Stephen McNaught wrote:
A couple thoughts on using a PDA with aviation moving map software,

snip

Some
GPS receivers were not built with aviation in mind, and for various reasons,
the altitude reported by the receiver to the software can be off, and the
software won't know, regardless of which software you use.

snip

I can't let this go by without pointing out the biggest reasons for GPS altitude
to be perceived as inaccurate. These are all *systemic* reasons and have nothing
to do with the quality of the GPS receiver, or whether it was built with
aviation in mind.

- GPS position calculation in the vertical axis has less precision than in the
horizontal plane because of the geometry. A small error in measuring the time of
arrival of the GPS signal translates into a larger error in the vertical axis
than in the horizontal. Users' expectation that the altitude will be as precise
as the horizontal location is misplaced.

- GPS altitude is usually presented as the height above whatever ellipsoid that
the GPS uses as an earth model. This is not the same as the height above MSL. I
think the difference can be a few hundred feet.