Still confused about Vista HCx wild altimeter errors

[hwyhobo]

I just got my Vista HCx and noticed that track point elevation was off by a large margin. I started at one point (nice, flat, sizeable spot with no tree cover) and returned to the same point one hour later. The difference on the track for the same point was over 100 ft. My entire hike was barely over 300 ft elevation change. The altimeter has autocalibration set to ON and barometric mode set to "Variable Elevation". I read other posts on this subject, but I am still not quite clear. Since autocalibration appears to be pretty much useless, should I set it to OFF? Is that going to use the barometric altimeter only?

 

What about the other setting? The other option for that setting is "Fixed Elevation".

[dogwalkers2]

If you turned your GPS on at the starting point and then immediately left without doing a manual calibration of the altimeter, then the autocalibration might not have had time to work and the altitude may have been wrong at the start and right at the end. It would take the autocalibration about five minutes to correct a 100 foot error, once you have aquired enough satellites.

 

If you turn off the autocalibration, you need to recalibrate your altimeter every hour to maintain accuracy, due to the air pressure naturally changing. Although this method is more accurate than the autocalibration, you need to know either your height (most likely from a topo) or the sea level pressure (less likely...generally if you are at or close to an airport or weather station) and it is a pain to recalibrate every hour.

 

Otherwise, it appears that you have it set up right for what you want to do with the altimeter: autoclaibration and variable elevation. Just either manually calibrate or wait a few minutes before beginning your hike.

 

The fixed elevation setting is used if you want to track pressure changes (useful for minitoring weather at a base camp) at a certian spot without as much influence of the GPS satellites. Because of the amount of altitude error this would generate, the altitude display is turned off on the trip computer in this mode, so you would not get any track elevation data in your track log. This doesn't sound like what you want to do.

[hwyhobo]

Thanks for the reply! So, it seems if I leave autocalibration ON, but wait for 5 minutes or so after initial calibration, it shouldn't have such wild swings then? I will have to test it, then.

 

Do I need to start tracking immediately after the initial calibration, or can I start a little later? It takes me about 5-6 minutes by car to get to the trailheads. So, if I do the initial calibration at my home, keep tracking OFF, leave the unit ON, drive up to the trailheads, wait 5 minutes at the trailheads, then start tracking ON, that should work? Or should I wait 5 minutes after calibration at my home (outside) but not at the trailheads? Or should I wait at both spots?

 

To be honest, absolutely precision is not my objective (although 100 ft is a bit too much off), but better consistency would be desireable, so my profile doesn't look like I fall off a 100 ft cliff upon return to the start point.

[hwyhobo]

Tested it tonight on a shorter hike (half the distance & elevation change - it was late), but I started and ended it at exactly the same point. This time I did both (just in case ), calibrated & then waited for a while to let autocalibration settle down (with WAAS ON). This time the discrepancy was 3 feet. THREE feet - instead of 100 ft. Holy smoke. Granted, perhaps it was a fluke, but even then the procedure explained by dogwalkers2 sure makes a difference.

 

Thanks!

Edited September 25, 2007 by hwyhobo