Garmin GPS - Barometric altimeter and elevation accuracy a bit of data

Author

Barometric altimeter and elevation accuracy a bit of data

Tom H.

2008-01-15, 10:33 pm

I recently got a GPSMap60CSx which has a barometric altimeter which the unit
uses to aid in altitude determination. From the best I have been able to
tell, in the absence of manually calibrating the altimeter with known
elevation, the receiver compares the barometric data with the gps elevation
data over some period of time and thereby derives a barometric calibration.
The result is supposed to be improved elevation accuracy and reduced scatter
in elevation data.

On a recent trip, the jeep trail I was on came within a few feet of a USGS
vertical control station, so I did a little comparison between my Geko 201
which I placed on the benchmark and the 60CSx a few feet away on the
windshield of my Jeep. The benchmark was at 1685 feet (NAVD88). Using
tracklog data from the two gps units I found the following:

The Geko gave an average height of 1668 ft; an error of -17 ft with a
standard deviation of 8 feet using 7 datapoints.
The 60CSx recorded 14 datapoints that gave an average elevation of 1678 feet
after correcting for the height above the benchmark for a -7 ft error and a
standard deviation of 2 ft.

WAAS was off on both receivers. I attempted to use WAAS on the Geko, but
was unable to receive the correction signal.

This is only one very simplistic experiment, but it may give some idea of
the usefulness of having the barometric altimeter. I only thought to do
this analysis after the fact, or I might have taken more data.

--
Tom
http://home.att.net/~tbharvey/

Happy Trails

2008-01-15, 10:33 pm

On Wed, 16 Jan 2008 00:32:35 GMT, "Tom H."
<tom_reader@att.nospam*.net> wrote:

>This is only one very simplistic experiment, but it may give some idea of
>the usefulness of having the barometric altimeter. I only thought to do
>this analysis after the fact, or I might have taken more data.

You also might have thought to use the benchmark to calibrate the
altimeter after you finished farting around.

Tom H.

2008-01-15, 10:33 pm

>
>
> You also might have thought to use the benchmark to calibrate the
> altimeter after you finished farting around.

Well yes, that's right, but when I was there I was really mostly interested
in horizontal accuracy, but I did note at the time how close the reading on
the 60CSx was to the elevation stamped on the benchmark. I figured that
when I got home I could search the NGS database and get all the data with
updated postion data, recovery dates etc. As it turns out, the benchmark
appears to only serve vertical control purposes and is not in the NGS online
database. Most vertical benchmarks are not in that database, and I had to
email USGS to ask for the precise data, and all they sent back was the
height data. That was when I decided to do the height accuracy comparison.
As a general rule I don't think much about elevation data for the things I
do with my gps. So, I keep learning more about benchmarks and datums,
geoids etc.

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