----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: "Michael Tope" <W4EF@dellroy.com>; "W3YY" <w3yy@cox.net>; "TowerTalk
List" <towertalk@contesting.com>
Sent: Thursday, January 13, 2005 6:33 PM
Subject: Re: [TowerTalk] RE: Downloading terrain data for use with HFTA
> I think it is probably really easy to overkill this problem. HFTA only
> uses one datapoint per 30 meters anyway.
>
> The USGS data are 1 arc-second resolution -- same as SRTM. I think the
> elevation accuracy is +/- 1 meter, or maybe it is 1-meter resolution, but
> either way the data points are pretty far apart.
USGS DEM data is at the same basic accuracy as 1:24000 maps (from which much
of the original database was derived, prior to SRTM). That's the usual
national map accuracy standards (i.e. 40 ft horizontal control).
1 arc second (in the north south direction) is about 10,000 km/(90*3600) is
about 30m (call it 100ft).
The 3 arcsecond data comes from the 1:250,000 scale 1x2 degree maps, and
holds to that sort of accuracy (i.e. 100meters or so). If you try and
coregister the terrain from the 3 arcsecond DEM/DTED data with things like
the National atlas (1:1,000,000), or, for that matter, with coordinates
measured off a 1:24,000 topo map, you'll often find displacements of 100
meters or so. (rivers that run on the side of the valley, for instance)
_______________________________________________
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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