@pimms.param<EOL><INDENT>def sphere_radius(sr):<DEDENT>
if sr is None: return None<EOL>if not pimms.is_real(sr) or sr <= <NUM_LIT:0>:<EOL><INDENT>raise ValueError('<STR_LIT>')<EOL><DEDENT>return sr<EOL>
proj.sphere_radius is either None or the radius of the initial sphere. Generally this is
left as None until projection is performed so that the sphere_radius can be deduced from
the mesh that serves as the projection domain. Because the MapProjection object that is
attached to the meta-data of the resulting map is modified to have this value filled in,
inverse projetions do not require that this value be set explicitly.