Elevation Help Required

[bill5439]

new to this,im unsure if i have to set my gps to sea level first as i seem to be getting wrong heights on my navigation out by 100 ft or more.......do i have to set this first, i have an etrex venture...to sea level to give me accurate readings or is there a quicker way as ive heard this can give a more accurate positioning..any help would be gratefully received and appreciated........ and i wish you all a happy xmas and an even better geocaching new year

[gm100guy]

I sure the information is on this page about that feature.

 

[+stu_and_sarah]

new to this,im unsure if i have to set my gps to sea level first as i seem to be getting wrong heights on my navigation out by 100 ft or more.......do i have to set this first, i have an etrex venture...to sea level to give me accurate readings or is there a quicker way as ive heard this can give a more accurate positioning..any help would be gratefully received and appreciated........ and i wish you all a happy xmas and an even better geocaching new year

I think I'm right in saying the Venture doesn't have a barometric altimeter. That means it calculates its elevation from the satellites alone. This is many times less accurate than the horizontal position, so you really can't rely on it.

 

The units with a barometric altimeter will be much more accurate, but require calibrating regularly (as the air pressure changes) - basically every time you're at a known height.

 

However... I believe, yes, you can set your elevation. If you're at a known elevation, go to the satellite info screen, choose the options menu which contains "New Elevation" and enter your value. This is how the Legend works, hopefully it's the same on a Venture.

 

Cheers,

 

Stu

[+Prime Suspect]

new to this,im unsure if i have to set my gps to sea level first as i seem to be getting wrong heights on my navigation out by 100 ft or more.......do i have to set this first, i have an etrex venture...to sea level to give me accurate readings or is there a quicker way as ive heard this can give a more accurate positioning..any help would be gratefully received and appreciated........ and i wish you all a happy xmas and an even better geocaching new year

The purpose of setting the New Elevation is to give you a more accurate latitude longitude reading at the position you're currently at, since your elevation can have a small effect on calculating your 2D postion. But it's not a true recalibration. It won't help you get a better altitude reading at other times in other places. So in your situation, it serves no purpose. After all, you have to know the elevation to enter it into the GPS, and if that's the case, you don't need the GPS to determine your elevation.

[+EraSeek]

Your GPS elevation reading can be up to about 100' off, depending on the accuracy of your triangulation. I also have a barometric altimeter on my watch. As much as the barometer varies around here, I find my GPS elevation as accurate. If you want better elevation accuracy for position (horizontal as well as elevation) enable your WAAS. Once you agument the sats with this correction, the accuracy of your elevation will be much better.

[tannhausergate]

I'm new to the sport and use a Legend for geocaching. I also have a barometric altimeter watch that I have primarily used to track stats while snowboarding. I've come to respect the altitude estimate of the Legend more than the altimeter on the watch (At least when geocaching. I haven't taken the GPSr to the slopes yet).

 

The barometric altimeter is only as accurate as it's last (and hopefully very recent) calibration and is very weather sensitive. I'd have to say that in my opinion barometric altimeters are more useful for measuring a change in altitude rather than the actual current altitude. Besides, constant calibration can be a real pain.

[+janni93]

Elevation retreived from GPS measurements is tends to be inaccurate since the reference used for this is the geoid (a theoretical system based on gravity), which is not the exact surface of the earth. There are anomalies up to 50mtrs as far as I remember.

 

Also, the satellite geometry on the sky makes it difficult to calculate precise elevation on the signal the GPSr receives, which adds another inaccuracy.

 

Whereas some 30 to 50ft accuracy might be considered OK for the 'flat' position on the earth's surface, such a difference in elevation is pretty much (and most probably even worse).

 

Every half-decent barometric device will give much better accuracy for the height with regular (i.e. daily) calibration - unless the weather is extreme unstable (air pressure rising/falling very fast)

[+rusty_tlc]

For a good explanation of how your GPS calculates your position in space (not "on" the planet, strictley speaking) go to

Trimble's tutorial site. To quote from that site;

 

"Triangulation???

 

We're using the word "triangulation" very loosely here because it's a word most people can understand, but purists would not call what GPS does "triangulation" because no angles are involved. It's really "trilateration." "

[+Stunod]

Elevation retreived from GPS measurements is tends to be inaccurate since the reference used for this is the geoid (a theoretical system based on gravity), which is not the exact surface of the earth. There are anomalies up to 50mtrs as far as I remember.

 

Also, the satellite geometry on the sky makes it difficult to calculate precise elevation on the signal the GPSr receives, which adds another inaccuracy.

 

Whereas some 30 to 50ft accuracy might be considered OK for the 'flat' position on the earth's surface, such a difference in elevation is pretty much (and most probably even worse).

 

Every half-decent barometric device will give much better accuracy for the height with regular (i.e. daily) calibration - unless the weather is extreme unstable (air pressure rising/falling very fast)

I have a MeriPlat and a NAVMAN Palm clip on. While the MeriPlat reads elevation very close to topo maps and benchmarks, the NAVMAN was always about 100' off. I contacted their tech support about it and was told that they use a very simplistic geoid model, which doesn't give very accurite results.

[Kerry.]

Since on a world wide basis ellipsoid-geoid separation can vary up to 50 metres it's certainly a very simplictic model that only interpolates to 100 feet (~33 metres).

 

At 100 odd feet difference it's really a wonder why they would even bother at all as really the height based on their "simplictic" model isn't worth much at all. Why bother at all, why not just provide ellipsoid height and be done with it as then at least ellipsoid height is something that has some meaning albeit not that reliable in the first place.

 

.... Elevation retreived from GPS measurements is tends to be inaccurate since the reference used for this is the geoid (a theoretical system based on gravity), which is not the exact surface of the earth. There are anomalies up to 50mtrs as far as I remember ....

 

"Anomolies" really isn't the best description for ellipsoid-geoid separation distances but for sure these differences can range up to a little more than 50 metres (in the US between about 10 to 50 metres +/- a few) so meaningfull heights (as per maps etc) from GPS do rely on the accuracy of the geoid model.

 

Primary elevations derived using GPS relate to the ellipsoid of the datum reference being used. These are not inaccurate due to the geoid, simply inaccurate being GPS derived heights, inaccurate is depending on equip/methods used of course. The accuracy of the geoid model then has an additional affect on accuracy of the orthometric height but probably not as much as the initial accuracy (inaccuracy) of the GPS ellipsoid height.

 

Elliposid-Geoid models are really much better at giving relative height differences (depending on distance etc) much more so than an absolute height as there are simply to many unknowns to absolutely take all factors into account.

 

But then as per Stunod's example the final height can never be any better (probably worse) then the inaccuracy of the ellipsoid height no matter how good (or bad) the geoid model is.

 

Cheers, Kerry.

Edited December 15, 2003 by Kerry.