Another accuracy question

On Mon, 26 Jul 2010 09:46:53 +0800, "Hoges in WA"


I've noticed this anomaly too between mine and other GPS owners. I bought
an eTrex Legend Cx on sale ($100 bundled with 12v power-adapter, suction
mount, and CNNANT DVD included), close-outs they were trying to get rid of
as they switched to the HCx models. The HCx models are supposed to have
higher accuracy and better reception, but mine has the same chipset in it
as the newer and much more expensive Venture HCx and Legend HCx models. I
tried geocaching a few times to see what the enjoyment was, but out of the
five I found my GPS put me within 3-7 feet of each one, one my GPS had me
stand right on top of the geocache. If the geocache owner is doing their
job by taking an good average to get higher accuracy on its initial
location, and mine is able to recreate that accuracy without even taking an
average, then what's the challenge? So I don't bother geocaching anymore.
The only time it would be interesting is if the geocache owner didn't
record the location with any precision.

I was having a property-line dispute with a neighbor. Each year they cut
further and further into my land. So slowly that during an 18 year period
they were over 120 feet into my property line. Knowing how accurately this
GPS was doing its thing, I thought I'd use it to find the property corner
in dispute. This time I took a many many point average, walking to and away
from that point about 100 feet from all manner of directions many times.
When I finally got no changes at all I used a sledge and pounded an 8 ft.
iron-rod deep in the ground under where I held the GPS. The iron-rod was
almost on the edge of their driveway. The neighbors called the sheriff, the
sheriff told them they'd have to get their land surveyed. Now granted, this
was pure luck for much of it, but when their surveyor planted his official
legal stake, it was only TWO INCHES away from where I planted my iron-rod
mark using my GPS to determine the location (my iron-rod marker was even on
my side of the property line by those two inches). They had to abandon
their new garden and everything. It was on MY land. This is also after
having projected that waypoint from an official geodetic marker deep in the
woods from over 1/4th mile away. How's that for accuracy? :-)

[The next time I saw the neighbors, I said, "Now that was an expensive
lesson for you, wasn't it?" (getting your land surveyed around here costs
about $3,000 to $5,000) They never replied, couldn't even look me in the
eye. The even funnier part is, the highly arrogant (and animal-abusive)
neighbors north of them lost about 50 ft. x 1,320 ft. of their land too,
land they had been plowing for 40 years. Their whole wooden fence running
for 1/4th mile is on the wrong land. All discovered due to my little $100
Legend Cx and my finally taking a stand on what is mine. LOL!]

I strongly suspect, just like cars, cameras, and any other man-made
equipment, there are some lemons and some that are gems coming off the
assembly-line randomly. For the every-now-and-then lemon there must be an
every-now-and-then gem. I think I got one of those gems. It also tracks
indoors just as accurately and under dense tree cover, yet it shouldn't
because it's an older and inexpensive Cx model, not an HCx model.

Small deviations in antenna fabrication dimensions, small deviations in
silicon quality in the circuitry, small deviations in circuit-board
manufacturing (too much flux remaining from overused solvents during
cleaning causing bleed-over, etc.), small deviations in ... whatever. If
they are all close to being out of spec the effect is cumulative. The
converse also being true.

Sometimes it's just a luck of the draw.

I have an old Garmin GPS 12 that I use for Geocaching...
no map or anything, just a blank screen with an arrow pointer.

My mode of operation is to get within range of the cache,
and then take a look at the exact Geocache coords
and use X/Y positioning to zero one aspect of the coords,
and then walk the line till the GPS zeroes on the other coord.

I tend to believe my old GPS 12 -
and use it in this X/Y grid manner...
Really irks me when the cache is not within
say a 10ft circle of the exact coords listed.

That seems fair. We may try that this weekend.

Hoges in WA

was curious -
so I grabbed both my old GPS 12 and my Nuvi 255
walked out to the driveway... clear sky view.
sats locked up - looked at the coords... walked around...

the least significant third digits tracked pretty close...

.992 vs .991
.271 vs .274

That's about as close as we were on the one I mentioned above.
There's another one that I can't find that we're going to test on the
weekend. Can't do it before then because my friend has gone off to
Melbourne for a couple of days.

Hoges in WA