size DELTAX DELTAY                    Size of the map in pseudo-angles

   Because of the non-linearities inherent in map projections, it is
   difficult to characterize the size of the map in units that make
   sense to the user (angles) in a way that "feels right".  The method
   we are using derives from imaging, where the size of a pixel at the
   center of the image is given (in effect the gradient at that point)
   as well as the number of pixels.

   The product of these two is not any real angle, but makes sense to
   the user and in the small angle limit is a good approximation for
   the angular extent of the image.

   Likewise here DELTAX and DELTAY are angles (in degrees) but they
   should be viewed as the product of the gradient of the map on the
   display surface at the map center with the extent of the display
   surface.


