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Garmin Vista Altimiter Calibration--- How?


Team FocalLength

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Bought my Vista last weekend and have logged two cache finds so far this week, now I'm curious about how to calibrate the altimiter on this receiver. How do I go about doing this accurately? The Vista tells me it needs to know its precise elevation to calibrate. Where can I go to do this, or what can I do from home that will be as precise as possible? Are there locations to take your GPSr for calibration/accuracy checks?

 

Any help appreciated,

 

-FocalLength, Annapolis, MD

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Find a benchmark--some are also known as vertical control markers in governmentese, and it should have the elevation (a.k.a. altitude) stamped on it. If you can't find one, find a topo map--most hilltops are marked with their elevation on the map. All you need to do then is go to that hilltop and use the elevation from the map to calibrate your gps-r. Happy cachin'!!! 15T

 

www.1800goguard.com

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The new 2.25 release http://www.garmin.com/products/etrexVista/agree.html for the Vista allows you to set the barometric altimeter value to the satellite determined position with the click of a button. While it's not as accurate as calibrating to a known physical position such as mentioned above, it's a quick and fairly acurate way as long as you have 4 satellites' recceving on the satellite page.

 

Alan

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quote:
Originally posted by MajBach:

I think altimeters in GPSrs are neat. You must have to calibrate them daily though, no?

 

MajBach

_You can't have everything.

where would you put it?_

http://www3.sympatico.ca/majbach/images/1compass.gif


 

Not really ... if your using the GPS altitude ... but like any instrument it's good to verify or calibrate it every once in a while. When backpacking I check it periodically against reference points I've set on my paper map. Not as definitive as a bench mark but close enough for my use. I've gotten in the habit of calibrating the Vista compass ... not only when I change batteries ... but when I start using it for the day.

 

348_2702.gif

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quote:
Originally posted by mwmm15T:

Find a benchmark--some are also known as vertical control markers in governmentese, and it should have the elevation (a.k.a. altitude) stamped on it. If you can't find one, find a topo map--most hilltops are marked with their elevation on the map. All you need to do then is go to that hilltop and use the elevation from the map to calibrate your gps-r. Happy cachin'!!! 15T

 

http://www.1800goguard.com

 

Great advice 15T, thanks. So then... how do I go about finding benchmarks in my area. Is there a resource available on the net to locate benchmarks? I'm just around the corner from the US Naval Academy, I can't help but think that there might be a benchmark around there somewhere...

 

Thanks for the replies guys!

 

-FocalLength

 

null

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Every airport has to know it's elevation. If there's a municipal or county airport nearby, drive out there and use that as a benchmark.

 

Since the altimeter in the Vista is barometric, you will need to do this every time you want it to be accurate, since barometric pressure varies with the weather.

 

Your local airport will have a weather office that will tell you the barometric pressure as well, usually on a taped weather announcement line.

 

Easiest for me is to simply use the GPS altitude to automatically calibrate the altimeter.

 

I suppose the calibrate function steps you through the methods in the order it does because they are of decreasing accuracy. A physical check at a known elevation would be most accurate, knowing the barometric pressure would be second, and the GPS would be last. Isn't GPS altitude a bit less accurate than horizontal? Like 30M or so?

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quote:
I suppose the calibrate function steps you through the methods in the order it does because they are of decreasing accuracy. A physical check at a known elevation would be most accurate, knowing the barometric pressure would be second, and the GPS would be last. Isn't GPS altitude a bit less accurate than horizontal? Like 30M or so?
With quite a few gps-r's on the market, the altitude isn't quite so accurate, since it gets altitude readings from the satellites, but with an eTrex Vista, there is a barometric altimeter in the unit. 15T

 

www.1800goguard.com

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I don't know exactly when that possibility was introduced, but the ability to calibrate the Vista against the GPS altitude is not something that came with firmware release 2.25. It has been there for a while.

 

The reported altitude, by the barometric altimeter, is likely to change often. Especially in areas where the weather changes a lot. It can easily change by 200 feet (60 m) during a single night, for example.

If you aren't away for too long, check the altitude of your house on a topo map. Then use that to calibrate you Vista. You can usually determine the altitude like that within a few meters, which should be good enough for most applications.

 

If you are very cautious about the altitude, think about that if you use your Vista in a car, the altitude depends upon the ventilation fan speed, not to speak about possibly open windows or moon-roofs. This changes the pressure in the cabin. You can try it yourself. Close the windows, and set the fan speed to zero. Check the altimeter. Then set the fan speed to max, and you'll see the altitude decrease, since the pressure inside the car increases. Also, using the GPS in windy conditions may cause dynamic pressures that makes the altimeter a little erratic.

 

Another thing is that squeezing the unit (hard) can also cause changes in the reported altitude.

 

There is no difference in the resulting accuracy if you calibrate to a known altitude or to a known pressure. The actual pressure measured by a Vista can't be calibrated by a user. However, the pressure you calibrate it to is the normalised pressure at that location. This is the pressure that the ambient pressure, at this elevation, would give at sea level. Since the unit measures the ambient pressure, telling it one of the two unknowns (altitude or normalised pressure), it can calculate the other. So, if you know either the altitude or the normalised pressure with the same precision, you'll get the same accuracy to your result.

Normalised pressure is renamed Barometer Pressure in release 2.25.

 

The calibration against the GPS altitude is of course no better than the GPS altitude reading. Still, it provides a more stable reading of the altitude after calibration, than units with GPS altitude only can give you. It's true that the GPS altitude has less precision compared to the position. Thís is due to the geometry of the satellites, which is more favourable in the plane than along the Z axis. If it had been possible to receive signals from satellites at the other side of the earth, it would have been something else.

 

If you enable the Auto Calibration of the altimeter in your Vista, it will calibrate the altimeter to the GPS altitude each time you turn it on, and then keep on checking the two altitudes against each other. Apparently, it makes a large calibration step immediately (if necessary), and the reduces the impact of the GPS altitude gradually when it's used. While this approach gives less accuracy compared to a barometric-only altimeter, it does get away with the problem of long-time drift of the altitude display.

It's possible to turn this automatic calibration off in the Setup/System screen.

 

As long as Garmin refuses to make it possible to select barometric versus GPS altitude for the altitude display, the unit will not report the correct altitude inside a pressurized cabin in an aircraft. The are typically pressurized to something equivalent to 1800 - 2500 meters of altitude.

The 2.25 firmware does allow you to see the GPS altitude from the satellite screen, though.

 

24148_200.jpg

 

Anders

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quote:
Originally posted by Hawk-eye:

 

Not really ... if your using the GPS altitude ... but like any instrument it's good to verify or calibrate it every once in a while. When backpacking I check it periodically against reference points I've set on my paper map


 

Actually, I was referring to the built-in barometric altimeter. Obviously, the computed altitude on a GPSr is often unreliable (as stated so often in these forums), let alone when you can achieve an accurate reading. I like the idea of an independent one that's built into the GPSr that could give you a reading even if you do not have a lock on any satellites. The concern I had was that without knowing the barometric pressure outside, this altimeter is very inaccurate. I was wondering if the unit allowed you to enter atmospheric pressure and it calculated altitude from there or if you calibrated it from a reference point in which it computed atmospheric pressure OR it compared real atmospheric pressure to what the computed altitude was from the satellites.

 

MajBach

You can't have everything.

where would you put it?

1compass.gif

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quote:
Originally posted by Anders:

As far as the Vista is concerned, there are no "ors", only "ands", since it can use any of these three methods.

...


 

icon_biggrin.gif and it does it with or without the fancy geo-suit! All kidding aside ... Anders is right and he is the man when it comes to the Vista!

 

348_2702.gif

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quote:
Originally posted by Anders:

As far as the Vista is concerned, there are no "ors", only "ands", since it can use any of these three methods.

...


 

icon_biggrin.gif and it does it with or without the fancy geo-suit! All kidding aside ... Anders is right and he is the man when it comes to the Vista!

 

348_2702.gif

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I'm getting into this discussion late in the game, but maybe this information will help some. In addition to hilltops (top of Mt. Baker, elevation 10,777 feet, puff, puff--much effort required!) and airports, one can find elevations marked on USGS topographic maps at many road junctions. If an elevation isn't given for the road junction at your location, you can interpolate between contour lines. Just ensure that you know the contour interval--20 feet is common--and estimate the proportion of the scaled distance (SD) between the junction and the next lowest contour line and the total SD between that contour line and the next highest. For example, the SD between your road junction and the 100-foot contour is 0.7 inch and the SD between the 100-foot contour and the 120-foot contour is 1.0 inch; divide 0.7 by 1.0 to obtain 0.7; multiply 20 feet by 0.7 to obtain 14 feet; and add this result to 100 feet to obtain an elevation of 114 feet for your road junction. This might seem more complicated to many than it really is--just do it a few times to learn how simple it really is. Don't worry about whether or not your calculation is accurate enough to set your barometric altimeter as contained in your Garmin eTrex Vista. During many periods in the outdoors barometric fluctuations cause elevation readings to vary minute by minute anyway--precision within 5 or 10 feet of "true elevation" is quite adequate, and should be fine for your purposes unless you're setting "brass cap" benchmarks. FE icon_razz.gificon_razz.gifnull

 

Barrat

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I'm getting into this discussion late in the game, but maybe this information will help some. In addition to hilltops (top of Mt. Baker, elevation 10,777 feet, puff, puff--much effort required!) and airports, one can find elevations marked on USGS topographic maps at many road junctions. If an elevation isn't given for the road junction at your location, you can interpolate between contour lines. Just ensure that you know the contour interval--20 feet is common--and estimate the proportion of the scaled distance (SD) between the junction and the next lowest contour line and the total SD between that contour line and the next highest. For example, the SD between your road junction and the 100-foot contour is 0.7 inch and the SD between the 100-foot contour and the 120-foot contour is 1.0 inch; divide 0.7 by 1.0 to obtain 0.7; multiply 20 feet by 0.7 to obtain 14 feet; and add this result to 100 feet to obtain an elevation of 114 feet for your road junction. This might seem more complicated to many than it really is--just do it a few times to learn how simple it really is. Don't worry about whether or not your calculation is accurate enough to set your barometric altimeter as contained in your Garmin eTrex Vista. During many periods in the outdoors barometric fluctuations cause elevation readings to vary minute by minute anyway--precision within 5 or 10 feet of "true elevation" is quite adequate, and should be fine for your purposes unless you're setting "brass cap" benchmarks. FE icon_razz.gificon_razz.gifnull

 

Barrat

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Also, if there is any new construction in your area, new house, etc..... When they stake out the house for position of the house and elevation on the ground floor, the surveyors will mark on a tree or other immoveable object a benchmark elevation above sea level. If you catch the job while the block layers or workers are on the site, they may be able to tell you where that benchmark is. Believe me, every new house going up will have this marked somewhere.

 

As far as airports, the official elevation of the airport is really just an average since they occupy so much land. If you call a NOAA weather location or an FAA Flight Service Station (FSS) located in the Blue pages. They can tell you the elevation of any airport in the area. They are usually there 24/7.

 

Thinking more on this, you can probably call an engineering firm or surveying company and they can tell you a location of a benchmark they use.

 

"Indecision may or may not be my problem"----J. Buffett

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Hawk-Eye is right, Anders is the Vista man. I enjoy reading fellow Vista users comments. I learn more here than on the Garmin website. The manual is useless since it hasn't been updated since version 2.0 came out. As for the Altimeter I like it it. It's one reason I spent the extra $$. But like you guys said it is after all only relative. If a cold front comes in while your climbing the mountain well it's nice to hit Sat page and get a quick check. Anyone done any tests on Sat vs known airport or benchmark altittudes with sub 20ft Sat accuracy. I think I will try my own experiments tommorow.

 

5399_300.jpg

Gig 'em Aggies

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One thing about hiking on level ground with the eTrex Vista, is that if the elevation suddenly goes way up, you need to seek shelter immediatly, since it could be a tornado. Would be neat if the Vista had an audible alarm.

 

If the Vista had Averaging, that would be one way to auto-correct the elevation reading in the Vista. Would be neat if you could turn on Auto-Averaging on a Vista.

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I'm sitting here, my face getting more and more red every second, from all the compliments I get from you fellow geocachers (or should I say GPS-geeks?). Actually, this color change will influence the next cache search, since by then my geo-suit will be pink, not leopard, I promise!

 

It's just that when I buy something, I want to know everything there is, since if I don't, I should have bought a cheaper thing, with less features.

 

It was the same with my programmable calculators, the old TI 99/4A computer and so on.

 

Thanks again for you appreciation.

24148_200.jpg

 

Anders

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