Garmin Altimeter Questions

I find the altimeter function on my Zuma interesting, especially here in Florida, where a couple of feet can make a significant difference.
My questions are:
1) How accurate is the altimeter?
2) Can it be calibrated ?

Thanks

Ride safe

I don't have a Zumo, but the altimeter on my older Garmin, a 76C, was spot on in most instances where I was close to a sign, indicating the altitude. it worked even in Death Valley, at Badwater, where it is 280 feet BELOW sealevel. it showed a negative value, which surprised me. I think it actually showed 279 feet below sea level.

Calibrated? Not likely. I would guess that the PC Board is potted and not very accessible.

richpeabody said:

1) How accurate is the altimeter?

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There is no altimeter, at least not in the conventional sense. GPS calculates your position based on an imaginary ellipsoid (flattened sphere) that approximates the shape of the earth but doesn't match it exactly. At any given spot, the surface may be above or below where you actually are. The ellipsoid was selected to minimize that as much as possible, but the difference in some places is significant.

Relative to the ellipsoid, the altitude is as accurate as the position and subject to the same kinds of errors. If your receiver thinks is has a fix to within, say, 10 meters, the altitude will be the same.

Some GPS units have barometric altimeters, but I don't believe the Zumo is one of them.

2) Can it be calibrated ?

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Short answer: No.

Longer answer: One of the things NASA has done with your tax dollars is make fairly precise measurements of the actual shape of the earth, which includes mean sea level and altitude of land masses. That data set could be used to correct GPS, but it's large enough that it's not going to be poured into a consumer GPS anytime soon.

--Mark

richpeabody said:

I find the altimeter function on my Zuma interesting, especially here in Florida, where a couple of feet can make a significant difference.

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Some smartphones have a barometer and there are plenty of barometer/altimeter apps at their respective stores. No idea how accurate they are but used along side a GPS they might be useful.

Blrfl said:

There is no altimeter, at least not in the conventional sense. GPS calculates your position based on an imaginary ellipsoid (flattened sphere) that approximates the shape of the earth but doesn't match it exactly. At any given spot, the surface may be above or below where you actually are. The ellipsoid was selected to minimize that as much as possible, but the difference in some places is significant.

Relative to the ellipsoid, the altitude is as accurate as the position and subject to the same kinds of errors. If your receiver thinks is has a fix to within, say, 10 meters, the altitude will be the same.--Mark

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This is more of an academic question. Were you to take a GPS on a boat in a storm with 40' waves, would the altitude vary as you descended into the troughs or is the refresh rate not fast enough?

SMSW said:

Were you to take a GPS on a boat in a storm with 40' waves, would the altitude vary as you descended into the troughs or is the refresh rate not fast enough?

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The practical limit on calculating a position is once every 20 ms or 50 per second, so in theory you'd be able to record the altitude changes if you weren't busy calling Ralph in Europe on the porcelain telephone.

Part of figuring out where you are involves integration of all the data about the signals that came in over the sampling interval. Integration is a lot like taking an average, where the more data you collect, the closer your answer is to reality. Less data collected over less time gets you more answers, but they're less accurate. Even on calm water, there are a bunch of factors that make the position vary over time, and that will add noise to the positions as well.

There are probably better ways to measure how much you're bobbing up and down; the orientation sensor and accelerometer found in most mobile phones come to mind.

--Mark

ST Gui said:

Some smartphones have a barometer and there are plenty of barometer/altimeter apps at their respective stores. No idea how accurate they are but used along side a GPS they might be useful.

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barometric altimeters are generally pretty accurate, its just that 99.99% of the population doesn't know how to use one, and they vary as weather changes.

For automotive/motorcycling applications where you travel hundreds of miles a day, barometric altimeters need a lot of periodic re-calibration to remain accurate. The part that most people don't understand is altimeters are all calibrated to what's called a 'standard atmosphere'. It defines the expected air density and temperature at various altitudes. Problem with that is the actual conditions are very rarely the same as the 'standard atmosphere', so the altimeter reads incorrectly, even though its working exactly as it was designed to work. For example, if you're riding one of the CA passes at 7,500 ft. the standard atmosphere conditions state that the temperature at 7.5k ft. should be right about at freezing, 32F or 0C. When you're riding there the actual temp is probably in the 70-80F range, so the air is far less dense than the altimeter is calibrated for. It will read almost 10% off in that case, because the 80F air is only 90% as dense as 32F air at that pressure.

So when you're riding, you have to mentally calculate the altitude based on how much you know the altimeter will be off, and it varies with altitude and temperature, so its more work than its worth. If you're hiking and things change more slowly, its do-able.

GPS altitude doesn't have any of those problems, but it has the problem Mark mentioned. The people who have posted that they got elevation accuracy to 1m were just coincidence, but its usually good to within 30ft/10m or so on average. For motorcycling you generally don't need surveyor quality readings, if its within 30ft its probably good enough.

dwalby said:

GPS altitude doesn't have any of those problems, but it has the problem Mark mentioned. The people who have posted that they got elevation accuracy to 1m were just coincidence, but its usually good to within 30ft/10m or so on average. For motorcycling you generally don't need surveyor quality readings, if its within 30ft its probably good enough.

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Unless, of course, you are doing Evel Knievel type jumps where a 30 foot uncertainty in altitude might lead to serious conseqences.:bow1:

Blrfl said:

Relative to the ellipsoid, the altitude is as accurate as the position and subject to the same kinds of errors. If your receiver thinks is has a fix to within, say, 10 meters, the altitude will be the same.

--Mark

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This is not true for the following reason. To get a fix, the GPS receiver needs to lock on to signals from at least four satellites, so that the built in computer can solve four simultaneous equations to get x,y,z and t where x = latitude, y = longitude, z = altitude and t = time. (It needs to solve for time because the clock in the receiver is nowhere near as accurate as the atomic clocks in the satellites and the whole system is based on the time-of-flight of a radio signal from satellite to receiver). You generally only get a decent fix in x and y when the satellites are spread around the sky and not too high above the horizon. Unfortunately these are the circumstances where the altitude measurement is at its poorest. I teach students to expect twice the error in altitude that they get in latitude/longitude.
Modern receivers can pick up both the US GPS system and the Russian Glonas satellites. The more range measurements you have to determine the position, the lower the error becomes. Many receivers also average multiple measurements over several seconds to further drive down errors. Even with all this the best accuracy you can expect out of a consumer GPS unit is 3-5m in x and y, 6-10 in z. To test this try leaving the receiver in the same place for 30 minutes or so. Periodic checking will usually show the position move around a few metres back and forth as one satellite goes below the horizon and the receiver picks up the signal from another one rising in another part of the sky.

Sorry for the long post - you just hit my GPSnerd button!

Thanks....
Here's one of the reasons that I asked:
Our condo complex is fairly large and one of the areas is in a Class A flood zone.....meaning, as near as I can figure, an elevation of less that 11 feet. With Irma approaching and a threat of a severe tidal surge, I sought to park a couple of cars on high ground.
I fired up the trusty Zuma ....lo and behold, it read 29 feet! I did notice that it increased a bit when I went to what I perceived as higher ground, and conversely, it decreased on what seemed to be lower areas.
I moved the fleet accordingly to the high spot.
As it turned out , there was no need....a NEGATIVE tidal surge! Followed by a "normal" high tide....all was safe.

richpeabody said:

Thanks....
Here's one of the reasons that I asked:
Our condo complex is fairly large and one of the areas is in a Class A flood zone.....meaning, as near as I can figure, an elevation of less that 11 feet. With Irma approaching and a threat of a severe tidal surge, I sought to park a couple of cars on high ground.
I fired up the trusty Zuma ....lo and behold, it read 29 feet! I did notice that it increased a bit when I went to what I perceived as higher ground, and conversely, it decreased on what seemed to be lower areas.
I moved the fleet accordingly to the high spot.

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here's a better alternative. Its android, but there's probably an Apple version or something similar. Instead of using GPS or altimeter data, this uses altitude data that has been collected by satellite and is usually accurate to within 1meter or so. Its a satellite view like Google Earth, zoom in to the location you want, and then just press on the screen at the desired location and it tells you the elevation for that location.

https://play.google.com/store/apps/details?id=com.peha.elevation&hl=en

Thanks!
I will give that a shot!
Ride safe!

I question it's inaccuracies at times!