This is the form used by many Global Positioning Systems . We use a simplified model of the earth. In this model the earth is a sphere whose radius is 6367 kilometers . Because the earth is not a sphere this model is somewhat inaccurate. For our purposes, it is a good first approximation.
Most Maps are Flat for more details.
The first step is to convert the measurements of latitude and longitude into a more usable form. In spherical coordinates we measure the angle phi from the north pole. Thus, the following formula converts from latitude expressed in degrees to phi also expressed in degrees. Both phi and theta as described above are measured in degrees . It is mathematically much better to measure angles in radians .