Vista HCx Altimeter Calibration and Error

[IndianaDan]

The manual for the Garmin Vista HCx is a bit thin on details, but there seem to be quite a few knowledgeable people here. I have a few questions about the barometric altimeter in the Vista HCx:

 

1. Does the unit do any self-calibration? Since it knows the GPS altitude and the error estimate, it would seem to be a simple thing to recalibrate anytime the barometric altimeter gives a reading outside the GPS altitude + error range. I would also imagine that it has to recalibrate itself every couple of hours just due to weather. Anything else would make it less accurate than the GPS altitude, right?

 

2. Is there a way to view the GPS altitude outside of the manual calibration screen? The barometric altimeter is nice, but it's useless if you're in a pressurized vehicle of some sort, like an airplane.

 

3. Is there no way to turn off the barometric altimeter (like with the compass)?

 

4. Which altitude value gets recorded in the tracklog? GPS, or barometric?

[dogwalkers2]

1. It autocalibrates if you have it set to. From Garmin:

"Using the autocalibration feature on the device, the accuracy is +/- 50-125ft (same as GPS elevation). When manually calibrating (autocalibration off) to a known elevation it is approximately +/- 10ft for the first 15 minutes. The unit should be calibrated hourly to maintain accuracy when using manual calibration because of possible preassure changes.

 

If using a locally reported pressure reading to calibrate the device, the accuracy of the altimeter would depend on the time and location of the reading.

 

With WAAS and good reception you can obtain an accuracy of +/- 25-50ft."

 

2. & 3. There doesn't appear to be a way to turn it off. However, the auto calibration mode filters with a significant emphasis on the GPS altitude. The rate of change will be slow, though. I did a test and calibrated with a wacked out pressure. The altitude corrected back to the true altitude at a rate of about 20 feet per minute. I suspect the same thing would happen in an aircraft. Definitely not usable for active flying, but OK if you are just curious and have time to wait (you could help speed it along by entering an approximate altitude in a manual calibration, but set up to auto calibrate). Anyway, GPS altitude is never used for air navigation (see point 1...it is similar for airborne GPS applications as well). The eTrex series were meant for "On the Trail" not "In the Air" so, obviously, not a major design consideration to use just GPS altitude. Personally, I think they did it right, given the other uses for pressure and the use of a blended altitude capability (auto calibration).

 

4. Since "There must be an active track log (not a saved track log or activated TracBack) to see the elevation profile. If the track log has been cleared, there will not be an elevation profile," I would hazard to say that the barometric altitude (as filtered with GPS altitude, if you are set up to auto calibrate) gets recorded.

[Rhialto]

I did a test at 660m and it was right on... see http://forums.Groundspeak.com/GC/index.php?showtopic=171442

I never played with calibration of it.

[Hertzog]

4. Which altitude value gets recorded in the tracklog? GPS, or barometric?

For the sensor models this is "almost" always the barometric value; I think the non-sensor models do record the GPX elevations in the tracklog.

 

I say "almost", because I accidently on purpose discovered that on the 60CSx putting the altimeter in the "fixed elevation" mode caused it to record the GPS elevation instead. This is definitely not documented, and I don't think it was really intended; I live in fear that Garmin will consider this a bug and "fix" it in a firmware update. The 60CS definitely doesn't have this "feature", and it may not have gotten into the HCx models either. To check it on your GPSr, just use the manual calibration function to some large offset, say 1000 feet, then switch between fixed and variable elevation and see what gets recorded in the tracklog.