[GPSNavX]

While iNavX does not display the VPE: (Vertical Position Error), I would suspect that it's similar to the HPE: (Horizontal Position Error). So a 0' report at an actual height of 6' would be in range of that vertical error. I assume you were using "Location Services" and not NMEA data from an external GPS. I could add VPE:, but I figured for marine use most are at sea level or know the altitude of the body of water they sail on so they can compare it to the ALT: (Altitude) that iNavX reports. FWIW: I checked the vertical accuracy of my iPad 3G and it's reporting 32'. For the test I am about 4,000' above sea level. VPE: would be a nice addition so it most likely will go in the next release. On another note, your post makes me realize how we take GPS for granted and just assume it's perfectly accurate when in fact it actually has some [relatively small] error. You can google GPS and HDOP or VDOP to learn about the error.