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calibrating the altimeter/barometer


jlw82
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Just got my Oregon 450 today, very nice unit. Even if the touchscreen is not iphone standard. ;-)

 

Just a question. How do I calibrate the altimtere/barometer. Lets say I place the gps right above sea level, and calibrate. Will it then be calibrated and show good result?

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Just got my Oregon 450 today, very nice unit. Even if the touchscreen is not iphone standard. ;-)

 

Just a question. How do I calibrate the altimtere/barometer. Lets say I place the gps right above sea level, and calibrate. Will it then be calibrated and show good result?

Close enough. At sea level (pick your favorite ocean), you'll be close enough when you enter 0 feet as the "known" altitude. :) Here in North America, the Pacific side is actually about 20cm higher, then there's tide issues, etc. Were you considering use of the Baltic Sea for your measurement? Edited by ecanderson
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Just got my Oregon 450 today, very nice unit. Even if the touchscreen is not iphone standard. ;-)

 

Just a question. How do I calibrate the altimtere/barometer. Lets say I place the gps right above sea level, and calibrate. Will it then be calibrated and show good result?

I live at 10 feet AMSL and just set my GPSr, a DeLorme PN-40, to that level.

Lasts until another storm, or weather, front comes through, however.

 

Or, I can use the ambient pressure data from the nearest commercial airport which is about eight miles away. It is about 50 feet AMSL, so I can adjust for that.

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Or, I can use the ambient pressure data from the nearest commercial airport which is about eight miles away. It is about 50 feet AMSL, so I can adjust for that.

Ditto here. I've been using the ATIS for a fairly nearby airport for the same purpose when I need baro readings. Of course, they always have to give it in inches, and the Garmin wants millibars...
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If you're not at sea level, how do you find out your current elevation in order to do the calibration?

 

Thanks,

 

Larry

 

Check to see if your county has an on-line GIS system. Most systems have LIDAR elevation data, accurate to within a foot or so. Find a convenient place and calibrate there. For example, the sidewalk in front of my house is at 2244 feet.

 

Dave

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Altitude is one of the toughest values for a GPSr to determine with good accuracy, but it does help if you set the calibration once in a while. Still, don't expect miracles

 

Just got my Oregon 450 today, very nice unit. Even if the touchscreen is not iphone standard. ;-)

.. try dropping the Oregon & iPhone onto a hard surface & let us now which screen survives. Suddenly it could be the iPhone screen that isn't up to Oregon standards :P

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Just visit a mountain that is 67.1 meter high and used for map making. Did zero out the Oregon 450, and then visit another spot I know is 16 meters above sea level and the GPS told me 15 meters, and the barometer told me 16 meters. I had connection to many Egnos satellites at the moment, maybe its there for the GPS data was almost spot on. Must test this out more. :-)

 

http://www.everytrail.com/view_trip.php?tr...4c552e381ddbba1

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I've been using the ATIS for a fairly nearby airport for the same purpose when I need baro readings. Of course, they always have to give it in inches, and the Garmin wants millibars...

Units used for pressure is a user setting (Setup>Units) on the 60/76CSx series. I would expect that it would be on all current Garmin units with barometric altimeters.
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I can use the ambient pressure data from the nearest commercial airport which is about eight miles away. It is about 50 feet AMSL, so I can adjust for that.

When using barometric pressure for calibration, it isn’t necessary or desirable to make any adjustment for difference in elevation between the reporting station and your location. If you do, you will introduce error.

 

The reported barometric pressure is the ambient pressure adjusted to the equivalent pressure at sea level. Sea level adjusted pressure doesn’t vary with elevation, because it’s what?…adjusted to sea level. Barometers at different elevations within a small geographic area will be calibrated with different offsets to yield the same sea level adjusted pressure. The sea level adjusted pressure is relatively constant over a wide area (generally about a 25-mile radius). Under normal conditions, a barometer at your home should report the same reading as the one at the airport only eight miles away. So the calibration pressure you put into your GPS should also be the same as reported at the airport.

 

Your GPS sensor reads the ambient pressure and calculates its elevation based on the difference between the ambient pressure it reads and what you tell it the sea level pressure is. The “adjustment” for the difference in elevation between the reporting station and your location is made by the ambient pressure sensor, because the pressure it senses will be different from what it would read if it were at the reporting station.

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When using barometric pressure for calibration, it isn't necessary or desirable to make any adjustment for difference in elevation between the reporting station and your location. If you do, you will introduce error.

Might help to explain it this way.

 

An altimeter in an aircraft displays feet, but it's really displaying the results (adjusted) of a barometric pressure sensor.

 

When the airport provides a barometric reading, it takes the actual barometric pressure (which varies with weather) and adjusts it to a number that will result in the correct number of feet above sea level for the runway. The rule of thumb (although not perfectly accurate) is 1000' per inch of mercury for these calculations.

 

If the runway is 300 feet above sea level, and the REAL local barometric pressure is 29.50, the tower will report the current barometric pressure as 29.80 (29.50 + (300'/1000)). That way, when you're landing, you don't discover the runway about 300' higher than you expected on a bad IFR day, and wind up pranging your aircraft.

 

When the pilot dials in 29.80 on his altimeter (0.30" higher than the real pressure), he sees 300' on his altimeter when he reaches the runway.

 

[What follows assumes that there aren't major local weather induced differences in the relative pressure where you are vs. the airport or wherever you're getting your baro info. Unless you're next door to the person doing the reporting, don't bother trying to calibrate with a storm front moving right through your area. Our weather here in the Front Range area of Colorado can be so variable over short distances at times that I wouldn't trust a Denver reading 50 miles from home.]

 

If, in the above example, you live on a hill near the reporting airport that is 200' higher (which causes your real baro readings to always be about 0.2 lower than the airport due to your increased elevation), and you plug the 29.80 that is being reported into your Garmin, the difference between the reported 29.80 and the 29.30 (airport real 29.50 - 0.20) where you live will be 0.50, or about 500'... 200' higher than the airport.

Edited by ecanderson
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I can use the ambient pressure data from the nearest commercial airport which is about eight miles away. It is about 50 feet AMSL, so I can adjust for that.

When using barometric pressure for calibration, it isn’t necessary or desirable to make any adjustment for difference in elevation between the reporting station and your location. If you do, you will introduce error.

 

The reported barometric pressure is the ambient pressure adjusted to the equivalent pressure at sea level. Sea level adjusted pressure doesn’t vary with elevation, because it’s what?…adjusted to sea level. Barometers at different elevations within a small geographic area will be calibrated with different offsets to yield the same sea level adjusted pressure. The sea level adjusted pressure is relatively constant over a wide area (generally about a 25-mile radius). Under normal conditions, a barometer at your home should report the same reading as the one at the airport only eight miles away. So the calibration pressure you put into your GPS should also be the same as reported at the airport.

 

Your GPS sensor reads the ambient pressure and calculates its elevation based on the difference between the ambient pressure it reads and what you tell it the sea level pressure is. The “adjustment” for the difference in elevation between the reporting station and your location is made by the ambient pressure sensor, because the pressure it senses will be different from what it would read if it were at the reporting station.

Thanks for the gibberish, I can't be bothered with it. Others will have to make their own judgements regarding its value.

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