How does Ardupilot do it?? GPS accuracy issues !!! Distance Calculations

Ok, this is a kinda double post. Yes i know i shouldn't but just bare with me, i put it in the sensors subject as it is a gps issue, but i believe it may also be a programming issue and think it would be relevant here, i'm sorry if i shouldn't be doing this, but i'm not sure how to link a topic to 2 discussion groups.

Anyway here's the link.....,50623.0.html

If anyone here has any ideas it would be really appreciated.