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Posted by Bill S on April 13, 2008 at 1:11:21 am EDT
In reply to Looking for new GPS Leaning toward eTrex Vista or Legend posted by WISam on April 11, 2008 at 11:35:14 pm EDT

Don't say you were not warned - the description is involved and rather technical. This is a much simplified explanation -

First problem with the barometric altimeter is in common with all barometric altimeters - you have to have a good understanding of how they work. A barometric altimeter does *not* measure the altitude. It measures the local air pressure, then converts that to an altitude corresponding to the local (absolute) air pressure in the ICAO Standard Atmosphere Table, with an offset that is programmed by either the user or in some cases through some automatic process. Problem one is that the real lapse rate (the variation of pressure with altitude) is only rarely the same as the ICAO table gives. Problem two is that the pressure at a given fixed location varies from minute to minute and day to day - a daily cycle plus a variation due to weather changes (Low pressure areas will indicate a higher altitude and high pressure areas will indicate a lower altitude, with variations as much as a couple thousand feet, or more during severe weather). This means you must recalibrate frequently at known locations (preferably surveyed). Problem three is that the terrain causes variations in two ways - because the ground heats up during the day, the ground-level lapse rate (variation of pressure with altitude, mentioned above) is frequently very different from the ICAO values, which were determined in "free air" (intended for use by aircraft), with the other being the effect of wind, most noticeable around narrow passes (lower pressure in the pass due to the venturi effect, meaning the altimeter will show higher altitude. I have seen deviations of several hundred feet in 3000-4000 feet of climb on summer days in the mountains, due to the ground heating, and jumps of a couple hundred feet while crossing a pass in high winds.

That was in general for barometric altimeters - you have to be aware of these effects to take account of them with any barometric altimeter, including those in Garmin's GPSRs.

The Garmin units with built-in barometric altimeters (mostly designated by an "S" in the model name, such as the GPSMAP 60CS and CSx, but also some specific units in the eTrex series, ike the Vistas) have some additional "features" designed in by Garmin. Those units display only the barometric altitude in the data display in each window - you cannot set them to display the GPS-derived altitude. So you have to recalibrate the altimeter fairly frequently. You can display the GPS-derived altitude in some of the units from the satellite screen by pressing "Menu" and scrolling down to "GPS ALTITUDE". However, this is only a small, temporary popup window. You can calibrate from the altitude graphing screen by pressing "MENU" and selecting "CALIBRATE ALTIMETER". You are then presented with 3 successive choices for calibration - "Known altitude"; "Known Barometer" (the "sea level" barometer setting, obtained from the local airport, NOAA Weather Radio, or similar calibrated source); or "Use GPS Altitude". You can also set the altimeter to "Autocalibrate" in the setup menu. In principle, the altimeter is automatically set to the GPS-derived altitude at set intervals. In fact, this only happens if the difference in altitudes is greater than a certain amount, and basically does not happen if you calibrate using a "known altitude" or "known barometer" setting. You can set the "known altitude" off by several thousand feet (higher) and never see it corrected (if you set it lower by more than a couple thousand feet, it will reset to the altitude given by the absolute pressure and ICAO table, which can be off by several hundred feet).

But there is the further peculiarity that I and others have encountered in high latitudes (more than about 70 deg N or S latitude) - since the air is thinner at a given physical altitude in polar regions than at lower latitudes, the Garmin GPSRs with barometric altimeters will refuse the surveyed or GPS-derived altitude calibrations and within 15 sec switch to the barometric altitude. Thus, at High Camp on Mt Vinson in Antarctica, surveyed at 12,139 ft, the 60CSx and Vista insist on displaying 13,720 ft (+/- a bit depending on the weather). I have never been able to get a good explanation from Garmin as to why they put this "feature" into their algorithm, but their basic response is that no more than a percent of their users would ever notice the error.

A further peculiarity is that if you take a waypoint and set it to "average", the altitude that is stored is the GPS-derived altitude, not the barometric altitude, even though the barometric altitude is displayed at the time you click "Mark".

So my advice is to avoid the units with the built-in barometric altimeter unless and until Garmin allows you to set the display to show only the GPS-derived altitude. If you want barometric altitude, you are far better off getting a "wrist-top computer" such as the Suunto watches, or a mechanical altimeter like the old reliable Thommens.

The units that have the barometric altimeter also have a fluxgate compass. This is a real battery-drainer that you can luckily shut off and should leave off most of the time. The compass can be de-calibrated easily by a number of things (like your car's electronic box that controls the engine functions or transmitting on a cell phone too close to the unit). It is rather annoying to have to recalibrate it while wandering in a whiteout - just use an inexpensive baseplate compass. They work better and don't depend on batteries.

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