The approximate conversions are:
- Latitude: 1 deg = 110.574 km
- Longitude: 1 deg = 111.320*cos(latitude) km
This doesn’t fully correct for the Earth’s polar flattening – for that you’d probably want a more complicated formula using the WGS84 reference ellipsoid (the model used for GPS). But the error is probably negligible for your purposes.
Source: http://en.wikipedia.org/wiki/Latitude
Caution: Be aware that latlong coordinates are expressed in degrees, while the cos
function in most (all?) languages typically accepts radians, therefore a degree to radians conversion is needed.