Re: GPSMap 60cs Altimeter calibration


dold@xxxxxxxxxxxxxxxxxxxxxxxx wrote:
> Hmmm. Interesting theory, but I can't quite subscribe to that one.

Not sure if you mean the whole thing or just the bit about pushing the
buttons changing the reading. You can easily verify that applying
pressure to the receiver case affects the altimeter reading as I
described. You can do this anywhere, since you don't even need to have
a satellite lock or have the altimeter calibrated. Turn on your unit
and turn GPS reception off. Go to a page where you have the altitude
displayed and watch it for a few seconds to get an idea of the range in
which it's fluctuating. (Mine fluctuates within a range of a few feet
even in steady-state conditions; for example, in my basement with very
little air movement.) Once you have established the general range of
elevation displayed, put your thumbs on the case and while watching the
display, press quickly, firmly and maintain the pressure. You'll see
an immediate reduction in the elevation (I get about 15 feet); but as
you maintain the pressure, you'll see the elevation return to its
previous reading. This takes only about 1 second. If you then quickly
remove the pressure, you'll see the elevation rise above the previous
reading, and return to it in about 1 second.

Since the elevation quickly returns to its previous reading after
applying or releasing the thumb pressure, I theorize that the pressure
in the case is equalizing through a small vent. This is obviously more
extreme than the pressure that is applied when pressing buttons; but it
shows that applying pressure to the unit does momentarily change the
elevation reading. Since pressing buttons doesn't put as much pressure
on the unit as the experiment just described, you wouldn't see as great
an effect. Maybe more like 5 feet ;-)

If you have other problems with my theory, I'd genuinely like to know
what they are. If I've overlooked something or there's something wrong
with my reasoning, I want to know. Otherwise, I may go on believing
something erroneous; and that, for me, would be the worst.

> There were five satellites, at least. I have the "color" satellite display
> turned on, and I don't know what the different colors are for, but it
> looked like a pretty good field. I think the EPE was +/- 18, but I didn't
> make real note of it. WAAS was turned off.

The number is important, but it is also important that they are not
more or less lined up in a straight line, as happens on occasion. But
if this were the case, you would probably have had an EPE of much more
than 18 feet. The colors don't mean anything. It's just a way to
relate the satellites you see on the sky view to the signal strength
bars.

> There are no buildings within several hundred feet. There is a north-south
> four foot high chain link fence about five feet west. There is a hill to
> the southwest, but no portion of it is above 20 degrees from here. Pretty
> flat to the other directions.

Wonder if the fence could reflect signals, causing multipath error.

> The one I saved before the altitude calibration
> is at 984 feet, and the one after the calibaration (to 965) is at 963, so
> it came back down a couple of feet while it was averaging, after me first
> seeing a climb of a few feet.

John Tyson's reply is relevant to this. It was starting to adjust
the elevation to the GPS elevation, which was almost certainly
different from 965. That doesn't mean GPS altitude is more accurate.
It may have been 940, but you didn't average long enough to get
there.

You probably know this, but just in case you don't: to see the
current GPS elevation on your receiver, on the satellite page, press
menu, select "GPS Elevation". The value displayed is a
"snapshot"; it won't update while on the display.

.

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