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

[Jim Lux]

----- Original Message -----
From: "Pete Smith" <n4zr at contesting.com>
To: "Michael Tope" <W4EF at dellroy.com>; "W3YY" <w3yy at cox.net>; "TowerTalk
List" <towertalk at 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)