On Fri, 27 Nov 2009 01:58:09 -0800 (PST), "
wrote:

The "virtual" compass requires you to move since it bases direction on
differential GPS readings. OK in a car, semi-OK on foot, worthless if
the going is slow since the delta of distance is not significant
enough to get a good calculation.

Actually, it's not too horrible for determining the general direction
of travel. However, totally useless for determining the azimuth of
something from a fixed location.

The newer GPSs have sensors in them. To null out the nearby magnetic
field, you need to slow spin it around. Two revolutions generally. Or
you can spin your body around and look like an idiot. The trouble is
the GPS has to be level unless you have a 3-D compass. [Garmin
doesn't, Magellan does. Too many complaints about Magellan gear, so
you just put up with Garmin.] The GPS has a threshold where it will
shift from magnetic sensor to virtual compass. You can really screw
yourself up if you don't know about this.

I spent some time trying to get sane readings from the fluxgate
"magnetic" compass in a borrowed Garmin GPS. The rotation method
worked quite well, until I moved. When standing next to a vehicle, it
was nearly useless.

Your technique is fine if you spot from the same location. If you are
on the move, you realy do need a compass.

Ummm.... orbital satellite spotting from a moving vehicle is rather
uncommon and dangerous.

Occasionally I'll see
something off in the distance.[OK, OK, in denied access area.]

Sigh. I suppose the alien technology that they're hiding is better
than a compass.

Log
your position, take a vector, then study it on google earth or a map.
Other times I am DFing radio signals. Again, the compass does the log.
Log everything in magnetic, then use your mag in magnetic, and you
won't go crazy. Attempt to correct your readings and you are either
correct or you just added twice the adjustment factor to your reading.
Really not a good idea.

Ok. I'll admit to having added my declination instead subtracted more
than once. Still, some additional practice and a few sanity checks
against known locations should help.

Aviation long ago decided on doing everything magnetic, probably to
get around the confusion.

Mostly true. All US runways are magnetic. So is the VOR. However,
all sectional maps are based on true north. GPS navigation devices
can be either way. Magnetic is a big help when using a magnetic
compass for finding the runway.

Canadian aviation is really strange. The northern half uses true
north, while the southern half uses magnetic.

http://en.wikipedia.org/wiki/Magnetic_declination

Of course, they still use AGL (above ground
level) and MSL (mean sea level) for altitude. You may recall either a
Thunderbird or Blue Angel getting into trouble with this.

Dunno. However, I've screwed up a few HAAT calculations on FCC
license applications when I was doing them with just a calculator:
http://en.wikipedia.org/wiki/Height_above_average_terrain

I use your spotting technique at times too. It depends on if I can do
the preparation. Your scheme works better in mountain areas, which is
where I use it since I know the various peaks and where the campsite
is located.

It works with any decent road map. You don't even need to sight the
distant mountain peaks. Just align the map with the local roadways
and it's already better than a magnetic compass. The most difficult
part is finding a large flat area on which to place the map. When
desperate, I use corrugated box cardboard and pins for markers.

Incidentally, I have a crude system of doing TDOA (time difference of
arrival) hyperbolic RDF location on a paper map using two nails and
some string. I'm slowly scribbling a web page on the subject, so no
details until I'm done.

Oh, if you want high tech location and mapping assistance, there's GPS
augmented reality.
http://www.youtube.com/watch?v=4Fl718QO_xQ


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558