Garmin Etrex 30 Altimeter Malfunction

[+JL_HSTRE]

I have had lots of success with the altimeter on my Etrex 30 in the year I've had it. I even tested it at one of the overlooks on the Blue Ridge Parkway and it matched the elevation listed on the sign there.

 

A couple days ago I had a strange problem occur. I was at Goldhead Branch Ravine in Florida. I checked the elevation at the top of the ravine (about 225 ft) then descended the stairway into the ravine. At the bottom, I checked the elevation again and couldn't get a steady reading. I stood at the same spot at the bottom of the ravine for several minutes and the elevation kept fluctuating between 190 ft and 225 ft the entire time.

 

When I returned to the top of the ravine, the elevation was again a steady 225 ft. The next day, I had no problem measuring elevation at Ravine Gardens while in a ravine - albeit a wider ravine than Goldhead Branch.

 

Has anyone experienced similiar altimeter problems? I assume it must be something about the location since it has worked fine everywhere else; any ideas what?

[+cnschulz]

I have had lots of success with the altimeter on my Etrex 30 in the year I've had it. I even tested it at one of the overlooks on the Blue Ridge Parkway and it matched the elevation listed on the sign there.

 

A couple days ago I had a strange problem occur. I was at Goldhead Branch Ravine in Florida. I checked the elevation at the top of the ravine (about 225 ft) then descended the stairway into the ravine. At the bottom, I checked the elevation again and couldn't get a steady reading. I stood at the same spot at the bottom of the ravine for several minutes and the elevation kept fluctuating between 190 ft and 225 ft the entire time.

 

When I returned to the top of the ravine, the elevation was again a steady 225 ft. The next day, I had no problem measuring elevation at Ravine Gardens while in a ravine - albeit a wider ravine than Goldhead Branch.

 

Has anyone experienced similiar altimeter problems? I assume it must be something about the location since it has worked fine everywhere else; any ideas what?

 

Gday, when you said "ravine" I immediately assumed that a wide view of the sky would not be available there. The etrex 30 has a barometric altimeter but the setting "auto calibration" means that the unit will sample the gps altitude to get an approximate reading. Electronic altimeters are good at measuring difference in altitude but over time (hours) they become unreliable due to many environmental factors.

 

In that ravine you proably could onlt expect +-50m vertical accuracy

 

Hope that helps.

[tr_s]

Some thoughts:

 

1: eTrex 30 GPS+altimeter combo = pretty bad, too simple algorithm, could sure use improvement

2: Ravines = typical place for severe GPS signal distortion (multipath).

3: Altimeter reading on a consumer grade GPS remains, in this day and age, pretty rough no matter what.

[Forkeye]

I would try it somewhere else,mine has always been pretty steady. Was it windy? Make sure auto calibration is off as maybe it is signal distortion and altimeter is set to variable.They are hard to find now,but you could buy a Thommen ,temperature compensated altimeter for 2x what yours cost,they were the gold standard ,accurate to +/- 5-10 meters.The accuracy people are expecting from these small units is abit much.Survey crews still spend thousands for their gear.

[+Redwoods Mtn Biker]

What are your settings?

[+JL_HSTRE]

Auto Calibration = On

Barometer Mode = Variable Elevation

Pressure Trending = Save When Power On

 

So I should turn off Auto Calibration when I experience false altimeter fluctuations like this, which are due to GPS signal problems?

[MtnHermit]

Do you calibrate your altimeter before each use?

 

It's easy for the altimeter to be off 1000-ft because of barometric variations. I always check my topo map for the correct elevation and then adjust as needed. I've never had a GPS to sea level so have no experience, rarely get below 5000 feet. I'm typical +/-20 feet from know elevations.

[+Team CowboyPapa]

I read this thread earlier this morning, but was reluctant to offer some input at the time. I certainly would never post information that I knew at the time to be erroneous. Additionally, I don't like the idea of presenting wrong data mistakenly thinking it was correct. In the past, I was accused of presenting a false opinion in this forum and, consequently, temporarily declined to offer something here. However, I think with posting a disclaimer, one can come to their own conclusions regarding the validity of that which I will post below.

 

DISCLAIMER: I was unfortunately subject to a strenuous disagreement with my opinion regarding a similar subject, determination of elevation, or altitude, several years ago.

http://forums.Groundspeak.com/GC/index.php?showtopic=189245&view=findpost&p=3438512

Please take this into consideration regarding the validity of my following contribution.

 

The effect surrounding vertical, natural surroundings on the accuracy of the horizontal location has been discussed on several occasions herein and on another forum. The consensus is that being at the bottom of a canyon, ravine, or similar is that results in a less accurate indication than being directly above, but in the same general area. Furthermore, if signal blockage due to canyon walls decrement accuracy of longitude and latitude determinations, IM(not so)HO, the determination of elevation, or altitude, will similarly, but not necessarily equally, be decremented.

[John E Cache]

It could be an atmospheric temperature/pressure inversion.

[Grasscatcher]

I read this thread earlier this morning, but was reluctant to offer some input at the time. I certainly would never post information that I knew at the time to be erroneous. Additionally, I don't like the idea of presenting wrong data mistakenly thinking it was correct. In the past, I was accused of presenting a false opinion in this forum and, consequently, temporarily declined to offer something here. However, I think with posting a disclaimer, one can come to their own conclusions regarding the validity of that which I will post below.

 

DISCLAIMER: I was unfortunately subject to a strenuous disagreement with my opinion regarding a similar subject, determination of elevation, or altitude, several years ago.

http://forums.Groundspeak.com/GC/index.php?showtopic=189245&view=findpost&p=3438512

Please take this into consideration regarding the validity of my following contribution.

 

The effect surrounding vertical, natural surroundings on the accuracy of the horizontal location has been discussed on several occasions herein and on another forum. The consensus is that being at the bottom of a canyon, ravine, or similar is that results in a less accurate indication than being directly above, but in the same general area. Furthermore, if signal blockage due to canyon walls decrement accuracy of longitude and latitude determinations, IM(not so)HO, the determination of elevation, or altitude, will similarly, but not necessarily equally, be decremented.

 

.....er....ahhh....what did he say?????

[+Team CowboyPapa]

...

.....er....ahhh....what did he say?????

I can't help those who can't read.

[Grasscatcher]

It's just a fact that Altitude is the least accurate axis of the GPS.

 

It's not rocket science, it's just common sense and physics.

 

GPS determines / calculates horizontal position by trianglutation. That takes (a min of) three signals.

Then there needs to be another signal, preferably from a different angular position (horizon to overhead) to calculate altitude.

 

Think of being directly over a table and then someone asks you how long the legs are....you would have to change your position to be able to tell.

 

Even horizontal position accuracy is affected by (satellite)signal position. Haven't you ever been out with your GPS and notice that your EPE seemingly refuses to get below 100-200ft when you are wanting to log an accurate waypoint? The next time that happens, look at your satellite screen. Very likely, a high percentage of the visible sats are clustered almost directly overhead. instead of being "scattered".

Come back an hour later and conditions (satellite positions) will have changed.

 

That also explains the bad results down in canyons. Blocked signals or signals reflected off of canyon walls result in "bad calculations"...... ie results.

 

Then doesn't it naturally follow that the best GPS results come when the user is "in the open" where the GPS has a clear view of the sky where it can recieve signals from multiple positions in the sky?

Edited December 18, 2012 by Grasscatcher

[+JL_HSTRE]

Point of clarification: the Etrex 30 does have a barometric altimeter and does not solely rely on GPS triangulation.

[Grasscatcher]

If accurate altitude/elevation is important to you then calibrate your altimeter at a known elevation point before you go out. You can determine the elevation at your house by topo map, checking a known elevation point like a Benchmark, or at a nearby airport then return home and see what your reading is. Do this several times and average the results and you'll have a reasonably good guesstimation for a number to set your unit on when you start out.

 

With your unit calibrated when you leave, and set on auto calibration and variable elevation, the GPS elevation data gradually over time corrects any errors it detects in the barometric elevation.

 

Any quick barometric pressure changes will still cause variations between the two (GPS Elev and Baro Elev)

 

Edited to add:

Look at the elevation displayed on your satellite page. I "believe" you'll find that to be the GPS elevation.

Now, look on your altitude/barometer. That is the barometric altitude/elevation which the user can set/calibrate.

Edited December 18, 2012 by Grasscatcher

[John E Cache]

Another guess to be ignored:

Winds funneled down a canyon can lower the pressure do to the Bernoulli effect.

[+Atlas Cached]

Auto Calibration = On

Barometer Mode = Variable Elevation

Pressure Trending = Save When Power On

 

So I should turn off Auto Calibration when I experience false altimeter fluctuations like this, which are due to GPS signal problems?

 

I use [barometer Mode | Fixed Elevation] - I also check/calibrate the altimeter at a location where I know the correct elevation.

 

You will have a lot of error in canyons and ravines for the very reasons others have listed.

[+JL_HSTRE]

How often does the barometer need to be calibrated? I have never calibrated it in the year I've had the GPS, but the one time I tested it (elevation sign on the Blue Ridge Parkway) it was perfect.

[tr_s]

How often does the barometer need to be calibrated? I have never calibrated it in the year I've had the GPS, but the one time I tested it (elevation sign on the Blue Ridge Parkway) it was perfect.

 

I'd say the answer to that one is "When it reads significantly different than a known good reference". Never saw an option to calibrate mine though.

[+Team CowboyPapa]

How often does the barometer need to be calibrated? I have never calibrated it in the year I've had the GPS, but the one time I tested it (elevation sign on the Blue Ridge Parkway) it was perfect.

Regarding recalibrating the barometric functionality, I offer:

We just had a weather front go through with a resultant ambient pressure change equivalent to 300 ft in elevation within a 24 hour period.

 

Consequently, I don't bother with the barometric and just use the GPS derived elevations.

 

(Hopefully the above is readable by most.)