----- Original Message -----
From: "W3YY" <w3yy@cox.net>
To: "TowerTalk List" <towertalk@contesting.com>
Sent: Thursday, January 13, 2005 4:20 PM
Subject: [TowerTalk] RE: Downloading terrain data for use with HFTA
> To All -
>
> Thanks to Pete for posting this, as the process has, indeed, changed.
>
> 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.
Don't forget to adjust for the correct datum (look in the lower left corner
of the USGS map)...
Most topo maps are on NAD27 datum, and GPS (by default) reads out in WGS84.
They're different by several tens of feet, typically. (It has to do with
what the assumed shape of the earth is and where "zero,zero" in the
coordinate system is).
>
> 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.
The statement at the bottom of the map "Meets national map accuracy
standards" means that features on the map are within 1/50th of an inch of
where they should be (thickness of a pencil point, for what it's worth). On
a typical 7.5 minute quad (1:24,000 scale 1"=2000 ft) that means that the
USGS map is good to 2000/50 ft or, about, 40 ft.
The most accurate generally available terrain databases are those from the
recent SRTM mission, and are on 30 meter posts. The horizontal control is
probably 1/10th of that in most places (that is, if the database says that
the elevation is Z meters at some X,Y coordinate, the elevation is within a
few meters of Z, at a location within 3-4 meters of (X,Y).
As far as GPS goes, whether it's military or not doesn't really affect the
accuracy all that much. Far more dominant effects will be things like
multipath, ionospheric disturbances, etc. A differential measurement can be
as good as 1ppm +/- 1mm relative to the reference point. In absolute terms,
though, a meter or so is as good as your going to get, and a single
frequency receiver, averaged over a fairly long time (days) would probably
be within a few meters, in absolute terms.
>
> 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
> >
> >
> > ----- Original Message -----
> > From: "Pete Smith" <n4zr@contesting.com>
> > To: <towertalk@contesting.com>; <antennaware@contesting.com>
> > Sent: Wednesday, January 12, 2005 8:29 PM
> > Subject: [TowerTalk] Downloading terrain data for use with HFTA
> >
> >
> > > This is intended for anyone who has tried to download terrain data
from
> > > USGS to use with HFTA, only to discover that several aspects of the
> > > downloading process have changed. N6BV has just posted an article
that
> I
> > > wrote on the ARRL web site,
> > >
> > > http://www.arrl.org/notes/9043/HFTA-Changes.pdf
> > >
> > > which gives the new procedure with screen shots and active links.
Both
> > > Dean and Peter Guth, the author of MicroDEM, were a huge help in
pulling
> > > this together and making things work again.
> > >
> > > 73, Pete N4ZR
> > > The World HF Contest Station Database
> > > was updated 13 December 2004
> > > 2843 contest stations at
> > > www.pvrc.org/WCSD/WCSDsearch.htm
> > >
> > > _______________________________________________
> > >
> > > 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
> > >
> >
>
>
> _______________________________________________
>
> 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
_______________________________________________
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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