----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: "W3YY" <w3yy@cox.net>; <towertalk@contesting.com>
Sent: Thursday, January 13, 2005 6:24 PM
Subject: Re: [TowerTalk] Downloading terrain data for use with HFTA
> 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.
The TIGER's probably not all that much more accurate than the USGS data (it
was probably drawn from it in the first place).
If you're looking to site an antenna on a terrain model, the best approach
is to match the terrain. If you've got some sort of hillock or such as a
reference point, use it. You can also do traditional surveying approaches.
Identify some hill tops on the terrain map. Measure the bearings to those
hilltops from your antenna site. Plot it out on the terrain map. Now you'll
have your antenna position in "terrain map coordinates" which is what you
really want.
>
> 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.
Precisely..
Two aspects to this... the shape of the spheroid (Clarke 1866 in this case),
which sets what the shape of the surface is and
Where the master reference point is, which is a benchmark in Meade KS.
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.
NAD 83 and WGS84 are fairly close (I can't recall the exact difference, if
any). WGS84 was designed to provide a consistent system for all the world
(as the result of satellite measurements). Prior to that, every country had
its own system, and they didn't align, so the physical location
corresponding to some lat/lon was not the same.
>
> 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.
_______________________________________________
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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