Garmin GPS Altitude/elevation accuracy(altimeter)

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pfredricks
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Garmin GPS Altitude/elevation accuracy(altimeter)

Post by pfredricks » Jan 15 2004 7:34 pm

ON a hike recently-(I am certain that some of you are aware of this)
someone wondered why their GPS didnt exactly match the map/benchmark readings, thinking that their Garmin should have been pretty accurate.

I found out recently that the Garmin uses barometric pressure not GPS triangulation measurements to determine elevation. SO, when the weather or barometric pressure changes, so would the accuracy of the GPS. I had wondered about that too.

Suunto watches work off of the same principle.

They are pretty accurate nonetheless and while not always accurate on any given day, altitude gain/loss should be nearly dead on unless an EXTREME weather shift occured.

I know that sunnto recommends not changing the factory altimeter settings as they are based on the best possible average. It probably would not be advised to change the factory settings on the GPS with a known altitude either.

Hope this helps!
"I'd feel better if we had some crampons. Oh, what the hell, let's go for it..." — Common climbing last words.

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napalm
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Post by napalm » Jan 15 2004 8:33 pm

Fascinating. I always assumed a GPSr would simply use triangulation to determine altitude as this seems more simple, considering the unit is already receiving satellite signals. It just seems to add another layer of complication to have the unit use a barometer.

Does this hold true across the entire Garmin line, or is it only for specific models?

For instance, I have an eTrex, and the unit is supposedly waterproof. If it's waterproof, shouldn't that make it airtight also? How does it read atmospheric pressure through a waterproof/airtight case?


Aircraft have a facility for calibrating their altimeter to reflect current barometric pressure; I guess it's just too much of a pain in the butt to do it for a GPSr.


In the incident that you spoke of, was the GPSr user directly on the actual benchmark, or just in the same contour line? Where'd you find the information about the Garmins using barometric pressure?

This makes me want to hike up to Superstition Mountain with my eTrex and check the accuracy directly on the benchmark up there.

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pfredricks
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your answer

Post by pfredricks » Jan 15 2004 8:48 pm

go to http://www.garmin.com
go to "FAQ" in the left hand column
type in altimeter

I know suprised me too.

They are only waterproof to a certain standart

Watches are similar
My suunto is waterproof to 50 meters, yet works on pressure sensors as well.
"I'd feel better if we had some crampons. Oh, what the hell, let's go for it..." — Common climbing last words.

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Davis2001r6
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Post by Davis2001r6 » Jan 15 2004 8:49 pm

Not all Garmin products have a barometric altimeter. Only the Geko 301, Vista, and the Summit have a barometer on them to measure altitude. The other Garmin products use the triangulation method to calculate altitude. It is my understanding that the barometric altimeters should actually be more accurate but they can be affected by major changes in weather.

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pfredricks
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garmins

Post by pfredricks » Jan 15 2004 9:20 pm

I just got my info from the garmin website
the product comparison page lists all the outdoor series handhelds as having barometric altimeters
so who knows, but, I know some people were having confusion and questions
maybe that helps, maybe that adds to it
heck, I dunno
"I'd feel better if we had some crampons. Oh, what the hell, let's go for it..." — Common climbing last words.

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napalm
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Post by napalm » Jan 15 2004 9:32 pm

It was also my understanding according to the FAQs that the barometric altimeter was not universal across the product line, but rather that the FAQs were speaking in general terms regarding the GPSr's that did in fact have barometric altimeters.

Also, the product comparision page for the eTrex series shows that only some of the units have barometric altimeters. Apparently units without the barometric altimeter use triangulation to determine elevation/altitude.


http://www.garmin.com/products/comparis ... oorPIC.jpg

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bzachar
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Post by bzachar » Jan 15 2004 9:32 pm

The GPS-derived altitude is unaffected by passing weather systems. Every GPSR provides sat-based altitude.

The other side of the coin:Because of the geometry of the satellites relative to the GPSR (GPS Receiver), the GPS altitude will have more error in it than the lat/lon (2D) fix or barometic altitude.

For those with a math/geometry/navigation background see http://gpsinformation.net/main/altitude.htm

For fun, power-up your GPSR, let it run for 5 minutes with a clear-view of the sky and then place it on the ground (still with a clear view of the sky )and watch the altitude change.

BIll

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pfredricks
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hmmmph

Post by pfredricks » Jan 15 2004 9:46 pm

thanks bill,
that seems to explain to problem either way.
I thought that they had one type of altimeter or the other or none at all.
At anyrate, I am glad to learn of why the flux on the readings.
THanks for 'splaining that to me.
-Pete
"I'd feel better if we had some crampons. Oh, what the hell, let's go for it..." — Common climbing last words.

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Glitter
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Post by Glitter » Jan 17 2004 5:29 pm

I've got a GEKO 301 and so far it has proven to be very accurate. It has always been within a few feet of the the benchmarks I have compared it too, but I haven't had weather affect it yet. Don't know anything about using GPS triangulation but you would think that it would be more accurate than barometric pressure, but I guess not. Either way, I don't need to be perfectly accurate.
Keep on truckin'
-Chris

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Chris86314
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Garmin - Altimeter

Post by Chris86314 » Jan 29 2004 9:49 am

I have an old Garmin III and am upgrading to a Magellan Meridian Platinum (on the way via UPS and delayed do to snow back east) because I wanted the electronic compass and electronic altimeter. Both of these features (in the Magellan) can be switched off at any time and the unit will switch to triangulation for both.
Anyway, I did a lot of research before deciding to go with the Meridian and finally decided to go with it do to the very fact you have been talking about in this thread. It seems that the flaw in the Garmin altimeter is just what you have mentioned, it uses the barometric pressure and certain allogrhythm (sp) for their altimeter to work and this can change drastically with the weather changes (see great reviews on GPS systems at this site, have been using it for years and they are spot on.. http://gpsinformation.net/ All the major GPS web sellers recommend this site for product reviews and they are non bias. See the Garmin GPS Map 76S review for what they say (they say it a lot better than I can).
Magellan uses a totally different (and much more accurate) function for their electronic compass and altimeter and the accuracy (according to the reviews) is dead on. And, both functions can also be switched off to go back to satelite triangulation. Garmin has not changed even with their newest models and according to the reviews is a big flaw in this function of their units with electronic compass and electronic altimeters. The one nice thing about this function in the Garmin units is that at a certain speed (not very fast) their units automatically switch from electronic to the GPS triangulation function.
I have been using my Garmin III hiking for years and have found it to be very accurate but very outdated (especially in the battery eating catagory).. thus the reason it upgrade. I looked at many units (Garmin and Magellan combined) and decided to go with the Meridian unit, even though it is an older model) because according to the reviews it still offers more standard features than Garmin does in their more expensive and newer models.
Here is the excerpt from the review directly from Garmin in answer to their question on this problem (note their statement several times "After the calibration" and "remains stationary after the calibration" (in other words, you have to recalibrate it every time you want an accurate reading)..
"The normalized pressure is only valid immediately after a calibration. In order for us to accurately compute it after a calibration, we would need to have an independent source of elevation. What the unit does subsequent to the calibration is to continue to assume that the unit is remaining stationary at the last calibration elevation, and that any ambient pressure variation that is measured is due to atmospheric pressure change, which causes normalized pressure to change as well.

You should see the altimeter functioning very accurately after you calibrate it, and that is our priority in this product. Normalized pressure was provided so it could be used to allow a user with accurate knowledge of his elevation to compute a normalized or sea level pressure. The normalized pressure that is displayed is NOT the
pressure that is used in the standard atmosphere model to compute elevation. There is no loss of accuracy in the altimeter portion of the unit just because the normalized pressure we display is incorrect. Remember, normalized pressure is based on the last calibration elevation.
With the recent release of National Geographic Topo software for the Mac I will also be plottiing and charting my hikes on my computer to check for accuracy. I have done a couple small hikes since moving to Prescott 3 weeks ago and charted them and it works great.

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