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Cellular forums Home > Archive > Garmin GPS > December 2006 > 60CSx altitude deviations
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60CSx altitude deviations
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| James B. Thies 2006-12-20, 3:33 pm |
| When I calibrate the barometric altimeter on my Garmin 60CSx to a known
altitude at a trailhead and then walk a closed circuit up and down hills
back to the same trailhead in less than an hour, it shows a different
altitude for the trailhead by 20 or 30 feet even on a calm day when there
should be very little change in barometric pressure in one hour. Even when I
calibrate the altimeter and hold the unit at a fixed position on a calm day,
the reported altitude drifts away from the calibrated value by 10 or 15 feet
in 15 minutes. Is this degree of deviation normal for a 60CSx or 76CSx? The
Garmin specifications say the barometric altimeter should be accurate to
within 10 feet and I am wondering whether my unit is falling outside specs.
I would appreciate any insights from more experienced users.
Jim
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| Rudolpho 2006-12-20, 10:33 pm |
| James B. Thies wrote:
> When I calibrate the barometric altimeter on my Garmin 60CSx to a known
> altitude at a trailhead and then walk a closed circuit up and down hills
> back to the same trailhead in less than an hour, it shows a different
> altitude for the trailhead by 20 or 30 feet even on a calm day when there
> should be very little change in barometric pressure in one hour. Even when I
> calibrate the altimeter and hold the unit at a fixed position on a calm day,
> the reported altitude drifts away from the calibrated value by 10 or 15 feet
> in 15 minutes. Is this degree of deviation normal for a 60CSx or 76CSx? The
> Garmin specifications say the barometric altimeter should be accurate to
> within 10 feet and I am wondering whether my unit is falling outside specs.
> I would appreciate any insights from more experienced users.
Even on a calm day you have variation of the barometric pressure.
--
Rudolpho
A program is never finished until the programmer dies.
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James B. Thies wrote:
> When I calibrate the barometric altimeter on my Garmin 60CSx to a known
> altitude at a trailhead and then walk a closed circuit up and down hills
> back to the same trailhead in less than an hour, it shows a different
> altitude for the trailhead by 20 or 30 feet even on a calm day when there
> should be very little change in barometric pressure in one hour. Even when I
> calibrate the altimeter and hold the unit at a fixed position on a calm day,
> the reported altitude drifts away from the calibrated value by 10 or 15 feet
> in 15 minutes. Is this degree of deviation normal for a 60CSx or 76CSx? The
> Garmin specifications say the barometric altimeter should be accurate to
> within 10 feet and I am wondering whether my unit is falling outside specs.
> I would appreciate any insights from more experienced users.
Do you have autocalibration turned on? Autocalibration is good when
you will be going a long time (say, several hours) between
calibrations, but for a period as short as 1 hour it may actually
degrade accuracy if you have initially calibrated to a known elevation.
It could account for the calibration drifting by 10 or 15 feet in as
little as 15 minutes. For more information, see Chris Malcolm's
excellent explanation of autocalibration here:
<http://www.dai.ed.ac.uk/homes/cam/g.../altgraphs.html>
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James B. Thies wrote:
> When I calibrate the barometric altimeter on my Garmin 60CSx to a known
> altitude at a trailhead and then walk a closed circuit up and down hills
> back to the same trailhead in less than an hour, it shows a different
> altitude for the trailhead by 20 or 30 feet even on a calm day when there
> should be very little change in barometric pressure in one hour. Even when I
> calibrate the altimeter and hold the unit at a fixed position on a calm day,
> the reported altitude drifts away from the calibrated value by 10 or 15 feet
> in 15 minutes. Is this degree of deviation normal for a 60CSx or 76CSx? The
> Garmin specifications say the barometric altimeter should be accurate to
> within 10 feet and I am wondering whether my unit is falling outside specs.
> I would appreciate any insights from more experienced users.
> Jim
If you switch on at your trail head and watch your elevation your see
it start at some value and then after 10mins or so it will have
corrected itself to GPS elevation (within error range). If the weather
has changed when you get back it will still be showing GPS elevation.
The barometric sensor only takes total control when you have lost
satellite contact. At all other times there is some algorithm to use
the sensor for short term pressure changes and the GPS for the
long-term. Wind gusts over ridges and through narrow features will
cause short term elevation changes (added to your Total Ascent).
There is no feature in the 60CS series for using barometric as absolute
reference. I regard the calibration options as of very limited use, if
you use any try calibrate to GPS.
It may even be that your fix differs from satellite elevation for some
reason. 10-15ft 3-5m is close to the measurement limit.
Ted
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Ted wrote:
> If you switch on at your trail head and watch your elevation your see
> it start at some value and then after 10mins or so it will have
> corrected itself to GPS elevation (within error range). If the weather
> has changed when you get back it will still be showing GPS elevation.
> The barometric sensor only takes total control when you have lost
> satellite contact. At all other times there is some algorithm to use
> the sensor for short term pressure changes and the GPS for the
> long-term.
That is only true if you have autocalibration turned on. The OP didn't
indicate whether he did or didn't have autocalibration turned on.
> There is no feature in the 60CS series for using barometric as absolute
> reference.
I believe you are mistaken. If autocalibration is not turned on, the
unit will use barometric pressure as its only reference.
>I regard the calibration options as of very limited use, if
> you use any try calibrate to GPS.
The only reason to perform an initial calibration to GPS is if you
don't have any other reference to use, whether you have
autocalibration turned on or off. However, if GPS altitude is
significantly different from the elevation you input for the
calibration, and autocalibration is turned on, the calibration will be
adjusted to the average of the GPS altitude over the course of about an
hour.
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| James B. Thies 2006-12-21, 3:33 pm |
| Roy
Thank you so much for the excellent reference and information. These
motivated me to find more information on the web about the "autocalibration"
function on the 60CSx. I read the owner's manual very carefully, but was
left with the impression that "autocalibrate" only used the GPS determined
altitude to calibrate the barometric altimeter at startup. Of course, that
was wrong. Autocalibrate updates the altimeter (in a weighted moving average
sense) continuously. I see no way I could have learned that from the owner's
manual. How can I learn more about the real functioning of my 60CSx than is
revealed in the owner's manual?? I wish some of you knowledgeable folks
would write a "Supplemental Users Manual" for sale at a "reasonable" profit
for the rest of us. Thank you again for your assistance!
Jim
"Roy" <roybassist@yahoo.com> wrote in message
news:1166676304.928972.11580@80g2000cwy.googlegroups.com...
>
> James B. Thies wrote:
>
> Do you have autocalibration turned on? Autocalibration is good when
> you will be going a long time (say, several hours) between
> calibrations, but for a period as short as 1 hour it may actually
> degrade accuracy if you have initially calibrated to a known elevation.
> It could account for the calibration drifting by 10 or 15 feet in as
> little as 15 minutes. For more information, see Chris Malcolm's
> excellent explanation of autocalibration here:
> <http://www.dai.ed.ac.uk/homes/cam/g.../altgraphs.html>
>
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Roy wrote:
> Ted wrote:
>
> That is only true if you have autocalibration turned on. The OP didn't
> indicate whether he did or didn't have autocalibration turned on.
>
>
> I believe you are mistaken. If autocalibration is not turned on, the
> unit will use barometric pressure as its only reference.
>
>
> The only reason to perform an initial calibration to GPS is if you
> don't have any other reference to use, whether you have
> autocalibration turned on or off. However, if GPS altitude is
> significantly different from the elevation you input for the
> calibration, and autocalibration is turned on, the calibration will be
> adjusted to the average of the GPS altitude over the course of about an
> hour.
I stand corrected, you are of course right. The default for
autocalibration is on and should probably stay there unless you have
very good reason to change it. Only yesterday I switched on at the
previous location and the altitude was -190m in error due to the very
high pressure (and fog) we have had recently had. It took about 20mins
to correct and an offset of that size is extremely unusual. Unless you
are a caver perhaps keep it on and let it fall back to solely
barometric for transient loss of satellite.
Ted
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