View Full Version : Setting the Altimeter on Garmin...

Wise Old Owl
09-12-2011, 22:28
Hate to say it - the worst part of the program is the altitude... it's all over the place! Maybe Mags Can get this right for us... anyway I have twice "tripped up the lies" GPS puts out... Altitude of -6 when you are on the second floor... well you can tell.

For the second year in a row I have "sat" on the beach in Delaware and waited for a while for the GPS to get a clue... then typed in 0000 for the altitude. This time I turned off Auto Calibration, Began auto calibrate and entered the correct alt..

I hope I understand it right I picked sea level between high & low tide. When I got home the Alt appeared right for the second time.....

Hey I realize that this is not a big deal for most of us,,,, but when the unit is reading barometric pressure... its important to know its accurate and right on most things....

09-12-2011, 22:39
Calibrate to a Survey Monument of known Altitude. ie: USGS Monuments in the states.

09-13-2011, 13:47
Airports are good place for calibrating altimiters, pilots frequently reset them on their planes before take offs.

09-13-2011, 15:52
You can get the current barometric pressure readings from METAR data at NOAA's website for various airports worldwide at http://weather.noaa.gov/weather/metar.shtml

The ICAO airport codes can be looked up at http://aviationweather.gov/adds/metars/stations.txt

An explanation of the coded report is at http://www.nws.noaa.gov/tg/code.html

The Old Fhart
09-15-2011, 00:23
If your GPS is using barometric pressure to calculate altitude, then it will change as the barometric pressure changes and there is no way around this. You will have to set the altitude on your GPS at a known benchmark or read the elevation from a topo map when you are at a known location. A barometric altimeter is just a barometer that displays altitude so it's really one instrument and you can switch the units from in-Hg to feet depending on your needs. If a front comes through a .2 in-Hg change will change the altimeter reading about 200 feet. How often you will have to calibrate your altitude will be determined by the stability of the air mass over you. If you're hiking in the mountains you may want to check it several times during the day at known landmarks.

If your GPS uses the satellites for altitude calculations, the earth isn't a perfect sphere and the WGS84 spheroid model can be off in some areas as much as 200 feet but it will not be affected by barometric pressure. ( http://en.wikipedia.org/wiki/World_Geodetic_System )

METAR data is rather cryptic and not that useful to the general public (I used it at the Mt Washington Weather Observatory) and airport data gets less reliable as your distance from the airport increases. If the weather channel has local weather for your city and displays the current barometric pressure that might be adequate.