Turtle, I haven't found what you are saying at all with my Garmin units, when you pass an elevation sign, the GPS unit is almost always within a few feet of the posted summit sign.
Your use of "at all" combined with "almost always" is puzzling, as "I haven't found... at all" implies
never, and "almost always" implies
not always. In any case, +/- 400 feet hardly mean that it absolutely, positively will be off by 400 feet every time, all the time, no exceptions. I have seen mine read within 5 or 10 feet of a summit sign, and I've also seen mine show more than 300 feet off from a summit sign, including those same variations at the same sign.
I am pretty sure Garmin has some sort of a geoid model loaded, but they don't say which one, likely EGM08 as that is the one that works with the native satellite datum, but really irrelevant as ALL geoid models in the last 20 years have less error than an autonomous GPS position does.
Back in June I sent an e-mail to Garmin Support and specifically asked how they model the difference between geoid and ellipsoid. The reply that I got wasn't all that helpful, and it was 5 paragraphs as opposed to the two paragraphs found in a
July update to their FAQ, but either the reply came from the FAQ, or the reply I was sent was later incorporated into it. The e-mail also noted that the barometric altimeters are accurate to +/- 10 feet, and displays an accuracy of up to the hundredths of an inch of mercury. even after inputting a known altitude or normalize air (despite one being able to infer from the accuracy of the altitude that the barometer would be accurate to one thousandth of an inch of mercury). Garmin does not publish accuracy for the barometric pressure sensor, but they do publish the accuracy of the barometric altimeter
readout. The accuracy is published as 10 feet with proper calibration. I usually get better than 10 feet with the GPSMap, as well as I usually get far better accuracy than 400 feet with the Nuvi.
But I wouldn't use either for a surveying job, and not just because I don't know the first thing about surveying. Or maybe that's the first thing you should know. I dunno.
I have never seen a handheld GPS that has a barometer built in, mine doesn't, it computes an elevation the same as both of my vehicle units.
Garmin has more than 20 handheld units which incorporate a barometric altimeter, including the Montana, eTrex, Oregon, and GPSMap series. I have a GPS 64 series unit.
To say GPS can't produce correct heights is incorrect information.
Yes it is. I never said that, however. I did say that vehicle GPS altitude readings are virtually useless, and I said it because they are not accurate enough to be relied upon. As Garmin will tell you straight up,
"Garmin's devices are designed as recreational GPS devices, as an aid to navigation. They should not be used for any activity requiring precise measurements such as surveying or weather prediction."
The elevation on my Garmin Nuvi is useful to me when crossing a pass or knowing when I reach the forecast freezing elevation, whether I am running on a flat grade, up, down, etc. are all informative uses in a vehicle navigation unit.
I use it for the same things, but I don't for a second believe that if it says I'm at 3252 feet that I think I'm at exactly 3252 feet. I'm going with somewhere in the neighborhood of +/- 200 feet, which is what "virtually useless" means. If it's not accurate, just how useful can it be on which to rely?
