On 08/08/2008 14:30, Fire Girl wrote: > I am working with OSM data, and would like to be able to spec out 5 mile > bounding boxes from certain GPS points. > > After research into this problem, I am to understand that each degree of > latitude is approximately 69 miles (111 kilometers) apart with a slight > variance (68.703 - 69.407 miles) between the equator and the poles, and > that each degree of longitude is widest at the equator @ 69.172 miles > (111.321 kilometers) and gradually shrinks to zero at the poles. : ) :) > > So what does this mean? If I want to take a input point, like lets say, > > 167.9 lat > -29.1 lon > > or > > -63.1 > 18.1 > > Can someone say with authority, what the 'calculus' would be to > definitivly construct a NSWE bounding box with a 5 mile radius around > those points?.... that would be basically close enough or accurate? :)
If you want accuracy, then you are asking the wrong question, because the "bounding box" will be a curved section of a surface of the earth (or, rather, the approximation to it defined by the ellipsoid for the datum you're using for the lat/lon coordinates - unless you want to start taking altitude and terrain into account), not a flat straight-edged box. So if you want to talk in terms of flat bounding boxes, you have to start asking "in what projection?" etc. If all you want is the lat/lon of the corners an approximate box parallel to lines of latitude and longitude approx 5 miles across, then you could indeed just add and subtract 2.5/69ths of a degree of latitude and 2.5/f of a degree of longitude, where f is the approx length of one degree at that latitude, i.e. f = 69 * sin(latitude) miles (with the reasonable approximation that the earth is a sphere, so the trigonometry is trivial). David _______________________________________________ dev mailing list [email protected] http://lists.openstreetmap.org/listinfo/dev
