Barometric Pressure and Altimeters
Modern GPS receivers (GPSr) often include a barometric altimeter. Barometric pressure is essentially a measurement of the weight of the air above a given point. When a high pressure weather system is in the area, barometric goes up because the air is more dense or heavier - this is what pushes the rain clouds away. Low barometric pressure usually means more clouds. Barometric pressure is typically reported in inches of Mercury (e.g., 29.92 inHg) or in millibars (e.g., 1013.25 millibars).
A barometric altimeter is tool that measures the amount of air pressure at that location. A GPSr with a barometric altimeter can provide more accurate elevation data (sometimes within 10 feet or so) than it can obtain from using the GPS satellites alone (sometimes within 100 feet or so - yes, elevation accuracy from satellites kinda sucks, and it gets worse as your elevation increases because you're closer to the satellites making it harder to determine your distance from them).
A GPSr with a barometric altimeter knows that if the pressure decreases, that there is less air above it. Thus one of two things has occurred - either the GPSr has moved to a higher elevation OR the natural barometric pressure for that location has decreased due to weather changes. The problem is that the GPSr doesn't know which has occurred.
To get accurate elevation readings, the GPSr must be calibrated so it can equate a pressure reading to an elevation.
There are four ways to calibrate the GPSr barometric altimeter:
1. Enter the KNOWN elevation when your barometric pressure is unknown.
2. Use the GPS-calculated elevation when your barometric pressure is unknown.
3. Enter the ADJUSTED barometric pressure when your elevation is unknown.
4. Let the GPS-calculated elevation help auto-calibrate the barometric altimeter over time.
Method #1 tells the GPSr that the currently measured barometric pressure in the GPSr is what should be expected for that exact elevation.
Method #2 does the same thing, except that it uses the rather inaccurate (+/- a couple hundred feet) GPS-calculated elevation.
Method #3 allows the GPSr to determine the current, accurate elevation by determining the difference between the measured pressure in the unit and the sea-level adjusted pressure you provide.
Once the GPSr has a good idea of what the accurate elevation is for the internally measured pressure, changes in pressure can more accurately be represented as increases or decreases in elevation. For example, a pressure change of .01 inch of mercury as measured by the internal barometer equates to ~10 feet of elevation change.
But, your GPSr assumes that the the only thing that changes pressure is it moving higher or lower - it ignores the fact that weather also affects pressure. Thus, if the atmospheric pressure around you changes, your elevation accuracy will suddenly be out of whack. This means you should only calibrate your altimeter using pressure or elevation if you want increased accuracy over short periods of time (shorter if the weather/pressure changes) and if you'll remain within a small geographic area (because changing locations is more likely to result in an atmospheric pressure change).
So which calibration method is best?
Methods #1 (known elevation) or #3 (known pressure) arguably provide the same level of accuracy, though using a known elevation is typically better because it is a finer value than the measures used for barometric pressure. Either way, the calibration values should only be entered outdoors, out of the wind (which can arguably affect barometer readings), and once your GPSr has been on, immobile, and well established for some time. All GPS altimeters require good GPS reception AND accurate pressure readings. Using the GPS altimeter in your car or indoors will not result in high accuracy - and could result in VERY poor accuracy (e.g., 1000's of feet off).
Letting the GPS auto-calibrate the altimeter is BY FAR the easiest - and by far the most accurate over long periods of time or distance or weather. This method uses the GPS-computed elevation to hone in on a 'best-guess' elevation and then uses the altimeter to help maintain accuracy and consistency of the displayed elevation over time. Most units recalibrate every 15 minutes using this method. Once your GPS location is well established, the accuracy of auto-calibrated altimeter readings are only slightly less accurate than manually calibrated readings. The advantage of auto-calibration is that you can be assured that natural pressure changes are not distorting elevation readings over time.
In short, there really are very few advantages to manual calibration over auto calibration. Perhaps the only notable advantages are increased accuracy within a short period of time after proper calibration and that most manually calibrated GPSr units can provide high accuracy almost immediately after turning them on - you don't have to wait for the unit to establish your position before getting a highly accurate elevation reading (e.g., the unit can read the barometric pressure much faster than it can triangulate your position).
If you calibrate your GPS altimeter with known pressure or known elevation, you must turn off "Auto-calibrate" function in your GPS otherwise it will ditch your entered value and go back to the best-guess GPS elevation in a matter of minutes. Most units prompt you to turn this off after manual calibration. But be sure to turn this function back on later otherwise the reported elevations will likely be WAY off because the pressure will likely have changed.
Some tips on using pressure calibration
If you choose to calibrate using a known pressure value, be sure to use sea-level adjusted pressure readings (sometimes referred to as ASL, MSL, or elevation adjusted). You can get these from local weather reports and from airport METAR reports. METAR reports for your local airport are available here - just find the numbers after the A and put a decimal point in the middle. For example, my local airport METAR contains A3035, so my current sea-level adjusted pressure is 30.35 - or 30.35 inches of mercury. METAR and weather station pressure values are typically accurate for perhaps 100 miles from the reporting station/airport (naturally less if the weather is changing).
Your GPSr expects an elevation adjusted pressure. The pressure can typically be entered in inches of Mercury (inHg) or in millibars.
If you're using a home weather kit, barometer, or get the pressure from another GPS system or weather station data feed, these will typically NOT report elevation or sea-level adjusted pressures. Using these values will cause great inaccuracies - higher inaccuracies the higher your elevation. Because one inch of change in mercury represents ~1000 feet of elevation, if you live at 5000 feet elevation, your elevation adjusted pressure might be 30.10 inches, but a barometer would probably show a local (unadjusted) pressure of 25.10 inches. If you enter 25.10 inches into your GPS, elevations shown on your GPS will be off by 5000 feet!!!
Some tips on using elevation calibration
The optimal method for calibrating using a known elevation is to use an elevation benchmark. Go to http://www.geocaching.com/mark/ and enter your zip code and try to find a benchmark you could use (U.S. only). Be sure to look for one that has recently been found in good shape (has a smiley face icon) and that has an adjusted (e.g., very accurate) elevation (check the description for "Altitude is ADJUSTED"). Benchmark elevations are VERY accurate - usually within a few 1/10s of an inch - pretty remarkable considering most were placed in the 20's and 30's.
Because the GPS unit itself is only accurate to within 10 or so feet of elevation at very best, you may be just as well off using a good topographic map or even Google Earth to determine your location's elevation for calibration. One good method is to use a benchmark initially then use that to determine your home's elevation - then use this elevation to calibrate your unit each time you leave home. Be sure to measure an elevation outdoors - taking it inside or calibrating inside will ruin your accuracy.
GPS altimeters and aircraft altimeters
It is important to note that airplane altimeters and GPS altimeters may vary a lot except when on the ground. One reason is that the airplane altimeter uses the pressure at some nearby airport to provide a basis by which the elevation is determined. This is why pilots are constantly updating the pressure setting on their altimeter - not only to ensure accuracy for when they land or fly over local terrain, but also so that their pressure setting is the same as every other plane's in the immediate vicinity.
When you're flying at 15,000 feet, the pressure will be significantly lower than at the airport below because there is less air above you than the airport (just like the local, unadjusted pressure at Denver is much lower - ~5 inches of mercury lower - than at sea level). But you want all airplanes in the immediate area to have one uniform pressure setting in their altimeters - otherwise you could risk collisions. One plane flying at an indicated altitude of 15,000 feet with a pressure setting of 29.75 and another plane with an indicated altitude of 14,000 feet but a pressure setting of 30.52 will be MUCH closer in ACTUAL altitude than 1000 feet.
Planes flying above 18,000 feet tend to fly faster and thus use a standard pressure setting of 29.92 so that all planes up there are reporting the same indicated altitude and don't have to update their pressure settings every few minutes as the pressure at the ground changes below them. Because of this standard setting, a plane flying at 35,000 feet with a standard pressure setting of 29.92 might actually be flying much higher or much lower than 35,000 feet above sea level. All that pilots care about is that what they think is a certain altitude is the same as what everyone around them thinks. They only care about really accurate altitude when landing.
If you take a GPS on your flight (make sure it's OK with your airline before using it in-flight!), you'll notice that the GPS elevation and the reported airplane altitude may vary a lot. Also note that GPS elevation accuracies decrease slightly as you gain elevation. Oh, and be sure to disable your GPSr barometric altimeter on a pressurized plane or the results will be, well, WAY off.
This post has been edited by apollosmith: 03 January 2010 - 05:27 PM