On 12/30/11 8:49 AM, Mike Ashby wrote:
> Bill; I just finished making application to the FAA and after about
> 60 days received back a "Notice of presumed HAZARD to air navigation"
> letter from them. This has since has been resolved and I now have
> received the go ahead to install a tower. QTH is 5572 feet south of
> a very small municipal airport. Tower location falls in the first
> ring around the airport marked by the FAA as "Horizontal Surface
> elevation: 150 feet". 10 feet further east on my 10 acre property
> begins the outer ring around the airport marked "Conical Surface
> 20:1" One would think that in the 1st ring the tower could be 150 ft
> in height and in the 2nd ring it could be 278 ft in height,
> (5572/20)...........I did. Started application by taking some quick
> elevation measurements with GPS unit and made application for a 120
> foot tower, it was rejected. After long discussion with FAA, took a
> trip over to the airport to talk with the manager. I wanted to find
> the airport elevation marker for the airstrip. They didn't know
> where it was. The sign at the entrance to the property posted the
> elevation as 5680 ft. The FAA shows it as 5637 ft. I asked a pilot
> of a small commercial jet, that was sitting of the airstrip what his
> altimeter was reading and he said 5605 ft. (That would be 32 ft
> below the official FAA surface of the runway elevation). Went into
> town and found an official USGS survey marker and calibrated my GPS.
> Went back to QTH and remeasured, nothing was looking good for me.
> Called FAA, discussed my measurements.........decide d I would use
> same thing that they were checking with...................GOOGLE
> EARTH! Plugged in all of the measurements, adjusted tower height
> down to 110ft, (could have gone as high as 113 ft but didn't want to
> press my luck), sent changes back to FAA and received approval next
> day. So much for the "150 ft zone" and the "20:1"
> zone.............never did get a good explanation as to why not, just
> happy to get what I got.
The first problem was in using GPS to measure the elevation. A 10 foot
(3 meter) variation wouldn't be unusual (unless you were using a dual
frequency surveying setup referenced to a nearby base station).
http://www.gps.gov/systems/gps/performance/accuracy/ shows 95%
confidence that the horizontal position for a high quality receiver is
about 2 meters.
The geometry of GPS is such that the vertical accuracy is typically 2-3
times worse than the horizontal accuracy (depending on where the
satellites are in the sky at the time you make the measurement). For
horizontal, you've got satellites all around you covering 360 degrees.
For vertical, they're only above you.
various correction schemes (WAAS) help with the ionospheric correction,
but don't help with things like multipath.
GPS is wonderful, but a handheld GPS is going to be limited to around
1-2 meter horizontal accuracy for a variety of reasons. You might be
able to do a poor-man's differential measurement over a distance of
10-20 meters and be down in the 10s of cm range, but that's when you
make the measurements within a matter of minutes.
Go out and measure a particular point today, and then go out and measure
it tomorrow, and the day after, and so forth, and you'll typically see
it move around by a few meters.
And some GPS units actually give you pressure altitude rather than GPS
altitude (my Garmin eTrex told me I was at 8000 ft when I fired it up in
a commercial jet at 30,000+)... I had to dig down into a bunch of menus
to find "GPS Altitude"
Then we get to the whole "datum" thing. The FAA doesn't really much
care how accurate the airport elevation is (at least until they do
precision GPS approaches). All they really care is that ALL pilots use
the same number (printed on the chart), so that when their altimeter
reads 1000 ft higher than the airport, they're really 1000 ft higher.
Whether they're at 6500 ft and the airport is at 5500, or they're at
6400 ft and the airport is at 5400 doesn't make much difference. That's
also why you set your altimeter to the current reported pressure (which
may not actually be what that pressure is..): everyone is using the same
base, and they use the same conversion from pressure to altitude.
I've been to small airports where they actually didn't have a barometer.
What they had was a calibrated aircraft altimeter, and they set the
pressure so that the altimeter read the field altitude (on the chart),
and used that pressure as the current altimeter setting.
So, when you get a GPS altitude, it's referenced to the WGS84 geoid.
The airport field elevation might be referenced to something else, and
the USGS topo map to yet something else (NAD27, typically).
So your strategy of going to the airport with your GPS and measuring it
there was a fairly sound one.. and probably would give you a good
differential elevation within 10 feet if you made the measurements over
the course of an hour. You'd have the same datum for both measurements,
the satellites in view are about the same, they haven't moved much in an
hour (15-30 degrees)
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