You should be able to do it with cartesian + pythag if your assuming that the coordinate system is a sphere. WGS84 coords are spherical from a datum of the centre of earths gravity so it ought to be ok. If your not in WGS84 then it depends on the datum used and you can be a few hundred meters out if you dont use the correct elipsoid. Us brits get collard by this since almost every map you see in the UK is OSGB which means you need to set up a GPS datum or else its out. Extrapolating the OSGB elipsoid to the whole earth and you would be miles out on the other side of the planet. I have the formula to convert OSGB to WGS84 and back but it turns out that there is no single step you can do it in. You have to run it itteratively ! Bizarre but true. I dont have any datum to WGS84 but I am sure GDAL would be able to do this tho I have never looked.