Elevation problem on my garmin

[yvon]

I have a Garmin 60CS and has always worked well.

But since two week I have a problem, I will try to explain it.

The problem is only that the recording of the elevation is wrong and I can see it only on my computer.
I did a hike and it work well for 5h30, it have the right altitude. But suddenly, for an hour the elevation is over 98,000 feet or more and then he returned to normal.

I re-downloaded the latest version of the 60CS, or 4.20 see if it would change someting.
I did a test yesterday and today from home to work, and he did the same thing but not the same value.

Is anyone can help me.
Thank you

 

[Jay H]

There is a very small porthole on the back of my Etrex Vista that if you cover it, completely messes up the barometric altimeter as it needs access to the air pressure. If you cover up this hole, or it loses access (dirt, sand, etc?), your elevation reading will be off.

Doodles and I found this out when hiking Allen in the ADKs, when Doodles put a sticker over it and when he took it off after realizing our elevation was off the charts, the elevation profile showed us taking like a 5000ft drop off a cliff!

So, check that, the hole should be very tiny and in our Etrex, it was in the back, above the battery compartment...

Jay

 

[MichaelJ]

Another way to test this theory is to catch it happening in real-time. When you see it going crazy, select the satellites page, press [menu] and choose [GPS Elevation]. If that value looks somewhat correct, then you know the problem is with the barometric altimeter.

 

[yvon]

There is a very small porthole on the back of my Etrex Vista that if you cover it, completely messes up the barometric altimeter as it needs access to the air pressure. If you cover up this hole, or it loses access (dirt, sand, etc?), your elevation reading will be off.

Doodles and I found this out when hiking Allen in the ADKs, when Doodles put a sticker over it and when he took it off after realizing our elevation was off the charts, the elevation profile showed us taking like a 5000ft drop off a cliff!

So, check that, the hole should be very tiny and in our Etrex, it was in the back, above the battery compartment...

Jay

It is not block Jay and I have a protective cover over it since I buy it.

 

[yvon]

Another way to test this theory is to catch it happening in real-time. When you see it going crazy, select the satellites page, press [menu] and choose [GPS Elevation]. If that value looks somewhat correct, then you know the problem is with the barometric altimeter.

Thank you MichaelJ, I will try to watch this next time. But I never know when this could hapen.

 

[Jay H]

It is not block Jay and I have a protective cover over it since I buy it.

Umm, may I suggest to try it without this protective cover.. it could just be that if it is very tight fitting it might effectively cover this hole.

Jay

 

[yvon]

Umm, may I suggest to try it without this protective cover.. it could just be that if it is very tight fitting it might effectively cover this hole.

Jay

I will try it, but it is a Garmin accessory and I have that for several years and it was ok since I buy my GPS.

 

[Jay H]

OK, I'd also just maybe just try to see if there is any dirt inside the hole, once you locate it. I think it supposed to come with some kind of goretex membrane over it so it keeps water out but allows the air/pressure in..

Jay

 

[Raymond]

This problem reminds me of a question I have had for a while now. If the GPS knows my location on the globe, why doesn’t it know what the elevation or altitude of that point is supposed to be? Sometimes my 60CSx thinks that my driveway is below sea level. Shouldn’t it know better than that?

 

[MichaelJ]

This problem reminds me of a question I have had for a while now. If the GPS knows my location on the globe, why doesn’t it know what the elevation or altitude of that point is supposed to be? Sometimes my 60CSx thinks that my driveway is below sea level. Shouldn’t it know better than that?

No, because even though the maps may have elevation data, how does the GPS know that you're standing on the ground vs. up on some feature (building roof, climbing up a cliff, etc.) vs. up at some altitude in a plane/helicopter/etc.

 

[Kevin Rooney]

This problem reminds me of a question I have had for a while now. If the GPS knows my location on the globe, why doesn’t it know what the elevation or altitude of that point is supposed to be? Sometimes my 60CSx thinks that my driveway is below sea level. Shouldn’t it know better than that?

If your GPS can "see" several satellites, then it should be able to calculate your elevation. I find my 60CSx usually displays a "+/- 25'" or thereabouts if I'm on the East Coast, and a "+/- 12'" or thereabouts if on the West. It tends to lock onto more satellites on the West Coast.

My 60CSx is setup to calculate altitude based upon satellite signals first, and then switch to barometric pressure if signals are lost. Have tried re-calibrating your GPS recently? My suggestion is try that - I think it will reset to a more believable altitude.

I'd have to disagree with Michael's explanation - I think the GPS calculates your altitude based upon 3 dimensions, not 2.

 

[MichaelJ]

Actually, Kevin and I don't disagree.

I was answering the (paraphrased) question, "If the GPS knows where I am on the globe, doesn't it know all the ground elevations based on the topo maps, and can't it just look up my elevation instead of using barometer or calculations that might be extremely wrong? Or at least assert that my driveway can't be below sea level?"

And my response was simply that the GPS has to rely on its own calculations, be they right or wrong, because the GPS doesn't know if you're on the ground or not.

 

[Kevin Rooney]

I understand your point, Michael, and agree with you. The satellites act as a point of reference, not a database repository.

 

[DougPaul]

Whenever a GPS is receiving 4 or more satellites in a decent constellation (places in the sky), it can compute a 3 dimensional location (lat, lon, elev), velocity (3 dimensions), and time (accurate to a few nanoseconds).

The elevation is computed as the distance from the center of the earth and the hight of the local geoid (hight of the local sea level) is subtracted to give the elevation. (One possible source of error is inaccurate geoid models in some GPSes or GPS software versions.)

Since the receiver can only see satellites that are above it, the vertical accuracy is not as good as the horizontal accuracy--the vertical errors are typically 2-3 times as great as the horizontal errors. As others have noted, the GPS does not try to interpret what you are doing (flying, walking, digging a hole, drowning, etc)--it just outputs its best estimate of the altitude.

Occasionally, a GPS will produce a wildly inaccurate location and altitude. Perhaps that is what happened to the OP's unit. There are also 2 solutions for the location based upon 4 satellites. Generally one is obviously impossible (for a terrestrial GPS user) and can be discarded. Perhaps this second solution was chosen and output.

A final possibility is that the barometric altimeter was calibrated incorrectly, malfunctioned, or (as suggested by others) the port was blocked.

Note: the 60CS, 60CSx, and 76CSx output the barometric altitude in all cases except when you specifically request the GPS altitude from the satellite page, which is then shown in a small pop-up window.

Doug

 

[KPMMBM]

thanks Yvon, good topic. so while we are on the subject of funky readings, my 60csx will read incorrect mileage totals while hiking but once the trac is downloaded to my pc, it corrects itself and is usually pretty accurate compared to book/map distances. any ideas?

 

[yvon]

Now I re-download version 4.20 of my 60CS and I have blowed in the opening Barometric altimeeter and I recalibrated my altimeter. And I tested it for the last two days from home to my work, and everything seems normal. I'll make the ultimate test on my next hike.

I wrote at the place who made repairs for Garmin in the Montreal area. And they saw that situation a few times. And normaly with a re-download of the internal program of the unit, have solved the problem. Or it is the Barometric altimeeter who is defective.

Thank you everybody, I will give you a report.

 

[MichaelJ]

Note: the 60CS, 60CSx, and 76CSx output the barometric altitude in all cases except when you specifically request the GPS altitude from the satellite page, which is then shown in a small pop-up window.

Which elevation is it using, GPS or barometer, in the track points, either in the regular tracks or the background GPX file written to the SD card?

 

[DougPaul]

Which elevation is it using, GPS or barometer, in the track points, either in the regular tracks or the background GPX file written to the SD card?

It is my understanding that the barometric altitude is used in both kinds of track.

You can check it pretty easily by:
* turn off auto-calibration of the altimeter
* manually calibrate it to something way off the current altitude
* record a minute or so of track, and while recording the track check the GPS altitude (to make sure it is very distinguishable from the barometric altitude)
* check the altitudes in your tracks
* (don't forget to restore the auto-calibration)

* And, of course, let us know what you find...

Doug

 

[yvon]

It is my understanding that the barometric altitude is used in both kinds of track.

You can check it pretty easily by:
* turn off auto-calibration of the altimeter
* manually calibrate it to something way off the current altitude
* record a minute or so of track, and while recording the track check the GPS altitude (to make sure it is very distinguishable from the barometric altitude)
* check the altitudes in your tracks
* (don't forget to restore the auto-calibration)

* And, of course, let us know what you find...

Doug

I just make a test in the street near my house. I set the altimeter at 10000ft and I turn off auto-calibration of the altimeter.
During my walk I looked on my trip computer page and it was always around 10000ft. And I looked also at the altitude (using the menu) on the satellite page, and it was between 60 and 75ft. And in my tracklog it was around 10000ft.

So, what is your verdict, doctor.

 

[DougPaul]

I just make a test in the street near my house. I set the altimeter at 10000ft and I turn off auto-calibration of the altimeter.
During my walk I looked on my trip computer page and it was always around 10000ft. And I looked also at the altitude (using the menu) on the satellite page, and it was between 60 and 75ft. And in my tracklog it was around 10000ft.

So, what is your verdict, doctor.

Somehow I rather doubt that anyone needs me to analyze this one... The barometric altitude is recorded in the tracklog.

Garmin has been asked a number of times to include a menu item allow the user to choose the barometric altitude or the GPS altitude for the displays and tracklogs, but they have steadfastly stuck to the barometric altitude. (The only way seems to be to buy 2 GPSes--one with the barometric sensor (eg 60CSx or 62s) and one without (eg 60Cx or 62)). The most obvious place where this shows up is in a pressurized aircraft where the cabin pressure is held at ~7K ft when the aircraft goes higher. (We should also note that by law aircraft fly by pressure altitude, not the true altitude above sea level so a pilot cannot use GPS altitude to control the aircraft.)

Doug