|To:||"W3YY" <firstname.lastname@example.org>, email@example.com|
|Subject:||Re: [TowerTalk] Downloading terrain data for use with HFTA|
|From:||Pete Smith <firstname.lastname@example.org>|
|Date:||Thu, 13 Jan 2005 21:24:00 -0500|
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 -
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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