Re: GPS units

singhals <singhals@xxxxxxxxx> wrote in
news:jNednT_yeK_HMv_anZ2dnUVZ_tGonZ2d@xxxxxxx:

Dennis Lee Bieber wrote:

On Thu, 13 Dec 2007 15:26:20 -0500, singhals <singhals@xxxxxxxxx>
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.

Yeah, it'll show me the street address and the map location,
but NOT the lat/lon.


Maybe the GPS's you've seen/used were dedicated to streets & driving
directions? Only a guess, because my Magellan Explorist 210 is for hiking
& geocaching and everything it does is based on the lat/lon or other
selected format of N-S-E-W position.

The waypoints & points of interest stay in the unit until you erase them
or write over them in the case of breadcrumbs, but even the breadcrumbs
can be saved to the internal memory storage.

And for what it's worth, Google maps, Google earth & others give you (and
you can search by) lat/lon coords.

Try this:
1. Go to http://maps.google.com/maps and
2. type this into the search box: 39.170098,-76.67362
then 3. click on the Satellite button on the map and zoom in as far as
you can. (It's the cenetery at the end of Runway 4 at BWI airport).

The coords came from me visually finding the airport in Google maps,
zooming in to the max and centering the map, then clicking the "Link this
page" and copying the link info with coords.


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 reporting
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)

Except, I _understand_ lat/lon (g).


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!

Understood. In my primary interest case, if you can get
within 30 ft you ought to see the fence ... and it ought to
still be there, it was built to LAST.


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)

There's a stream nearby but nothing else guaranteed not to
change in the next decade. S'why I'm so anxious to get a
lat/lon on the place. They're talking about re-routing a
major road and if they do I might not find the place again
without lat/lon.

Cheryl

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