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Accuracy of Vista Altimeter?


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quote:
Originally posted by nincehelser:

I would expect that it would change a bit depending on the weather, but I imagine that would be on the scale of inches, correct?


 

If not periodically recalibrated barometric altimeters can vary by hundreds of feet (or even over 1000' in extreme cases) as a result of changing weather conditions.

 

The Vista can either be recalibrated manually when at places with a known altitude or automatically by using GPS altitude measurements to compare with the barometer and averaged over time. Therefore it should be quite accurate in normal operation. But if used in circumstances where GPS reception is blocked and without manual recalibration it could drift considerably due to weather changes.

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While not a conclusive or scientific test by any means, I've found pretty good accuracy with my Vista. I have a survey certificate for my house which shows the elevation at the end of my driveway, and the Vista shows the same elevation there within 1/2 metre (I have it set metric and it only shows whole metres). On the beach it has shown 1-2 metres (at high tide, so I'm above the mean high tide level which is what elevation is usually based on, as far as I know). I've also checked it at a few mountain passes in the Rockies at known elevations (as well as a couple of airports, with published elevations), and it always seems accurate as long as you've had a decent degree of GPS accuracy since turning the unit on. That is, if you turn it on and get 22 metre accuracy, it may not be quite right until you wait for more sat reception to get better GPS accuracy so it can calibrate the barometric altimeter with data it's getting from the GPS altimeter. I have it set for auto calibration, as it seems to calibrate itself as well as I can manually by telling it the air pressure or a known elevation. That said, I haven't taken it through any major weather systems that might confuse its barometric altimeter; I'll have to try that some time while playing with GPS accuracy and see what happens!

 

Hope that helps!

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If I stand on the floor in my house, and hold the Vista in front of me, I'm 146 meters above sea level. This is according to a triangulated point nearby.

 

If I turn the Vista off, or use it with GPS off for a while (a day or so), when the weather changes from high to low pressure, or vice versa, I have been able to see that the house has lifted to 250 meters, or sunk down to some 70 meters.

 

One note about auto-calibration: It starts too early. When you turn your Vista on, it's eager to start the auto-calibration. It seems to commence as soon as the unit has a 3D lock. But the very first lock, with the minimum four satellites required, is usually not too good, especially not in the vertical aspect. Hence, it may very well blow itself from the (in this case correct) 146 meters up to perhaps 190 meters, then gradually, over 30-45 minutes, move down to something that's near to correct again.

 

Two possible remedies: Turn auto-calibration off normally. Enable it when you have a lock at several satellites. Or, after the unit has started to auto-calibrate, calibrate it manually. This seems to override the auto-calibration and put it on the right level again, literally.

 

Garmin's programmer ought to put in a delay there, so that the auto-calibration doesnt' start until a reasonably low HDOP has been obtained. Are you listening today??

 

Anders

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If you enter known barometric readings, it is very accurate (I've frequently compared the Vista altimeter to barometric altimeters in planes).

 

However, it does not take much change in barometric pressure to effect readings. A 1" change (ex. 29.92" to 30.92") is about 1000' change is indicated altitude.

 

The altimeter also does not seem to correct for non-standard temperature and humidity (most barometric altimeters aren't). Usually, these errors are relatively small, but they can account for hundreds of feet in extreme cases.

 

I've always found the auto calibrate to be pretty useless. I usually either calibrate from GPS, a known elevation, or get the current barometric pressure for an area from a FSS or a website like adds.aviationweather.gov. One problem with auto-calibration is altimeter loses some of its usefulness as a weather instrument.

 

I do wish you could chose which alitude information to store in tracks our output NMEA (GPS or altimeter). Actually, maybe you can now, I'm always behind on firmware (if it ain't broke...)

 

-jjf

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According to my experience, and I've tried both versions, calibration manually is better than autocalibration, but auto-calibration is much better than no calibration at all.

 

If you know nothing better, you may just as well use the auto-calibration. You would probably have to calibrate against the GPS altitude anyway.

But when I start out from my home, where I know the elevation, I calibrate according to that.

 

No, you still can't set which altitude to use for anything. Neither the track log nor the elevation plot (well, basically that's the same thing, since it's the track which is plotted).

By the way, after the 10000 trackpoint upgrade, you still can't see the elevation plot of more than the original 3000 points. More doesn't fit into the memory of the unit, so nothing to do about that.

 

Upgrading the firmware is one of the funniest things with these units! Always new and interesting functions as well as bugs to investigate! icon_wink.gif

 

Anders

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