MATLAB dted function calculation

If latitude and longitude limits are specified, the MATLAB dted function will return a map contained wihin those limits. It also returns a variable called maplegend that goes with the map. Can someone tell me how the maplegned varialble is calculated? I know that it represents the top left corner of the map, I just want to be able to calculate it for myself.

ex: [map maplegend] = dted(filename,scalefactor,latlim,lonlim)

How is 'maplegend' calculated by the dted function?