Vista Barometric Altimeter Calibration

[Guest Buck8Point]

The Vista Manual states that when calibrating the Barometric Altimeter, finding a 3rd Order or better Bench Mark will help.

I have searched TOPO's of my area looking for one to no avail.

Does anyone in the Baton Rouge, Louisiana area happen to have the coordinates of a 3rd Order or Better Bench Mark?

[Guest Buck8Point]

Ok....What about in the State of Louisiana??

[Guest Pote]

I don't know about benchmarks, but if you head to the gulf, I'd be pretty confident that the water is within a foot or 2 of -0- feet above sea level. I'd measure it somewhere halfway between high and low tide.

 

Or check with your town's public works or city engineers. They'll be able to provide you a fixed reference point.

 

[This message has been edited by Pote (edited 05 October 2001).]

[Guest Geoffrey]

Ive been calibrating it according to the topo maps loaded in the Vista. I just look at the map page with topo, and estimate what the altitude value I need for calibrating the compass. Then i check the Barometric pressure reading on the GPS to the Weather report on the radio, and it usually pretty close.

[Guest athiker]

Another couple things you may want to try.

1. Go to your local airport. Their altitude is known.

 

2. Use the barometric pressure from you local weather. That is the pressure at sea level..

 

I have the Suunto Vector watch with a Altimiter much like the Altimiter on your Vista. That is how I calibrate it.

 

[Guest martinp13]

Did a quick Google for "3rd Order Bench Mark" and got the following link for Wisconsin 3rd Order Bench Marks: Wisconsin. Keep looking... the data should be out there somewhere.

 

------------------

> Martin

Magellan 330 (1.56/WAAS enabled!)

Don't have time to program and record your shows while geocaching? Get a TiVo !

[Guest Prime Suspect]

The Vista's automatic zone calibration works pretty well. I've never manually adjusted it, and on those occasions where I've been able to check it against a known elevation, it's been correct to within 10'.

[Guest k2dave]

I have the suunto altimeter watch and use weather report barimetric pressure or known altitude. I find it is more useful to just use it for relitive elevation - actually I like the gps altitude feature from the sats - not as accurate but in the ballpark and using it for relitive alt. changes works great - and the best part about it is no calibration.

 

I usually find it accurate to about 100 ft.

[Guest webjester]

I got somewhat late to this thread but you get all you need for calibration here:

http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl

[Guest LarsThorwald]

It's my understanding that the elevation of any town - as listed on the "Welcome To" sign or on a map - is that of your city hall. Luckily, my town's only two miles across, so I can always pass by and give a check if I want to...

[Billy Bunter]

The Vista Manual states that when calibrating the Barometric Altimeter, finding a 3rd Order or better Bench Mark will help.<BR>I have searched TOPO's of my area looking for one to no avail.<BR>Does anyone in the Baton Rouge, Louisiana area happen to have the coordinates of a 3rd Order or Better Bench Mark?

 

Hi,

Goto the satellite page, then press menu, this should bring an option that allows you to get a GPS altitude fix (one off) make sure you have good geometry with the sats.

[+Phipps Family]

I went to my local airport and used its elevation.

[+apersson850]

Learn the elevation of your home turf. Often you start off from there, don't you?

[FiveNines]

The Vista Manual states that when calibrating the Barometric Altimeter, finding a 3rd Order or better Bench Mark will help.<BR>I have searched TOPO's of my area looking for one to no avail.<BR>Does anyone in the Baton Rouge, Louisiana area happen to have the coordinates of a 3rd Order or Better Bench Mark?

 

Go here:

 

http://www.geocaching.com/mark/nearest.aspx?rnd=0.8495004

 

And enter your zip code. Look for any benchmark that has "altitude adjusted." You can also read people's reports of finding/not finding the BMs. Searching for BMs is a side-hobby of geocaching with reports and photos and everything.

 

But since you need to calibrate your altitude about once an hour in real life, this method won't work for you. You really need to find the correct altitude where you start at -- at home. Either check a topo map for your home to get the correct elevation or get the barometric pressure from the National Weather Service for the closest weather station and calibrate using that.

[+julianh]

But since you need to calibrate your altitude about once an hour in real life, this method won't work for you. You really need to find the correct altitude where you start at -- at home. Either check a topo map for your home to get the correct elevation or get the barometric pressure from the National Weather Service for the closest weather station and calibrate using that.

I always calibrate my altimeter when starting out at the beginning of the day - either because I know my home elevation (to within a couple of metres), or else using topo map contours if in unfamiliar territory. If all else fails, you can calibrate using GPS elevation - but make sure you get a good 3D lock first - and your instantaneous elevation can still be out by 20 metres or more!

 

I also have "Auto Calibration" on. - this means you DON'T need to recalibrate every hour or so - the unit does it for you, continuously, "on the fly"!

 

I don't know the exact algorithm, but basically the unit compares the altimeter elevation against the GPS elevation every 15 minutes or so, and adjusts the barometer to compensate for variations in air pressure. The correction is not instantaneous - it is basically following trends, not instantaneous values, so the auto-calibrated barometric elevation is MUCH more stable and accurate than the instantaneous GPS elevation.

 

To prove this works, try deliberately calibrating your altimeter incorrectly by 50 metres, then put it down and watch the reported elevation (after first making sure auto-calibration is on!) - the barometric elevation will trend to the correct elevation within about half an hour (correction speed may vary on different units), and should then oscillate by no more than about 5 metres from the true value, whereas your GPS elevation can easily deviate by 10 to 20 metres or more.

 

Using this approach, my altimeter is good to about +/- 5 metres all day. Make a note of elevation at the end of the day, and you're set to start all over tomorrow!

 

(Above comments apply to a Summit HC, but I think it is pretty much the same approach on all Garmins with barometric altimeters.)

 

Hope this helps!

Edited February 25, 2008 by julianh

[FiveNines]

I don't know the exact algorithm, but basically the unit compares the altimeter elevation against the GPS elevation every 15 minutes or so, and adjusts the barometer to compensate for variations in air pressure. The correction is not instantaneous - it is basically following trends, not instantaneous values, so the auto-calibrated barometric elevation is MUCH more stable and accurate than the instantaneous GPS elevation.

True, but the high accuracy is only if you have a steady ambient air pressure. If it's rising or falling, then it will correct to a false number -- the last one you calibrated for. If you want the highest accuracy -- to a meter or so, then calibrate hourly and record your elevation many times. After a several hours over several days (so as to get a different constellation geometry) you should have your home elevation down to perhaps a foot or so accuracy. From then on you won't need to find a benchmark to calibrate to when starting out on a find -- you can use your home elevation with certainty. I know for sure that the surface of my kitchen table is within a foot of 708 feet above MSL from interpolating from a 1:24K topo quad, checking against a known BM by using the GPSr, and by calibrating it to current air pressure from an NWS station 5 miles away and letting it repeatedly autocalibrate.

[+julianh]

I don't know the exact algorithm, but basically the unit compares the altimeter elevation against the GPS elevation every 15 minutes or so, and adjusts the barometer to compensate for variations in air pressure. The correction is not instantaneous - it is basically following trends, not instantaneous values, so the auto-calibrated barometric elevation is MUCH more stable and accurate than the instantaneous GPS elevation.

True, but the high accuracy is only if you have a steady ambient air pressure. If it's rising or falling, then it will correct to a false number -- the last one you calibrated for. If you want the highest accuracy -- to a meter or so, then calibrate hourly and record your elevation many times. After a several hours over several days (so as to get a different constellation geometry) you should have your home elevation down to perhaps a foot or so accuracy. From then on you won't need to find a benchmark to calibrate to when starting out on a find -- you can use your home elevation with certainty. I know for sure that the surface of my kitchen table is within a foot of 708 feet above MSL from interpolating from a 1:24K topo quad, checking against a known BM by using the GPSr, and by calibrating it to current air pressure from an NWS station 5 miles away and letting it repeatedly autocalibrate.

I don't think that's quite correct.

 

From what I can see, I think that auto-calibration on the current generation Garmins works on the basis that time-averaged GPS elevation can be assumed to oscillate around the true elevation. The GPS elevation is therefore used to "anchor" the barometric elevation, adjusting for changes in air pressure as well as changes in elevation.

 

That is, when set to "Auto-calibrate On", the unit is recalibrating itself every 15 minutes or so, using GPS elevation as the benchmark. The algorithm seems to try to allow for the real-time fluctuations in GPS elevation by calibrating towards the GPS elevation "trend-line", rather than locking on to the actual instantaneous GPS elevation. Whatever the unit is actually doing internally, the effect is that the barometric elevation seems to stay accurate within about 5 metres over the whole day, provided reasonable satellite lock can be maintained most of the time.

 

If by "high accuracy", you mean "actual elevation at a single fixed location to significantly better than 5 metres", then I don't know if this is possible on a Garmin barometric altimeter, unless you are prepared to re-calibrate against a true elevation or true barometric pressure periodically (every hour or so). This isn't practical for most real-world GPS applications.

 

However, if you can live with elevation accuracy of about plus or minus 5 metres as you move around, then my experience is that this can be achieved easily with only a very approximate initial calibration, followed by half an hour or so of auto-calibration, and then keeping auto-calibration on all day.

Edited February 25, 2008 by julianh

[+IVxIV]

The Vista Manual states that when calibrating the Barometric Altimeter, finding a 3rd Order or better Bench Mark will help.<BR>I have searched TOPO's of my area looking for one to no avail.<BR>Does anyone in the Baton Rouge, Louisiana area happen to have the coordinates of a 3rd Order or Better Bench Mark?

 

Hi,

Goto the satellite page, then press menu, this should bring an option that allows you to get a GPS altitude fix (one off) make sure you have good geometry with the sats.

 

Umm,, the OP asked his question 7 years ago(??) I hope he's figured it out by now

[FiveNines]

If by "high accuracy", you mean "actual elevation at a single fixed location to significantly better than 5 metres", then I don't know if this is possible on a Garmin barometric altimeter, unless you are prepared to re-calibrate against a true elevation or true barometric pressure periodically (every hour or so). This isn't practical for most real-world GPS applications.

Perhaps we are discussing different things. I'm trying to give a way to establish a calibration point at home by using various techniques to narrow down the elevation variation so as not to have to find a bench mark to calibrate to whenever you need to calibrate. You are explaining how to do a routine calibration when starting out on a day of geocaching.

[+julianh]

If by "high accuracy", you mean "actual elevation at a single fixed location to significantly better than 5 metres", then I don't know if this is possible on a Garmin barometric altimeter, unless you are prepared to re-calibrate against a true elevation or true barometric pressure periodically (every hour or so). This isn't practical for most real-world GPS applications.

Perhaps we are discussing different things. I'm trying to give a way to establish a calibration point at home by using various techniques to narrow down the elevation variation so as not to have to find a bench mark to calibrate to whenever you need to calibrate. You are explaining how to do a routine calibration when starting out on a day of geocaching.

OK - I agree! That is basically how I found the "exact" elevation of my own home. By leaving the unit monitoring elevation for a few sessions of 6 to 12 hours each, recalibrating the barometer to Bureau of Meteorology figures periodically, I have concluded that my front veranda handrail (which makes a pretty convenient "benchmark", with good satellite visibility) is at 50 metres elevation (plus or minus about 1 metre).

 

(Yes - I'll admit that I'm the kind of guy that likes playing with these toys, to determine scientifically reproducible results with no practical value, just because I can! We are known as "Engineers"! )

 

However, for those who are not quite so anally retentive, but would like to know their elevation with as much accuracy as is reasonable, my point is that by using auto-calibrate, and with a good 3D satellite fix (i.e. put the unit outside, not on the kitchen table, where multi-path errors can give a poor quality 3D fix, even with the high-sensitivity receivers), you can get a 5 metre precision very quickly - within half an hour or so.

[+Prime Suspect]

The Vista Manual states that when calibrating the Barometric Altimeter, finding a 3rd Order or better Bench Mark will help.<BR>I have searched TOPO's of my area looking for one to no avail.<BR>Does anyone in the Baton Rouge, Louisiana area happen to have the coordinates of a 3rd Order or Better Bench Mark?

 

Hi,

Goto the satellite page, then press menu, this should bring an option that allows you to get a GPS altitude fix (one off) make sure you have good geometry with the sats.

You're replying to a question from 2001? Posted by someone who hasn't been on the site in years?

[+julianh]

You're replying to a question from 2001? Posted by someone who hasn't been on the site in years?

No, we're having our own conversation right now, which just happens to have been initiated by a question from seven years ago! (And if the OP happens to be still watching, they might learn something anyway! )