[android-developers] Re: Altitude Discrepancies - Any Secrets

Thank you. I have read more about the Elipsoid -geoid differences. That fits with what the customer was seeing.

Nathan

On Thursday, September 27, 2012 5:51:23 PM UTC-7, Rudolf Hornig wrote:

Hi, the 45m difference is quite plausible and it comes because of the different 0 points used for altitude measuring.

GPS is using the WGS84 ellipsoid as a reference for altitude, while people (and maps) are using altitudes relaive to the mean sea level around their location. The two reference point can differ as much as 150m in some places of the Earth, tough the MSL - WGS84 difference is 40-45m in Europe. (It's 41m here in Hungary). The altitude returned by the location change listener is WGS84 relative...

As people usually expect MSL relative altitudes its usually required to convert the two... The correction value is not provided by the android API, but most handsets provide it in the NMEA sentences so you can pares it out and use that value to correct the WGS84 value to MSL.

You can check the MSL correction value in "GPS Status and Toolbox". There is a small number near the altitude label.

Rudolf

On Thursday, September 27, 2012 10:56:02 PM UTC+2, Nathan wrote:

Once in a while, I get a message from someone in Spain or Italy that the altitude in my app is too high.
Of course, I don't expect altitude from GPS to be terribly accurate.
Then they'll say that some other app is getting better numbers.

So I tried an experiment.
Three apps running at the exact same time, stationary phone, all getting altitude from the GPS.
They all showed different numbers.

Actual altitude: around 398 ft. 
My App: 392 ft.
MyTracks: 350 at first. Between 350 and 370
Another GPS App: 425.

Mine was closest, through sheer luck I think. I know I am using Altitude from the Location. Since I register for location updates >4m I don't get any altitude updates when standing still.
MyTracks, I looked at the code, I know it is recording the altitude directly. It does do filtering for stats but not the altitude itself. 
Other apps that are not open source may be adjusting the altittude, but I don't know what it is.

Then the customer wrote back to say he was comparing against two other GPS devices (ie other hardware) and the phone GPS was consistently 45 m higher than the rest. While that's not good, it does make more sense. Unfortunately, I can't forget the experiment I did in the meantime.

Any tips?

Nathan

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