gpsman writes:

Altitude has always been very important to field soldiers and "ships"
whose responsibilities include flinging large projectiles long
distances.

The altitude of a ship is sea level. The altitude of an infantry unit
is marked on its maps.

The altimeter/GPS altitude accuracy debate is beyond my expertise but
my experience includes a different altimeter indication after landing
at an airport I had left earlier in the day while my handheld $150 WAAS
capable GPSr indicated its identical reading to 1/10ft.

WAAS isn't part of GPS.

Barometric altimeters are analog and the finest divisions of the scale
IME are 20', so I would not expect them to be any more accurate than +-
10'. I think WAAS combined with DGPS will soon provide digital
accuracy... far beyond any ordinary need.

There's nothing inherently accurate about digital systems. No digital
system can be more accurate than the best analog system.

"Both horizontal and vertical changes in position can be measured to an
accuracy of a few millimeters (horizontal) to several millimeters
(vertical).
http://volcanoes.usgs.gov/About/What...ation/GPS.html

Not in a tenth of a second. You can average readings over long
periods and get extremely accurate measurements, but that technique is
useless to aircraft.

The GPS altitude accuracy "problem" is mostly attributable to the
limitations and generality of the WGS84 datum geoid height
http://www.colorado.edu/geography/gc...gif/geoid2.gif
which could have easily been overcome by WAAS correction and simply
adding more satellites (and postioning them optimally). Economics have
precluded this rush to solution.

The problem is linked to the basic design of the system.

The bottom line is, IMO, WAAS altitude is far more accurate than
altimeter... depending on how many birds are in view.

WAAS isn't part of GPS.

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