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paulkbiba
USA
5064 Posts |
 Posted - 07 déc. 2005 : 18:46:52
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Altitude Comparison - Putting GPS altitude to the test
INTRO
We get a lot of questions about altitude accuracy on the site, so I thought I would post this little test. It consists of data I collected in two separate runs about a month apart. As you will see, altitude is not a consumer-level GPS's strong point.
SETUP
What I did was to run a copy of Outdoor Navigator on my Palm with the local topo map of my home area. I then stopped to take measurements at places along the road which intersected a topo line exactly. Thus I always knew exactly what my real altitude was when I took a reading. As sort of a "control" I used my Kestrel 4000 weather instrument. This contains a barometric altimeter which I calibrated before each run. I wanted to see how its accuracy compared to a "high tech" substitute. Here is a picture of the Kestrel. it is one of the most fun gadgets I own and if you want to learn more about it check out their site here: http://www.nkhome.com/ww/wwindex.html - It runs about $330.
I pitted the Kestrel against three GPS units *. The Globalsat BR335 wired unit, provided by GlobalSat, the Rikaline 6033, provided to me by www.buygpsnow.com, and the Mobile Crossing BT-308, provided to me by www.mobilecrossing.com. The Mobile Crossing unit is a SiRF IIe/LP unit and is advertised as having better altitude accuracy than other units. People are always interested in what our various setups look like, so here is a shot of the equipment as set up in my VW Jetta:
The easiest way to describe the results is simply to look at the data. In the charts below you will see the topo altitude. In all cases I was stopped exactly on the topo line as shown in Outdoor Navigator on my Palm. Next, you will see the altitude reported by my Kestrel, which had been calibrated just before I started out. Also presented are the actual altitude readings of each of the three GPS units. Next to each unit is a "Difference" column. This is the difference in altitude reported by the unit and converted to an absolute value (essentially that means that the + or - sign is ignored). By adding up the sum of all of the absolute values you can get a total error for each unit. This method of arriving at a total error was suggested to me by Calvin Chu of Mobile Crossing and I confirmed it with the local high school math teacher. Here are the results
DAY ONE
As you can see, the Kestrel is the clear leader, followed by the Mobile Crossing, the Globalsat and then the Rikaline. Note, however, how the readings bounce all over the place with all the units.
DAY TWO
This was done about a month later using exactly the same equipment
In this case we see that the Rikaline was the leader, followed by the Kestrel, then the Mobile Crossing and then the Globalsat.
OUTRO
One thing is clear from this little test. To all those pilots who have asked in the forums about using these consumer GPS units for altitude readings - DON'T DO IT. I used to fly gliders at one time and I clearly would not want to rely on this type of data in a critical situation. For the rest of us, altitude readings are fun, but that's about it.
* To best reflect the end-user experience, no MSL corrections were applied to the altitude readings than those reflected in the standard NMEA data, see this article for the impact.
Moderator
Don't forget the GPSPassion Club![/i] |
Edited by - paulkbiba on 11 déc. 2005 18:53:48
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Moris
87 Posts |
Posted - 12 déc. 2005 : 05:37:18
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Thanks for the test. So, it means the altitude accuracy can vary from 0 to 100 m. brrrr, risky business for flying.
Do you think the GPS r (like GPS Map 60CS) with barometer are more precise? |
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paulkbiba
USA
5064 Posts |
Posted - 12 déc. 2005 : 06:32:29
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I don't have any personal experience with that unit, so I can't comment.
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igurevich38
USA
764 Posts |
Posted - 12 déc. 2005 : 16:52:40
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This is interesting.
I had a weird experience a few months ago with the altitude readings from my Haicom 303MMF.
I was driving in Virginia and Outer Banks, NC on a bunch of roads that go right along the Ocean shore. Obviously I am driving above sea level, otherwise I would be in the ocean. But my GPS was constantly showing that I was 50-100 ft below sea level. Really strange. |
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gpspassion
93392 Posts |
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redcloud
USA
427 Posts |
Posted - 16 déc. 2005 : 15:34:21
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| Would I be incorrect in assuming the solution is fairly simple. The difference in the geoid vs the ellipsoid is known for various regions. From my observation, I'm assuming that the geoid variance is not a smooth transition, but more like a lumpy geoid. The empircally derived data for altitude variance between regions is entered into a matrix which is stored in GPS units. The units would require additional power and storage to compute the correction. Short of this, the only solution would be the current method of barometric pressure readings at Mean Sea Level to obtain accurate readings. The disadvantage is the constant need to calibrate barametric pressure. |
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joenoone
USA
2 Posts |
Posted - 26 janv. 2006 : 19:44:57
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This is something I've been looking into recently, since I hang glide. As cheap as GPS systems are I can't understand the HIGH price of altimeter/variometers other then they don't make enough to get a break in the manufacturing prices.
I noticed you didn't use any receivers that have a barometer which would give you more accurate readings, such as the Garmin's have. Do you know if any of any other GPS receivers that have a barometer? If so, wich ones? Try retesting with one of those - I'd be interested. |
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paulkbiba
USA
5064 Posts |
Posted - 26 janv. 2006 : 20:33:55
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We'd need to be provided with one to do a test. Can't afford to buy everything on the market :-)
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Don't forget the GPSPassion Club! |
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gpspassion
93392 Posts |
Posted - 26 janv. 2006 : 23:10:39
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Not a good excuse ! Anyway the kestrel has a barometer that you recalibrated regularly so that gives a good sense of how a GPS with barometer would perform.
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joenoone
USA
2 Posts |
Posted - 29 janv. 2006 : 17:51:37
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| Does anyone know which, if any, GPS receivers have a barometer? I'd especially be interested in a bluetooth one. |
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paulkbiba
USA
5064 Posts |
Posted - 29 janv. 2006 : 18:19:21
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I'm afraid that none of the receivers have a barometer. If you want one you would have to look at a unit like the new Garmin 60 or 76 series, but they don't have Bluetooth.
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Don't forget the GPSPassion Club! |
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DementedManiac
Australia
2 Posts |
Posted - 26 juin 2008 : 01:38:19
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The images are no longer being served. I'd be really interested in seeing either the raw data, or the images if you still have either available.
Thanks
Dawson |
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gpspassion
93392 Posts |
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DementedManiac
Australia
2 Posts |
Posted - 08 juil. 2008 : 09:51:10
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Thanks for fixing the images.
One further question: Can you confirm that the altitudes shown are in Metres as Moris has assumed above, or are they in feet?
Thanks again,
Dawson |
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paulkbiba
USA
5064 Posts |
Posted - 08 juil. 2008 : 16:22:00
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| All altitudes are in feet. |
Don't forget the GPSPassion Club! |
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* Altitude Comparison *
