To: | "W3YY" <w3yy@cox.net>, towertalk@contesting.com |
---|---|
Subject: | Re: [TowerTalk] Downloading terrain data for use with HFTA |
From: | Pete Smith <n4zr@contesting.com> |
Date: | Thu, 13 Jan 2005 21:24:00 -0500 |
List-post: | <mailto:towertalk@contesting.com> |
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. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk |
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