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

[Pete Smith]

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.

I like overlaying TIGER to get the same effect as Mike gets with his aerial 
photo data -- either way it can help you get your tower base in the right 
place.

As N6BV says in his HFTA documentation, trying to infuse too much meaning 
into a .1 or .2 difference in the weighted average gain (figure of merit) 
is unjustified anyway.

73, Pete N4ZR

At 09:15 PM 1/13/2005, Michael Tope wrote:

>----- Original Message -----
>From: "W3YY" <w3yy at 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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