[Top] [All Lists]

Re: [TowerTalk] Downloading terrain data for use with HFTA

To: "W3YY" <>,
Subject: Re: [TowerTalk] Downloading terrain data for use with HFTA
From: Pete Smith <>
Date: Thu, 13 Jan 2005 21:24:00 -0500
List-post: <>
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

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" <> To: <>; <> 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, > > > > 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 > > > _______________________________________________ > > See: 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. > > _______________________________________________ > TowerTalk mailing list > > >


See: 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.

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>