I was working with a sextant on land and wanted to know "height of eye", that is height of sextant above the water. So my eye is almost six feet and I was standing on a small dune, perhaps six feet above the water. So I checked the altitude on my navx operating on my iPad 3G enabled. It said my altitude was 0.
So why is that?
Copasetic
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.
I totally agree regarding our over reliance of the accuracy of GPS data. In this vein perhaps having an accuracy 'circle' as used in the iPad Maps app may be of some use?