Forum: General Topics

Hello,

I suspect your strange altitude readings are due to the fact that GPS altitude is calculated relative to a simplfied 'ellipsoid' shape for the Earth, and not relative to local ground level. ie, the Earth is assumed to be a smooth slightly-flattened sphere, and the altitude is given as the height above this surface. In reality the Earths surface is much 'rougher' than this, so some areas can be quite a bit higher or lower than this idealized simplification. More info here: http://en.wikipedia.org/wiki/Geoid

One other factor is that GPS altitude is generally much less accurate than the latitude and longitude readings. Often if I go for a walk I'll leave the house at, say 50m altitude, but on returning I'm apparantly at -6m altitude! Since the GPS satellites are always moving, the precision at any one time will vary. Do you find that you're consistently being shown as -180ft, or does this vary during the day/week? There isn't currently any 'altitude calibration' function in TMJ, but I could probably add this if you think it'd be useful. (It would basically just be a 'Set Altitude to Zero' function, which would calculate the offset from zero when the function is used, and then apply this to all subsequent readings...)

I've noticed when testing friends' Blackberrys that the GPS update frequency does seem to vary somewhat. I assumed this was due to the GPS signal being stronger at some times than others, so the phone is able to calculate its location more accurately/frequently. (TMJ 'asks' the phone to update its location as often as possible, but it doesn't seem to always play ball!). Overall, I've generally found external bluetooth GPS devices to be far more reliable and much more accurate. (I'm currently using a GlobalTop G66 touch gps which gives 5 updates per second - far better than any internal GPS that I've seen!)

Cheers,
Stephen

Thanks for the info!!