Hi Dave, > Try browsing chapter three here- > >http://www.gmat.unsw.edu.au/snap/gps/gps_survey/principles_gps.htm > >I have trouble getting my head round some of this stuff as well and >found it a useful read. >DaveB That looks really excellent, I'll go off and study - thank you for the link. And thanks again to Matt for his further explanation. Peter