I found the answer to my own question. This is a must read for any pilot who
uses a GPS.

http://www.esri.com/news/arcuser/0703/geoid2of3.html

"Wyatt Emmerich" wrote in message
...
My Garmin 296 indicates 330 MSL at KHKS, which is correct. My Echo Flight
GPS indicates 140 MSL. Both are locked on to WAAS.

How can I have such a large discrepancy? Both units seem to be functioning
properly other than this.