[TowerTalk] Downloading terrain data for use with HFTA

[Pete Smith]

This is one reason why I prefer to overlay the TIGER data on the terrain 
before I decide precisely where the base of the tower should be set on the 
terrain.  To be honest, I have never tried to locate my tower through GPS 
-- I position it relative to my driveway, which shows fine on the TIGER 
data, and the road in the next block.  I then apply a sanity check relative 
to elevation changes that I can see.  I am satisfied that in the case of my 
tower the positioning is good within +/- 10 feet or so.

Another potential source of error -- the paper USGS topo map I have is 
based on the "1927 North American datum", which I think is a mathematical 
model of the shape of the earth.  In the margin of the map it says "to 
place on the predicted North American Datum 1983, move the projection lines 
7 meters south and 29 meters west as shown by dashed corner ticks."  I 
think, but don't know for sure, that both GPS positions and the USGS 
on-line data are based on what became the 1984 Datum.

Don't forget, HFTA uses data points that are 30 meters apart, so a small 
horizontal shift can make a fairly big difference in the perceived 
elevation if the slope in the area of the data point is at all pronounced.

73, Pete N4ZR

At 06:39 PM 1/13/2005, W3YY wrote:

>To All -
>
>Thanks to Pete for posting this, as the process has, indeed, changed.
>
>I want to note a related observation.  I have very detailed local
>topographic maps of my property and the surrounding area.  These were
>obtained from my County.
>
>Using a GPS receiver I "located" my two towers, then downloaded the USGS
>data, and ran elevation profiles.  Something seemed wrong.  When I checked
>the USGS elevation at the base of each tower, the elevation figures didn't
>seem right.  This conclusion was based on where I know the towers are
>located relative to my property lines and my physical topo maps.  I suspect
>the errors in the GPS are causing the problem, i.e., the location as
>specified by the GPS receiver is not sufficiently accurate and might be off
>by 25ft or more.
>
>The topography on my property is quite irregular, so "moving" the towers
>even 25 or 50 ft can make a noticeable difference in elevation and elevation
>profiles.
>
>The bottom line - if you are on highly irregular property, I'm not sure I
>would trust results based on the USGS data.  This is not because the the
>USGS data is bad, it's because you can't really determine the correct
>position of your towers, unless you have a military-grade GPS unit.
>
>Maybe there is another explanation.  Other thoughts are welcome.  I'd like
>to be able to confidently use the USGS data, if possible.
>
>73, Bob - W3YY
>
>
>----- Original Message -----
>From: "Pete Smith" <n4zr at contesting.com>
>To: <towertalk at contesting.com>; <antennaware at contesting.com>
>Sent: Wednesday, January 12, 2005 8:29 PM
>Subject: [TowerTalk] Downloading terrain data for use with HFTA
>
>
> > This is intended for anyone who has tried to download terrain data from
> > USGS to use with HFTA, only to discover that several aspects of the
> > downloading process have changed.  N6BV has just posted an article that I
> > wrote on the ARRL web site,
> >
> > http://www.arrl.org/notes/9043/HFTA-Changes.pdf
> >
> > which gives the new procedure with screen shots and active links.  Both
> > Dean and Peter Guth, the author of MicroDEM, were a huge help in pulling
> > this together and making things work again.
> >
> > 73, Pete N4ZR
> > The World HF Contest Station Database
> > was updated 13 December 2004
> > 2843 contest stations at
> > www.pvrc.org/WCSD/WCSDsearch.htm
> >
> > _______________________________________________
> >
> > See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
> >
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> >