Gps Altitude Reading

[+Skyman]

Thinking this would be a decent topic for discussion tried searching but couldn't find anything conclusive. I do know about the Altimeter and the Kollsman Window, but not excluding units without a barometric sensor, the altitude reading on a handheld gps would it be ( MSL Mean Sea Level ) or ( AGL Above Ground Level ) of course the latter would be presuming your in some sort of aircraft or whatever.

[+tflight]

The short story is it is MSL. But since the earth isn't perfectly round some interpolation needs to happen. You can read more of the long story on this page.

[+Tahoe Skier5000]

msl

Edited July 29, 2005 by Tahoe Skier5000

[+Timpat]

The short story is it is MSL. But since the earth isn't perfectly round some interpolation needs to happen. You can read more of the long story on this page.

Most excellent reference to answer the question tflight! You must work in the GIS field.

[+GOT GPS?]

MSL

[+Skyman]

The short story is it is MSL. But since the earth isn't perfectly round some interpolation needs to happen. You can read more of the long story on this page.

Most excellent reference to answer the question tflight! You must work in the GIS field.

Ah yes I agree a great link and info thank you. My curiosity started a long while back and have discussed this only a few times with only a couple of pretty well read and informed people. One said nether at least not in the sense of a pressure altimeter, that the altitude on a gps correlates to the center of the Earth, and I just went so here is an excerpt from the link above.

 

The GPS receiver uses a theoretical sea level estimated by a World Geodetic System (WGS84) ellipsoid, which does not perfectly follow the theoretical MSL. The MSL, approximated by an ellipsoid, is related to gravity or the center of mass of the earth.

[+fizzymagic]

The GPS receiver uses a theoretical sea level estimated by a World Geodetic System (WGS84) ellipsoid, which does not perfectly follow the theoretical MSL. The MSL, approximated by an ellipsoid, is related to gravity or the center of mass of the earth.

By itself, this could be misleading. It could lead you to believe that the elevation reported by GPS units is the height above the ellipsoid. That is only true for old GPS units. New ones correct for the geoid. The relevant quote from the same source:

 

However, most users expect accurate elevation readings that are related to MSL. Consequently, newer GPS devices output orthometric (geoid) height measurements as a product of "behind the scenes" calculations based on a combination of formulas, tables, and matrices that use geographic coordinates as inputs.

[+tflight]

Most excellent reference to answer the question tflight!  You must work in the GIS field.

No, but I do run a GPS Review Blog and my sister works for the USGS. I'm also a private pilot so having a small understanding of how GPS calculates altitude is helpful.