|To:||"TowerTalk List" <email@example.com>|
|Subject:||Re: [TowerTalk] RE: Downloading terrain data for use with HFTA|
|Date:||Sat, 15 Jan 2005 02:10:36 -0000|
Oh, your experience at NT1Y sounds so familiar to my experience here!
Good to hear that someone else has experienced something similar to myself.
73, Bob - W3YY
----- Original Message ----- From: "Dick Green WC1M" <firstname.lastname@example.org>
To: "'W3YY'" <email@example.com>; "'TowerTalk List'" <firstname.lastname@example.org>
Sent: Friday, January 14, 2005 2:15 PM
Subject: RE: [TowerTalk] RE: Downloading terrain data for use with HFTA
I agree that consumer GPS readings may not be accurate enough to compensate for terrain with lots of variation. Elevations on my steep hillside location can vary considerably in just a few feet, and I've spent many an hour trying to verify the tower locations against Tiger landmarks by interpolation. Although I use a GPS unit with WAAS and averaging (I usually take at least 1000 samples), the accuracy is probably no better than 10 feet, which can make for interesting differences in HFTA.
However, there's another possible source of error. Did you use the "seamless" NED data or the straight USGS DEM data? A while back several of us were looking at proposed tower locations at NT1Y and found that when we overlaid the Tiger data the tower locations were considerably off from visual observations. For example, NT1Y took a GPS reading on one side of a road, but the location showed up on the other side of the road in MicroDEM. The error was at least 30 feet, more in some cases. At first we questioned the accuracy of the GPS readings, but further analysis by N6BV led to speculation that the seamless NED data was not lining up with reality. Dean thought that the smoothing algorithms used to produce the NED data might lead to inaccuracies in certain areas, particularly with very irregular terrain, which is the case at NT1Y.
I've not done an exhaustive study, but my recollection is that the NED data didn't match my tower locations very well, either. For this reason, I use the straight USGS data. The disadvantage is that you might have to combine several files if the location is near a map segment boundary. This is not hard to do in MicroDEM.
My feeling is that the inherent inaccuracy of consumer GPS and USGS data mean that HFTA results should not be taken literally -- e.g., "If I move the tower five feet I'm going to pick up 3dB!" I think the results are really best thought of as relative. For example, I have two main tower locations on my property, separated by 200 feet or so. There are fairly dramatic differences between the two locations, attributable both to a significant difference in elevation and the angle to hills between my QTH and Europe. Moving the two towers around in HFTA can result in some different gain readings, sometimes big ones, but the average relative performance is pretty consistent and correlates very well with on-the-air observations of gain in all directions.
73, Dick WC1M
-----Original Message----- From: W3YY [mailto:email@example.com] Sent: Thursday, January 13, 2005 7:20 PM To: TowerTalk List Subject: [TowerTalk] RE: Downloading terrain data for use with HFTA
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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