Elevation Spikes in Expedition Mode [Bug 51]

Discussion related to the Garmin GPSMAP 66s/st GPSr
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GPSrChive
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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CheminerWill wrote: Wed May 08, 2019 5:15 am
gpsrchive wrote: Wed May 08, 2019 3:41 am There you go. The GPSMAP 66 is auto calibrating 'Once' per power cycle, when the unit is first powered up. If you had performed a manual calibration at the point where you were starting the second half of the track recording, or performed a manual calibration at the exact same starting point from the first half, then moved to the second half before recording data, the elevation profiles likely would have been very similar.
I can do that but if it is calibrating once upon starting up, should it not be correct for that whole power cycle? So if I power on the unit and do the first 1/2 of the track, it should be calibrated and correct on the elevation. If a week later I then power up and it does a calibration again, why would it be off from the first half? I must be missing how this works, but if it calibrates automatically each time it is powered on, why would that be a problem?
Imagine the first time you went out you powered it up at the trail head and it performed it's one time calibration there.

But maybe the next time you went out you power it up in the vehicle getting to the trail head, and it is a hot day out and you have the windows up and the A/C on full blast....

Or, imagine each visit uses a different group of satellites, different days, different times, and one track log has many overhead satellites while the next has the majority of satellites closer to the horizon....

Or, maybe on Day one the sun is shining and the air is warm and dry, but on a follow up visit the clouds have moved in, the air is cool and damp...

So many variables for what really is an amateur hobbyist piece of equipment. These things are not professional surveyor grade scientific instruments.

If one wanted to make multiple tracks near the same location over multiple visits, one would be best served by choosing a common point near where the track log recordings were being done, and manually calibrating the unit to the same elevation for that point each time. Then the resulting recordings would all be centered off that one common point... a "Benchmark" if you will.... (wait, that sounds familiar)

8^)
CheminerWill
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Re: Elevation Spikes in Expedition Mode [Bug 51]

Unread post by CheminerWill »

gpsrchive wrote: Wed May 08, 2019 12:57 pm
Imagine the first time you went out you powered it up at the trail head and it performed it's one time calibration there.

But maybe the next time you went out you power it up in the vehicle getting to the trail head, and it is a hot day out and you have the windows up and the A/C on full blast....

Or, imagine each visit uses a different group of satellites, different days, different times, and one track log has many overhead satellites while the next has the majority of satellites closer to the horizon....

Or, maybe on Day one the sun is shining and the air is warm and dry, but on a follow up visit the clouds have moved in, the air is cool and damp...

So many variables for what really is an amateur hobbyist piece of equipment. These things are not professional surveyor grade scientific instruments.

If one wanted to make multiple tracks near the same location over multiple visits, one would be best served by choosing a common point near where the track log recordings were being done, and manually calibrating the unit to the same elevation for that point each time. Then the resulting recordings would all be centered off that one common point... a "Benchmark" if you will.... (wait, that sounds familiar)

I understand that you are saying these things are toys and we really should not count on the elevation being accurate with these units. I was thinking it was at least as accurate as the GPS app I have on my phone but I guess not. In my case the inaccuracy I have seen between tracks has come when indeed the unit has been powered up in the same parking lot each time. Not as in your example of: "in the vehicle getting to the trail head, and it is a hot day out and you have the windows up and the A/C on full blast....."

But because each time has been several days to a week apart I suppose there would be different satellites. Still I would not think that would cause one elevation to show on the track as 3,800' and then 4 days later the same location to show 4,600'.
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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CheminerWill wrote: Wed May 08, 2019 6:16 pm But because each time has been several days to a week apart I suppose there would be different satellites. Still I would not think that would cause one elevation to show on the track as 3,800' and then 4 days later the same location to show 4,600'.
No, of course not, that's why we need to determine what other mitigating factors were affecting your GPSMAP 66 those days.

Many other variables play small roles, like temperature, humidity, current weather patterns...

First question, what is the correct elevation for that location? And from what source?

I actually check my GPSMAP regularly at the same location (home - easy to repeat), and it is always within ~50 feet of what is believed to be the correct elevation for this location, using atmospheric pressure to determine elevation. When using satellite only, they are usually about ~100 feet apart, give or take. (I have both displayed on one of the Trip Computer pages along with atmospheric pressure, barometric pressure, temperature, etc.)

If you can find an easy to access or common location to test, as track logs do not record all of this information, you could frequently stop by and take a screen capture of a trip computer page showing all these data fields, as well as battery level, GPS Signal level, date, time and location (coordinates).

This might make an interesting story over a series of screen captures at the same location, once a day, or every other day, after a week or two (or more) of recording information.

Also, do not allow any firmware or GPS software updates during this period as that would require starting the test again fresh. And I suspect you will be seeing another update for the GPSMAP 66 very, very soon.....
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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Posted as Bug 51 in Common Issues!
CheminerWill
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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gpsrchive wrote: Thu May 09, 2019 1:03 am need to determine what other mitigating factors were affecting your GPSMAP 66 those days.
I will pay closer attention. The first time I saw this I just made an mental note that it seemed odd. The next time I then went back to some saved tracks in Basecamp and looked at the elevation profiles. On the couple that I had recorded at two different times, I could easily see the change in elevation at the junction of the two different tracks. It may not be easy for me to detail as the few times when I checked elevation on my unit when standing at ground zero of a known or signed elevation, it was accurate.
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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The Satellite page shows the GPS Elevation always, so you can compare your Barometric elevation reading with the Satellite page for a basic reference! Of course, the GPS elevation is not always very precise either.
rogerm
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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I presume the spikes in expedition mode are due to expedition mode using GPS altitude exclusively while when not in expedition mode it's using, possibly horribly miscalibrated, barometric altitude. At least my altimeter appeared horribly miscalibrated by an amount that was comparable in size to the spikes.

Which brings me to the question as to what auto calibration does (if anything). If it's set to auto-calibrate once per power cycle, if I power it off and on, shouldn't the GPS elevation and elevation readings be about the same? Mine differ by 110m. If I set it to calibrate continuously they still differ by 110m and don't appear to change.

(this is all with firmware revision 3.00)

I powered up my 62s and deliberately miscalibrated its altimeter so it showed the same as my 66s (110m too high). Both are set to continuous calibration. The 62s is steadily dropping and is now 30m below the 66s which has remained at the same wrong elevation for at least half an hour and shown no signs of doing anything.
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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rogerm wrote: Fri May 10, 2019 11:59 pm Which brings me to the question as to what auto calibration does (if anything). If it's set to auto-calibrate once per power cycle, if I power it off and on, shouldn't the GPS elevation and elevation readings be about the same?
No.

Barometric Elevation and GPS elevation will almost never agree.

One is based on atmospheric pressure while the other depends heavily on satellite placement.
rogerm wrote: Fri May 10, 2019 11:59 pm Mine differ by 110m. If I set it to calibrate continuously they still differ by 110m and don't appear to change.

(this is all with firmware revision 3.00)

I powered up my 62s and deliberately miscalibrated its altimeter so it showed the same as my 66s (110m too high). Both are set to continuous calibration. The 62s is steadily dropping and is now 30m below the 66s which has remained at the same wrong elevation for at least half an hour and shown no signs of doing anything.
I'll do some testing with my unit....
rogerm
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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gpsrchive wrote: Sat May 11, 2019 2:26 am Barometric Elevation and GPS elevation will almost never agree.
But surely it's not barometric elevation. It's an elevation that is computed from a a number of things including barometric pressure and (if you're using continuous calibration) GPS altitude. I think the whole point is that they want to agree on average. Assuming you don't usually have an accurate, independent measure for how high you are the only thing you have to go on is GPS elevation.
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Re: Elevation Spikes in Expedition Mode [Bug 51]

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rogerm wrote: Sat May 11, 2019 2:54 am
gpsrchive wrote: Sat May 11, 2019 2:26 am Barometric Elevation and GPS elevation will almost never agree.
But surely it's not barometric elevation. It's an elevation that is computed from a a number of things including barometric pressure and (if you're using continuous calibration) GPS altitude. I think the whole point is that they want to agree on average. Assuming you don't usually have an accurate, independent measure for how high you are the only thing you have to go on is GPS elevation.
Elevation = "The distance to your current location from sea level, based on ambient pressure."

GPS Elevation = "The altitude of your current location using GPS."
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