Garmin eTrex Vista HCx Altimeter

[+A.T.Hiker]

I just bought this model GPS and really love it. It's nice and compact for hiking and the barometric altimeter is excellent for reading total ascent and descent. I do notice the unit wanders a bit, and over a 20-30 mile car trip, will begin to differ from known elevations. I know you can simply recalibrate to the GPS elevation, or topo map elevation, but I see this autocalibration option, and I'm wondering, how does it work? Does the unit automatically recalibrate itself after a known distance / time?

[+coggins]

I just bought this model GPS and really love it. It's nice and compact for hiking and the barometric altimeter is excellent for reading total ascent and descent. I do notice the unit wanders a bit, and over a 20-30 mile car trip, will begin to differ from known elevations. I know you can simply recalibrate to the GPS elevation, or topo map elevation, but I see this autocalibration option, and I'm wondering, how does it work? Does the unit automatically recalibrate itself after a known distance / time?

It will recal the unit to the GPS elevation. Your unit in wandering because it measures air pressure, and that is changing as you drive. To see this in action, place it on the dash board of your car with the heater on and watch your reading, then open the window and watch what happens.

[+Barrettg]

I just bought this model GPS and really love it. It's nice and compact for hiking and the barometric altimeter is excellent for reading total ascent and descent. I do notice the unit wanders a bit, and over a 20-30 mile car trip, will begin to differ from known elevations. I know you can simply recalibrate to the GPS elevation, or topo map elevation, but I see this autocalibration option, and I'm wondering, how does it work? Does the unit automatically recalibrate itself after a known distance / time?

 

Team R2, Coggins stole a little bit of my thunder but............... Given that your Vista Hcx is a barometric altimeter, it relies on ambient, external conditons. Riding inside of a car is different than being on the ground outside of the car.

 

If you want to autocalibrate your altimer and barometer, it will take about 30 minutes, standing or sitting still, outdoors.

 

The wandering that you refer to could be the result of a number of factors, all of which would be humanly induced, ie., while riding in the car.

[+julianh]

I just bought this model GPS and really love it. It's nice and compact for hiking and the barometric altimeter is excellent for reading total ascent and descent. I do notice the unit wanders a bit, and over a 20-30 mile car trip, will begin to differ from known elevations. I know you can simply recalibrate to the GPS elevation, or topo map elevation, but I see this autocalibration option, and I'm wondering, how does it work? Does the unit automatically recalibrate itself after a known distance / time?

Just to add my $0.02 worth - I have described the auto-calibration process as I understand it here:

http://forums.Groundspeak.com/GC/index.php...p;#entry3337672

 

I find I can maintain around 5 metre vertical accuracy all day, provided you do a basic calibration at the start of the day, and can maintain a reasonable GPS "fix" for the whole day.

 

You can get apprent height fluctuations of 5 or 10 metres when the GPSR is in a moving car - I give my take on it here:

http://forums.Groundspeak.com/GC/index.php...p;#entry3275375

 

Hope this helps!

Edited March 11, 2008 by julianh

[Hertzog]

I just bought this model GPS and really love it. It's nice and compact for hiking and the barometric altimeter is excellent for reading total ascent and descent. I do notice the unit wanders a bit, and over a 20-30 mile car trip, will begin to differ from known elevations. I know you can simply recalibrate to the GPS elevation, or topo map elevation, but I see this autocalibration option, and I'm wondering, how does it work? Does the unit automatically recalibrate itself after a known distance / time?

When you are in the autocalibration mode the GPSr increments the measured elevation toward the GPS elevation by a small amount each second. The effect is that the elevation output will approach the GPS elevation exponentially with a time constant of about 30 minutes. You can see this by calibrating the altitude by a large offset, say 1000 feet, to create a "step function" and watching the elevation return exponentially to the correct value. There is also a faster "catchup" mode in which the time constant is reduced to about 30 seconds or so. The unit will enter this mode when it feels it has lost track for a while and regained it (I don't recall the exact conditions in which it does this).

 

Hope this edit isn't too confusing; I tried to edit my original message but accidently replied to it, then hopefully backed out of the reply and did an edit. If you see only one message from me, then it worked

Edited March 11, 2008 by Hertzog

[Hertzog]

I just bought this model GPS and really love it. It's nice and compact for hiking and the barometric altimeter is excellent for reading total ascent and descent. I do notice the unit wanders a bit, and over a 20-30 mile car trip, will begin to differ from known elevations. I know you can simply recalibrate to the GPS elevation, or topo map elevation, but I see this autocalibration option, and I'm wondering, how does it work? Does the unit automatically recalibrate itself after a known distance / time?

When you are in the autocalibration mode the GPSr increments the measured elevation toward the GPS elevation by a small amount each second. The effect is that the elevation output will approach the GPS elevation exponentially with a time constant of about 30 minutes. You can see this by calibrating the altitude by a large offset, say 1000 feet, to create a "step function" and watching the elevation return exponentially to the correct value. There is also a faster "catchup" mode in which the time constant is reduced to about 30 seconds or so. The unit will enter this mode when it feels it has lost track for a while and regained it (I don't recall the exact conditions in which it does this).

 

Sorry for the multiple post (see explanation on my previous post).

Edited March 11, 2008 by Hertzog

[+julianh]

When you are in the autocalibration mode the GPSr increments the measured elevation toward the GPS elevation by a small amount each second. The effect is that the elevation output will approach the GPS elevation exponentially with a time constant of about 30 minutes. You can see this by calibrating the altitude by a large offset, say 1000 feet, to create a "step function" and watching the elevation return exponentially to the correct value. There is also a faster "catchup" mode in which the time constant is reduced to about 30 seconds or so. The unit will enter this mode when it feels it has lost track for a while and regained it (I don't recall the exact conditions in which it does this).

I am sure the basic process of auto-calibration is basically as you have described it, but I think the algorithm may have been significantly refined in the newer (high sensitivity) models.

 

On my old B&W Vista, it worked pretty much as you described it, and the barometric elevation would pretty much track with changes in the reported GPS elevation. I found that it was not uncommon for GPS elevation to fluctuate by 20 to 50 metres over an hour or two, and the auto-calibrated barometric altimeter elevation would do likewise. That is, the barometric altimeter didn’t add a great deal of value compared to the reported GPS elevation, other than giving you a continuous elevation reading, even if the 3D GPS fix was a bit “flaky”.

 

My new Summit HC is rather different in its response. It seems to be much more heavily “damped” in the way it tracks against the GPS elevation. Now, even if the GPS elevation fluctuates significantly over short durations, the auto-calibrated barometric altimeter is much slower to adjust the barometer pressure. Accordingly, I find that provided I give the unit a reasonable calibration at the start of the day, I can maintain about 5-metre vertical accuracy almost all of the time; 10-metres absolute worst case. I was never able to get this level of consistency with my old Vista.

 

Maybe they just modified the algorithm to limit the rate of change of the calculated sea-level barometer pressure (which is the basis of the whole calibration)? The rate of change of barometer pressure which would be required to generate an elevation fluctuation of 20 to 50 metres in an hour is pretty extreme. Just limiting the maximum rate of change of barometer pressure to something a bit more feasible would have the effect of better damping the auto-calibration process.

 

Of course, the reported GPS elevation is also a lot more stable on the Summit HC compared to the B&W Vista, as the high-sensitivity receiver will keep a good 3D fix under much worse reception conditions, and this probably helps in the whole auto-calibration process too.

 

Hope this helps!

Edited March 11, 2008 by julianh

[Hertzog]

 

I am sure the basic process of auto-calibration is basically as you have described it, but I think the algorithm may have been significantly refined in the newer (high sensitivity) models.

 

On my old B&W Vista, ...

You could be right; I started looking at this originally on the B&W Vista and continued on the 60CS and 60CSx, but haven't any experience with later models. It's always possible that they have also changed it recently in the 60CSx firmware; when I have a chance I'll look at your post in more detail and compare that with the 60CSx.