On Thu, 13 Dec 2007 15:26:20 -0500, singhals <singhals@[EMAIL PROTECTED]
>
declaimed the following in soc.genealogy.computing:
>
> Which models in which brands will allow me to retrieve
> lat/lon a week after I mark the spot? Not all of 'em do,
> apparently.
>
Every GPS unit I've owned (and I'm up to my fourth generation unit
now) retained all way points until manually deleted by the user.
The /number/ of retained way points varied -- I think my first unit
could hold 50 way points and one route of 20 or 30 of those points. My
current unit has some 500-1000 way points, and between 20-50 routes of
30 or 50 way points each.
A GPS unit without way points is basically useless -- it can show
you where you are NOW, but can not guide you to a location.
Took me forever to clear out stray way points on my last trip to the
former ConiFur NorthWest (furry convention)... I had the GPS plugged
into a Kenwood D7 radio running in APRS (automatic position re****ting
system); Every 2 minutes my call-sign and position, as retrieved from
the GPS, were transmitted, and as I recall the call-sign and position of
any received signals were recorded as way points on the GPS [It's been
five years so memory could be wrong -- maybe it was just the radio
message log that took forever to clear out].
For a new GPS unit, in CONUS, things to look for: 12-channel
parallel receiver (really old units -- my first -- were 8-channel
sequential). WAAS enabled. Desirable features: averaging (you leave the
unit on in averaging mode for some time without moving it and it refines
the location over time, rather than having instantaneous position that
changes with each update as the NAVSTAR birds move in orbit)
I'd also suggest using UTM rather than Lat/Long... Since UTM is a
metric readout, you can easily compute things like: 10 meters true north
of "xyz mausoleum gate", 5 meters east... a description easier to
visualize than a pair of lat/long values that differ in some decimal
place -- especially as an arc minute of longitude at the equator is
about a nautical mile, but maybe only half a nautical mile at latitude
60 (and only a few inches near the pole)
And you /should/ reference to some distinctive, and unlike to move,
landmark, as just recording a lat/long (or UTM) directly from the GPS
unit can still be off up to 10meters (though with WAAS and clear sky,
more likely the extreme drift is 3-5meter). Even a 5m error for your
recorded position, combined with a 5m error for someone coming back next
year, could result is their position being 30 feet away from where you
were standing...
GPS1...............Actual
your 5m error Actual...............GPS2
a year later
30 feet could be a LOT of graves!
Using UTM with a reference landmark means that the GPS2 person can
compare their reading to your record and determine "I'm reading 10
meters to the east of the recorded values... so if I add 10 to the
recorded UTM eastings, I get numbers that my unit should display today"
(I emphasize the "today" as even a few hours could result in drift)
--
Wulfraed Dennis Lee Bieber KD6MOG
wlfraed@[EMAIL PROTECTED]
(Bestiaria Sup****t Staff: web-asst@[EMAIL PROTECTED]
)
HTTP://www.bestiaria.com/
|
