How to adjust GPS altitude?

[SouthshoreM6]

So, I have the new Nav software, and it's nice! But I live at 15 feet over sea level and the GPS is saying I am 100 feet above. I've gone to the service screen, but there are no adjustments I can see to fix the problem. Do others have the same problem?

[Bart Carter]

I think the earth has shifted and you are now 100 feet above sea level. You can verify this with your M5's GPS.

[AMP]

Single receiver GPS is very good at a two dimensional location, but sucks when you add a third dimension. In order to get a good 3D (lat, lon, elev) location you need to use differential GPS with multiple stations...blah...blah...blah...

In other words the mathematics is letting us down. You'll find that over time your elevation will fluctuate even if you don't move.

... I knew I'd use that geophysics degree for something, someday

[PhilSeastrand]

Also, the accuracy of the non-military GPS system is limited. I think it's something around +/- 50 feet. The system in our car uses the map to correct for this problem in two dimensions, but can't do anything for the third.

[tonyo]

The accuracy of civilian GPS was enhanced a couple years ago when the selective availability feature was turned off, thus giving us all the accuracy that the civilian channels of the GPS can provide. The accuracy of the information transmitted by the GPS satelites is probably much better than +-10 ft, but atmospheric distortions and other factors will impair that.

As far as altitude is concerned, there are really two major issues.

The first problem is that the GPS system doesn't know or care where sea level is. The GPS receiver independently uses a mathematical formula to estimate sea level at any given position on the planet. The problem is that the planet (which is irregular and not spherical) doesn't bother itself to precisely conform to any known mathematical formula. That difference is a source of error, since your "altitude" is a distance calculation from your 3D position to that imaginary sea level.

The second problem is harder to explain.

In principle, the GPS system is perfectly capable of good 3D positioning, but because it works by distance triangulation, to get the best 3D positioning, you want GPS signals coming from all directions, or more precisely, you want a GPS signal to be substantially aligned with every direction that you care about.

However, since the GPS satellites are in relatively low orbits, there are few that are nearly directly overhead at any given point in time. Further, all the GPS satellites very near to or below the horizon are unable to help. This results in a sort of "saggy trampoline effect" where positions along the surface are well known, but elevations are not.

Imagine that you had well-calibrated strings running to a few "stationary" satellites, all somewhat low in the sky. The spot where you are holding all of the strings in one hand is your position, which you can calculate by reading the markings on your strings and doing some math. Any movement along the ground causes changes in at least one of the strings, which gives you great "2D" position information. However, if none of your strings is going "up", you can't measure small changes in altitude very well, since you are just moving perpendicular to all of the strings and the numbers don't change. But when you do have a satellite nearly overhead, your altitude can be more precisely determined, subject to the first error described above.

[PhilSeastrand]

Thanks, Tonyo... I knew there would be someone on this board that would have the right answer!

[SouthshoreM6]

Excellent info, thanks to all!

Merry Christmas!
Bill

[Steve Beswick]

Tonyo,

Great description of GPS and triangulation. This board is so valuable, there are so many knowledgable people from so many different areas of expertise, I am constantly in awe !

Merry Christmas and Happy New Year to All !

[doug]

Since GPS receivers are now seeing use in aviation I wonder how they are dealing w/ vertical accuracy? Perhaps they have a built in altimeter-- don't know, but if you look on Garmin's web site you can see handhelds for pilots. It seems to me vertical accuracy was simply not a concern when it came to choosing a Nav system. These cars do fly, well... er, sort of. Moreover, the system accuracy is probably within, or not far from SouthShore's stated error in altitude (100-15=85ft).

[Bart Carter]

Quick question.

My GPS reads 2020 for Las Vegas. When I drive to my house, 10 miles, it reads 2680.

Is the realative distance accruate?