so it might very well be that it
>truncated the data using only every third one and all this time I had been
>believing I was using 10 meter points!
>
>My plots look quite undulated and match a topo map  ... but I can now easily
>see that it could very well be 30 meters since the major demarcation points
>are 1000'!
>
>This would be the first thing I'd like to see changed - take it, change it
>so that it can use the 10 meter data out to the full extent.
>
>Gary
>
>-----Original Message-----
>From: Jim Brown
>Sent: Saturday, July 16, 2016 7:28 PM
>To: towertalk@contesting.com
>Subject: Re: [TowerTalk] HFTA Disc....
>
>Gary,
>
>When I was running HFTA on my QTH in the Santa Cruz mountains, Dean
>advised me to to out to 10-15 miles. This ignores small variations, like
>the gullies being described.  The radial data is a plain text file, and
>it is possible to generate your own data from topo maps. When I started
>using HFTA, I did a few radials this way as an exercise.
>
>73, Jim K9YC
>
>On Sat,7/16/2016 3:03 PM, StellarCAT wrote:
>> I use the 10M range data with HFTA exclusively! The 30 is far too long of
>> a sample point. I'd prefer even smaller if available. I set up microdem at
>> 5° increments and never have a problem getting the full data set. Works
>> great.
>
>
>
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