Many modern GPS receivers (GPSr) are equipped with a barometric altimeter. A barometric altimeter measures ambient air pressure and compares it with a stored Mean Sea Level (MSL) value to calculate the devices elevation. A GPSr with a barometric altimeter can provide more accurate elevation data (~10 ft) than can be obtained using GPS satellite signals alone (~100 ft).

• Barometric Pressure is the term used to describe Ambient Pressure corrected to Sea Level. Barometric pressure is typically reported in inches of mercury (29.88 inHg) or in millibar (1011.85 mb).
• When a high pressure weather system is present, the ambient pressure increases because the air is denser and therefore heavier. A high pressure system is what pushes rain clouds away.
• When a low pressure weather system is present, the ambient pressure decreases because the air is thinner and therefore lighter. A low pressure system usually indicates clouds are present.

A GPSr equipped with a barometric altimeter knows that when the ambient pressure changes, one of two things have occurred. Either the GPSr has been moved to a different elevation, or the barometric pressure for the current location has changed due to atmospheric conditions (a change in the weather). Unfortunately, the GPSr doesn't know which of these two scenarios has actually occurred.

Calibration Methods:

To calculate accurate elevation values, the GPSr must be calibrated so that it can equate a pressure reading to an elevation. There are four methods available to calibrate a GPSr barometric altimeter:

1. Use a known elevation value when the barometric pressure is unknown.
   
   • This method uses the currently measured ambient pressure for a specified elevation value.
     
     
2. Use a known barometric pressure value when the elevation is unknown.
   
   • This method uses the difference between the currently measured ambient pressure and a specified barometric pressure value.
     
     
3. Use the GPS calculated elevation value when the barometric pressure is unknown.
   
   • This method equates the currently measured ambient pressure with the GPS calculated elevation value.
     
     
4. Allow the GPSr to auto-calibrate the barometric altimeter over time using the GPS calculated elevation value.
   
   • This method uses both ambient pressure and GPS calculated values to determine the current elevation.
     
     

Once the GPSr has been calibrated, changes in atmospheric pressure can be more accurately represented as increases or decreases in elevation. The GPSr can now equate each ambient pressure reading change of 0.01 inHg to ~10 feet of elevation change. However, this is only true until the current weather system changes, as the GPSr is assuming that any and all changes in ambient pressure are a result of a change in elevation. This means any changes in the ambient weather will directly result in an artificial and incorrect indication of elevation change by the GPSr.

Now that we know calibrating our GPSr altimeter using a known pressure or elevation value is only effective for a period of time as long as the ambient weather conditions remain stable, which calibration method is best?

• Method #1 (known elevation) should arguably provide accuracy levels equal to Method #2 (known pressure). However, using a known elevation is preferred here because this value uses a finer scale of measurement. Either way, the selected calibration value should only be entered outdoors, away from any wind, and after the GPSr has been powered on for a period of time. It should be noted that using the GPSr inside any enclosed space (in a building, vehicle, aircraft, etc.) will result in poor altimeter accuracy.

• Method #2 (known pressure) should arguably provide accuracy levels equal to Method #1 (known elevation). However, using a known elevation is preferred here because this value uses a finer scale of measurement. Either way, the selected calibration value should only be entered outdoors, away from any wind, and after the GPSr has been powered on for a period of time. It should be noted that using the GPSr inside any enclosed space (in a building, vehicle, aircraft, etc.) will result in poor altimeter accuracy.

• Method #3 (GPS calculated elevation) can vary wildly in accuracy as it depends solely on a constantly changing GPS satellite constellation configuration which is susceptible to all of the common GPS signal errors (obstructions, atmospheric deflection, signal bounce, etc.). Therefore, this calibration method is the least desirable and should be avoided.

• Method #4 (auto-calibration) is the easiest and most accurate over longer periods of time, distance, and changing weather conditions. This calibration method allows the GPSr to use a combination of the GPS calibrated elevation value (coarse adjustment) and the ambient pressure value (fine adjustment) to determine an accurate and consistent elevation value. While this calibration method is only slightly less accurate than using a manually calibrated reading, the advantage of being assured that any changes in ambient weather conditions are not distorting elevation values over time makes it the most desirable calibration method to use.

Elevation Calibration Tips:

1. The most accurate method to calibrate a GPSr altimeter using a known elevation value involves locating a nearby elevation Benchmark. Look for a Benchmark that has recently been found in good shape and that has an adjusted elevation (very accurate). Check the description for 'Altitude is Adjusted'. Benchmark elevations are very accurate - usually within a few tenths of an inch, which is pretty remarkable considering most were placed in the 20's and 30's.
2. As most consumer grade GPSr are only accurate to ~10 ft of elevation under ideal conditions, using a quality topographic map or even Google Earth to determine the correct elevation for a given location is generally acceptable.
3. Tip: After locating a Benchmark near your home to calibrate your GPSr altimeter, immediately return to your home location and record the indicated elevation value (outdoors), then use this value to calibrate your altimeter each time you leave your home.

Pressure Calibration Tips:

1. When choosing to calibrate your altimeter using a known pressure value, the GPSr expects an elevation adjusted pressure, so be sure to use a sea level adjusted pressure reading (ASL, MSL, or elevation adjusted) obtained from local weather reports, such as the NOAA Aviation Weather Center.
2. Pressures obtained from a home weather station, barometer, or other data source typically will not report elevation or sea level adjusted pressures, which will lead to extreme inaccuracies. For example, if you were located at 5,000 ft elevation, your elevation adjusted pressure might be 30.10 inHg while a barometer would most likely show a local (unadjusted) pressure of 25.10 inHg. Entering this value into your GPSr would result in an elevation error of 5,000 ft!