Vista Altimeter

[The Way Finders]

I'm from Texas, so I haven't paid much attention to the altimeter on my Vista HCx until last week. I did some hiking in the Rockies and got interested in tracking my elevation.

 

My altimeter, without calibrating, was over a thousand feet off when using just the GPS elevation, even though I was tracking several satellites. I calibrated to a known elevation (9405ft) and began my 3 mile round trip hike with about 400 ft of elevation change at the halfway point. When I returned to my starting point, my altimeter showed 9475ft...doing the math, that's 70 ft off!

 

I had similar or worse results in different areas around Rocky Mtn Nat'l Park over the next few days.

 

I've been very happy with my Vista's lateral accuracy, so I was really surprised that the altimetry is so poor. Am I being too picky? Are there tips or tricks to the altimeter setup? Anyone else having this issue with the Vista?

 

Thanks

 

PD

[+coggins]

My altimeter, without calibrating, was over a thousand feet off when using just the GPS elevation, even though I was tracking several satellites.

You mention "without calibrating" um... lets start there. You have to calibrate it for it to work.

I calibrated to a known elevation (9405ft) and began my 3 mile round trip hike with about 400 ft of elevation change at the halfway point. When I returned to my starting point, my altimeter showed 9475ft...doing the math, that's 70 ft off!

This is a rookie mistake, during your 3 mile round trip, the air pressure is sure to have changed throwing off your calibration. This is one of the reasons pilots always recalibrate their altimeters before landing, even when returning to the same airport they took off from. For it to work, as the air pressure changes, you would have to recal the unit, generally this is done with either the contours on a TOPO map or some people have the unit set up to recal automatically from the gps derived elevation. Edited August 10, 2009 by coggins

[+julianh]

My altimeter, without calibrating, was over a thousand feet off when using just the GPS elevation, even though I was tracking several satellites.

You mention "without calibrating" um... lets start there. You have to calibrate it for it to work.

I calibrated to a known elevation (9405ft) and began my 3 mile round trip hike with about 400 ft of elevation change at the halfway point. When I returned to my starting point, my altimeter showed 9475ft...doing the math, that's 70 ft off!

This is a rookie mistake, during your 3 mile round trip, the air pressure is sure to have changed throwing off your calibration. This is one of the reasons pilots always recalibrate their altimeters before landing, even when returning to the same airport they took off from. For it to work, as the air pressure changes, you would have to recal the unit, generally this is done with either the contours on a TOPO map or some people have the unit set up to recal automatically from the gps derived elevation.

My advice: For general hiking use you should ALWAYS have the altimeter set to "Auto-Calibration: On", and ALWAYS have the Barometer Mode set to "Variable Elevation". Finally, ALWAYS manually calibrate your altimeter at the start of each day, and leave your GPSr turned on all day. If you don't know your current elevation, and don't have a good topo map, just get a good 3D lock, then use your GPS elevation to calibrate the altimeter.

 

Set up properly in this way, the auto-calibration routine should keep your elevation accurate to about plus or minus 5 metres (or better) all day, even taking into account significant barometer pressure changes as the weather turns nasty, etc.

 

As long as auto-calibration is enabled, even if you forget to calibrate at the start of the day, you will usually find the unit has self-corrected within half an hour or so, and will then maintain good elevation accuracy for the rest of the day. To have seen an error of 70 feet at the end of the day strongly suggests to me that you don't have auto-calibration activated, so it won't self-correct for sea-level barometer pressure changes.

 

Hope this helps!

[The Way Finders]

Coggins-

 

Thanks for the reply. I don't think I said what I meant to say. I calibrated using the GPS elevation prior to seeing an elevation sign. I had calibrated the GPS without a known elevation or barometer setting. I was surprised that the GPS elevation was so far off when I saw the sign.

 

I don't think the barometer is the culprit. The altimeter setting would have to have dropped .07 inches of mercury in the hour I was hiking. That is a fairly rapid rate of change for the weather (or lack thereof) we had that day.

 

I assumed that the Vista uses only triangulation to determine elevation. Is this not true? Is there a barometric sensor in the unit?

 

Julian-

 

Thank you, too. I'll try your recommendations next time.

 

PD

[+julianh]

Coggins-

 

Thanks for the reply. I don't think I said what I meant to say. I calibrated using the GPS elevation prior to seeing an elevation sign. I had calibrated the GPS without a known elevation or barometer setting. I was surprised that the GPS elevation was so far off when I saw the sign.

 

I don't think the barometer is the culprit. The altimeter setting would have to have dropped .07 inches of mercury in the hour I was hiking. That is a fairly rapid rate of change for the weather (or lack thereof) we had that day.

 

I assumed that the Vista uses only triangulation to determine elevation. Is this not true? Is there a barometric sensor in the unit?

 

Julian-

 

Thank you, too. I'll try your recommendations next time.

 

PD

.07 inches of mercury = 2.37 hPa change - significant, but not exceptional in my experience. Note that your local air pressure is also affected by wind speed, air temperature etc. Turn on auto-calibration, and your unit will manage all of these factors for you.

 

Yes, the Vista HCx has a barometric altmeter - but it has to be calibrated to be useful. Auto-calibration is far and away the best option for hiking use etc.

 

Hope this helps!