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Thermometer
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Pear Head
Past MnGCA President


Joined: 04 Apr 2004

Posts: 5711

PostPosted: Thu Apr 24, 2008 11:33 am    Post subject: Reply with quote

Marsha and Silent Bob wrote:
Pear Head wrote:
On units without a barometer it does use a satellite fix to determine altitude, although this method is actually less accurate than a pressure fix (that's why those with the "S" model of Garmins paid more for the pressure sensor).

If you ever plot your tracks with altitude from a GPS that uses the sats for altitude, you will notice large (100s of feet) fluctuations in altitude over the course of several miles.


Personally I've never found the altitude functions of GPS units to be accurate, whether it's determined by satellites or pressure. Same goes for the magnetic compasses.

I paid an additional $10 to get the "S" unit when I bought mine. It was a gamble that the compass features had been improved from earlier units that I had tried. I wasn't impressed and rarely turn it on.
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msteffel
Geocacher


Joined: 15 Jun 2007

Posts: 415

PostPosted: Thu Apr 24, 2008 2:22 pm    Post subject: Reply with quote

Sorry, I know I'm getting off the thermometer topic, but it kind migrated toward the accuracy of altitude in GPSs...

One attempt at a solution to getting an accurate altitude is motionbased's Gravity. http://wiki.motionbased.com/mb/Gravity It cross references the horizontal position (latitude/longitude) provided by the GPS with an elevation that has been acquired by professional survey.

Granted this is "after the fact" [it takes the .gpx file and goes from there] and cause it's following a survey it wont help if you were off the ground in a plane/helicopter/etc when you took the readings. But it supposedly more accurate, especially if your GPS doesn't have a Barometric Altimeter http://wiki.motionbased.com/mb/Barometric_Altimeter
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Arcticabn
Past MnGCA Board


Joined: 30 Nov 2003

Posts: 1846

PostPosted: Thu Apr 24, 2008 3:16 pm    Post subject: Reply with quote

Both of the last two posters are correct. My point is that even with the Csx if you turn off the barometer you can still get altitude off the sat fix. If you are interested in a baro altitude fix, you need to calibrate it that day and not turn it off. You have to calibrated it near the location you intend on being.
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spinowner
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Joined: 25 Nov 2004

Posts: 588

PostPosted: Thu Apr 24, 2008 8:41 pm    Post subject: Reply with quote

And if you are in a changing weather pattern (in other words when the barometric pressure is rising or falling) you may need to calibrate more than once a day.
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