On Tue, 26 Apr 2005 10:02:22 -0400, Dave Butler wrote:

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.

Additional data follows...

Many years ago, prior to WAAS, and prior to SA being turned off,
altitude would regularly be several hundred feet different than
indicated on the altimeter.

However, with SA shut off, and a WAAS enabled receiver, the accuracy
in the Z domain is much improved. As an empirical reference - I have
a Garmin 295, which has WAAS capability. The delta between indicated
altitude and GPS altitude is always less than 500 feet, and usually
is less than 100 feet.