----- Original Message -----
From: "W3YY" <w3yy@cox.net>
> I want to note a related observation. I have very detailed local
> topographic maps of my property and the surrounding area. These were
> obtained from my County.
>
> Using a GPS receiver I "located" my two towers, then downloaded the USGS
> data, and ran elevation profiles. Something seemed wrong. When I checked
> the USGS elevation at the base of each tower, the elevation figures didn't
> seem right. This conclusion was based on where I know the towers are
> located relative to my property lines and my physical topo maps. I
suspect
> the errors in the GPS are causing the problem, i.e., the location as
> specified by the GPS receiver is not sufficiently accurate and might be
off
> by 25ft or more.
>
> The topography on my property is quite irregular, so "moving" the towers
> even 25 or 50 ft can make a noticeable difference in elevation and
elevation
> profiles.
>
> The bottom line - if you are on highly irregular property, I'm not sure I
> would trust results based on the USGS data. This is not because the the
> USGS data is bad, it's because you can't really determine the correct
> position of your towers, unless you have a military-grade GPS unit.
>
> Maybe there is another explanation. Other thoughts are welcome. I'd like
> to be able to confidently use the USGS data, if possible.
>
> 73, Bob - W3YY
A couple of comments. With regard to locating your towers, I
suggest you checkout the program Radio Mobile:
http://www.cplus.org/rmw/english1.html
It allows you to overall 1 meter resolution aerial photos from the
Terraserver and maps from mapquest on top of elevation data.
I use the NASA SRTM database as my elevation data source.
It provides 1 arcsecond horizontal resolution. This amounts to
approximately 100 x 100 ft blocks. Worst case relative elevation
resolution is on the order of 30 feet, but I think in most cases it's
much better than that (30ft was the design spec on the radar
instrument that acquired the data). The advantage of the Radio
Mobile program is that you can compare visible features from the
Terraserver aerial photos overlays with the position readout from
your GPS to make minor corrections.
The other thing to keep in mind is these terrain databases may
not capture very fine structure. The STRM database for instance
is pixelized into 100ft x 100 ft blocks. If you have an elevation
feature that is smaller than this like a narrow mesa for instance,
it will get smeared a little bit by the sampling process. Not sure
how good the resolution is on the USGS data in MicroDem, but
I suspect it suffers from the same limitation. In those cases, you
may have to do some local surveying to supplement the terrain
data in the database with the missing fine structure. It's very
painstaking, but, for instance, you could use your country maps to
generate the terrain profile manually. It's just an ASCII file with
two columns. The first column is distance from the tower in feet,
and the seconds column is the height above sea level at that
point. I dump the terrain profiles files from Radio Mobile into
Microsoft Excel, reformat them to match the HFTA format, save
them out as ASCII text, and then pull them into HFTA.
73 de Mike, W4EF..................................................
_______________________________________________
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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