The Garmin position on GPS and barometric altitude is quite subtle. A mechanical altimeter is very accurate and reliable once it is set at a local elevation. But it will vary slowly over time in altitude reading if QNH (sea level) pressure drifts. Using a mechanical pocket altimeter such as a Thommen, you would normally expect to reset altitide whenever you reach an accurate known altitude (landmark, map reading). With GPS, when you are wandering through valleys, overhangs, canyons or among trees and buildings the GPS altitude flops all over the place. But the mecanical altimeter stays accurate. At this point it gets a bit tricky because the Garmin philosophy, says, Forget the map or the geopgraphical altitude marker; instead, we’ll feed in a GPS altitude once every 15 minutes and we hope it will be a good one. If it isn’t, then for the enxt 15 minutes your absolute altitude will be out by a fixed amount but your CHANGES of altitude will be very good. In other words you have a smoothing mechanism. This is great for hiking and biking but will not be any use in A presuurized aircraft and definitelly should not be relied on in an unpressurized light aircraft

It is easy to see that in rough terrain where the exposure to satellites changes rapidly, GPS altitudes can change iin value and in accuracy moment by moment; but the barometric reading will be very reliable. I can see that this is the kind of thing that would confuse many people if you put it in the instruction manual. This barometric-GPS feature is excellent for moving relatively slowly over rough terrain. It is totally inappropriate for use in planes and not much use in road vehicles. My Oregon has this feature but my vehicle satnav 1340 does not. And I find that my Thommen (mechanical) barometric altimeter and my Oregon agree closely nearly all the time if each is set and used as it should be.

One final point: checking from time to time on how many satellites, and at what strength, are being picked up by your GPS is good practice.

But the barometric altitude is subject to any kind of change in pressure, such as whether it’s in the wind or shielded from the wind. I suppose you could count on it if you ensure it nor the air around it moves. /-: —Jeffrey

I came across this as a long time Garmin user, from an eTrex Legend Cx –> 60 Csx –> Colorado 300 and now since my Colorado just broke (don’t get me started on that) I will probably buy a GPSMAP 62. Ultimately I have the same problem as Mike. As much as the rest of the information is quite true, honestly who cares if the thing is on or not as long as it is rendering the most accurate elevation reasonably possible. Personally I have found it to be no better or worse in normal circumstances, however I travel quite a bit and in a pressurized plane cabin the thing goes completely nuts. The Garmin logarithms can’t seem to deal properly when the GPS and the barometric altimeter are far apart. I have read in the past that this is due to the fact that the system averages the change from the altimeter to the GPS so that it isn’t either one overriding the other, but that the two work in conjunction. However when you are in a plane all this seems to cause is the GPS to spend several hours giving you the wrong altitude and then several more hours giving you the wrong altitude once you are back on the ground. Yes, yes I know you can manually reset it yourself or go to the satellite screen to see the GPS altitude or set it to “variable elevation mode” to have it log the GPS elevation rather than the Altimeter elevation, but then to the whole point of the thread why have it in the first place if I have to do this kind of thing.

Ultimately this brings me to the main point. When evaluating the GPSMAP 62 series Garmin effectively has 2 models.

GPSMAP 62 (basic) and the GPSMAP 62s. At first glance when you look at the products it seems great, the basic 62 has no barometric altimeter, awesome, however Garmin then proceeds to remove several other features which make this almost unpalatable for the price.

Who the hell doesn’t support “Hi-speed USB” in 2011, especially with 1.7 GB of memory. And no expandable SD slot (maybe that is why you don’t need to Hi-speed)? There are a few other features removed as well, but they are of less importance to me personally.

What I would like is a helix antenna version of the eTrex Legend Cx (or HCx for the new ones). All the features except the compass and barometric altimeter. Since this has been an outstanding issue for several years now I won’t hold my breath though.

NOTE: @Jeff, I have never found the eTrex and the 60CSx to be significantly different for reception in most circumstances except if I tried to put them in my pocket. The patch antenna in the eTrex doesn’t seem to deal with that at all, where the Helix antenna on the 60 would loose some accuracy, but still keep a good satellite lock. the III+ definitely has a better antenna than the eTrex so that is probably the issue.

–Toronto, Canada

As far as auto calibrate option, that seems to be self defeating. (If you set the unit based on the GPS number it will be no more accurate than the GPS unit from then on?) Off course there are times when you don’t know the elevation so using the GPS unit to calibrate is useful. The auto part though adds some uncertainty on what it is using. The advantage of the auto part might be to have the unit do it when it has a good signal which you might not notice if doing it manually.

I do have a Thommen barometric altimeter. It usually works very well even when I don’t recalibrate it. It is also quicker to use (no start up time etc. since I leave my GPS off). I bought a used one to replace one I lost, but it appears to be defective so I can’t use it to verify what is happening with the Garmin, which I expect is less accurate anyway.

(The Land Navigation Handbook states ” If you have watched barometers regularly , you’ll seldom have seen movements of more than 1 inch,. . . in 24 hours .. .which translates into about 40 feet (12 meters) elevation difference at 3000 feet (1000 meters) above sea level. . . .On most days the hourly change in barometric pressure . . .will be a good deal less”) Of course this assumes you are on foot and even so it recommends you reset at known elevations which there are usually a number. And of course occasionally there can be fast moving fronts etc.

I can’t say I put either one out a car window, but I have never read that wind is a factor. Not sure why air pressure would change though from a car and not a natural wind.

It is theoretically possible that rapid weather changes could outpace the automatic calibration, but I’ve never seen it happen. The time scale of the automatic calibration is 20 minutes, so the weather would have to change significantly within that time for the errors to become appreciable.

The 60CS remains accurate after a long day of hiking and weather changes becuase, by default, it’s recallibrated via the GPS altitude every 15 minutes. That’s your 15-20 feet of accuracy.

As for how it can change in the wind, since it responds to minute changes in pressure, wind can have a large influence on it. Try holding it out the window of a moving car and turning it this way and that, and watch the altitude display change. I’ve seen it change substantially depending on how I held it in a brisk breeze. Any barometer would act the same way. I just wish that the unit could be set to ignore the barometer and take altitude from the GPS receiver — this would be very convenient on windy days, or on a flight (where the in-cabin pressure simulates much lower altitudes than the 38,000 feet crusing altitude).

BTW, I’ve heard that there are new GPS-related receivers that latch on to a signal much more quickly (1 second vs. 30 seconds). I would like that a lot, so I should investigate if they are used in any hand-held consumer products. I’d appreciate pointers if anyone has any….

Jeffrey

for many decades, mountaineers have use barometric altimers (as well as a compass) for finding their way in the mountains.
Yes – you do have problems with changing weather (moving the unit or wind has no influence- at least from my experience)- on the other hand, a changing barometric pressure can alert you on an incoming bad wetaher front.
From my personal experience, the built in barometric pressure sensor in my 60CS is very accurate – even after a day that involves many thousands feet of altitude change I’m within 20 or 30 feet of the altitude that I can derive from a map – including dramatic weather and temperature changes.
The built in barometer was a major reason for me buying the 60CS because my old GPS relied on GPS data for the altitude and in the mountains you sometimes have problems getting a fix on enough satellites for an accurate altitude measurement.

Hope this helps

Daniel