Garmin Barometric Altimeters

[+Rubberhead]

One more thing, as GPS units get more accurate, the GPS altimeter's accuracy gets better. It's to the point, that the barometric altimeter is mostly useful as a barometer or to track altitude when the unit isn't reading satellites.

[+Team CowboyPapa]

Oh yeah, we had a low tide this AM about 8AM PDT. I guess I'll walk my dogs down there to check it out....back later.

OK, back now.

 

I was at a somewhat inland tidal pool, so there were no waves but it is directly connected to the ocean.

 

The low tide, 0.3', was actually at 9AM and I was down there about 10:25AM. With a high tide of 3.7' projected for 2PM, the water level was about 1 - 2' at that time. I was standing with my feet about 2' above the water and holding my GPSr about waist high, or about 5' above the current, observed SL. My GPSr displayed a GPS derived elevation of 0 - 1' while the autocalibrated barometrically derived value toggled between 4 and 6'.

 

I can live with that.

[sparksd]

One more thing, as GPS units get more accurate, the GPS altimeter's accuracy gets better. It's to the point, that the barometric altimeter is mostly useful as a barometer or to track altitude when the unit isn't reading satellites.

 

GPS altitude accuracy has improved tremendously in the last few years. Even after SA was turned off, for a long time altitude accuracy was horrendous and I always had one of my baro altimeters handy when out in the mountains and never used GPS altitude. I'd been away from playing with my GPS units for a few years until I recently bought the 62st and I've been astounded by the improvements in GPS altitude accuracy. Newer (and more) satellites; wide adoption of WAAS; improved receivers, antennas, and more powerful processing chips in handheld units; and likely improvements in s/w have all been contributors. GPS altitude has actually become usable!

 

Dave

[+khumbu_calypso]

I hunted down a few more benchmarks yesterday and at all three the elevation number reported by my 62s was never more than 2m off from the Canadian Government published elevation. Two of the three were only 1m off but I could easily say that was due to rounding and the exact position of my 62s which due to obstructions around the benchmark disk could not be placed exactly in the same vertical plane as the disk.

 

Still I haven't fully pieced together in my mind how the 62s arrives at that elevation number. I really wish the owner's manual was a bit more explicit. Is there a good tutorial on this somewhere? Is it using satellites to calculate the elevation, the barometer, or a combinbation of both? Is the barometer reading corrected for altitude based on satellite measured altitude? So if I see a rise in pressure that is strictly due to changing weather and not a drop in altitude? Noob questions I know.

Edited June 8, 2011 by khumbu_calypso

[+julianh]

I've found humidity affects mine, especially at sea level. I'll set mine at the landing (sea level) and be, just like you said, 30-40 feet below sea level once I'm on the water.

Hmmmm - I've never seen that - on the open (sea) water, my Summit HC always reads around 0 m - 3 m or so, which is about right for the tidal range around here.

 

What sort of boat do you have - a power boat? As I noted above, air speed affects local pressure (do a Google for "Bernoulli"); if you are moving quickly, you can get an error of several metres, depending on whether your unit is in an area of high or low local pressure zone - this will depend on the layout of the boat.

 

Also - what unit do you have, and does it correct for the deviation between the ellipsoid and the geoid - see my following post for more information.

Edited June 8, 2011 by julianh

[+julianh]

One more thing, as GPS units get more accurate, the GPS altimeter's accuracy gets better. It's to the point, that the barometric altimeter is mostly useful as a barometer or to track altitude when the unit isn't reading satellites.

 

GPS altitude accuracy has improved tremendously in the last few years. Even after SA was turned off, for a long time altitude accuracy was horrendous and I always had one of my baro altimeters handy when out in the mountains and never used GPS altitude. I'd been away from playing with my GPS units for a few years until I recently bought the 62st and I've been astounded by the improvements in GPS altitude accuracy. Newer (and more) satellites; wide adoption of WAAS; improved receivers, antennas, and more powerful processing chips in handheld units; and likely improvements in s/w have all been contributors. GPS altitude has actually become usable!

 

Dave

There is still an issue on many units, such as car sat-navs and mobile phones, where the manufacturer / programmers have "decided" that accurate elevation information is not important - the difference between the "ellipsoid" and the "geoid".

 

The GPS processor unit calculates your elevation relative to the standard ellipsoid; however, local Mean Sea Level (as defined by the geoid) can deviate by a considerable height above or below the standard ellipsoid - see for example:

http://www.esri.com/news/arcuser/0703/geoid1of3.html

http://www.esri.com/news/arcuser/0703/graphics/geoid3_lg.jpg

 

Some units (e.g. most if not all Garmin outdoor recreational hand-helds) correct for the deviation between the ellipsoid and the geoid; however, many units do not. Where I live (Brisbane Australia), Mean Sea Level is about 30 metres ABOVE the ellipsoid. My Summit HC corrects for this, so when I go to the beach, it reads around 0 m to 5 m. However, my car sat-nav and my Android phone don't correct, so they tell me I am at about 20 - 30 m elevation when I am standing at the water's edge. Plots of elevation loss / gain are fine, but your absolute elevation may be in significant bur relatively consistent error.

 

(This issue may not affect all brands, but it is certainly an issue with my no-name sat-nav and my HTC Wildfire phone.)

[+julianh]

There is still an issue on many units, such as car sat-navs and mobile phones, where the manufacturer / programmers have "decided" that accurate elevation information is not important - the difference between the "ellipsoid" and the "geoid".

 

...

Some units (e.g. most if not all Garmin outdoor recreational hand-helds) correct for the deviation between the ellipsoid and the geoid; however, many units do not. Where I live (Brisbane Australia), Mean Sea Level is about 30 metres ABOVE the ellipsoid. My Summit HC corrects for this, so when I go to the beach, it reads around 0 m to 5 m. However, my car sat-nav and my Android phone don't correct, so they tell me I am at about 20 - 30 m elevation when I am standing at the water's edge. Plots of elevation loss / gain are fine, but your absolute elevation may be in significant bur relatively consistent error.

 

(This issue may not affect all brands, but it is certainly an issue with my no-name sat-nav and my HTC Wildfire phone.)

Interestingly, OziExplorer for Android http://www.oziexplorer3.com/android/oziexplorer_android.html is in Beta testing now; version 1.05 and earlier versions did not correct for the Geoid / Ellipsoid error, but this correction is now applied in version 1.06, so my Android now reports true elevation much better in OziExplorer.

Edited June 12, 2011 by julianh