Re: Air and Water pressure measurements

John Perry wrote:

linnix wrote:
Rich Grise wrote:


But, I'm also thinking; isn't there differential GPS, that can get
within centimeters? How hard would it be to get Z axis information from
their signals?


DGPS can get millimeter precision within small areas. Horizontal, at
least. I know that GPS favors horizontal accuracy over vertical, but I
don't really know how much it can be corrected. You could make use of
it by using two GPS receivers and calibrating them at a reference
location, then moving one around to make your measurements. That's how
public DGPS works.

Using a Garmin C320 and sampling hundred points (while eliminating some
points) , I can position horizontal locations within a meter. So,
given enough samplings, horizontal positioning is not a problem. In
theory, vertical positioning is at best 10% of horizontal. I don't
even bother to find out how to get that from my C320.

But if vertical DGPS is comparable to horizontal DGPS, you can get
sub-meter precision at least. Maybe even millimeter precision. Does
anyone here have enough knowledge to say?

Very roughly, with full view of the sky you can expect vertical accuracy
to be about 2-3 times worse than horizontal - given by the geometry.

If you have a DGPS system which can do sub-cm accuracy, you'll get
sub-3-cm vertical accuracy.

To the OP, please don't expect 1m accuracy by "sampling 100 points and
eliminating some". GPS is not that predictable. The ionospheric errors
have a tendency to 'wander', slowly. In other words, you may have a
whole hour where your handheld is consistently off by 5m or more.

Some handheld GPS devices are cheating - if they think you're not moving,
they "nail down" whatever position they have, to look more accurate. If
you can, switch off filtering. If you can't, power cycle the device for
each measurement.

You can indeed get good positions out of many samples, but you need to
sample over a few days.

Samples can be eliminated if you have access to the residuals from
the position solution.

Kind regards,

Iwo

.

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