Stefan Steiniger a écrit : >>... but geodesy is a bit more complicated than converting feet to >>meters, and converting a coordinate to the central wgs84 may be a waste >>of energy if you just wante to change the projection but stay in your >>local datum (in this case, your local datum with your local ellipsoid >>should be used as the pivot, and coordinates should just be converted to >>this system then reprojected in the new projection. >> >> > >right.. so we need to be aware that we do not convert if the ellipsoid >stays the same - on the other hand.. it is just "some math".. and if >avoid rounding, then things should be the same for dx=(0,0,0) > >stefan > > Yes, may be it is not worth doing if/else, but if you just need to reproject (without datum change), you can avoid a transformation from geographic lat/lon to geocentric and back, which are not instantaneous functions.
To take an even more trivial example, imagine you just have to change the length unit (meter to feets) or the angle unit (degrees to grads) : transforming the local data to wgs84 before any operation would be a very heavy transformation compared to what is really need. Michael ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Jump-pilot-devel mailing list Jump-pilot-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel
