'Returns the default SRID for the given geometry, taking into account
the SRID set for the field.  For example, if the input geometry
has no SRID, then that of the field will be returned.'

def get_srid(self, geom):

    gsrid = geom.srid
    if ((gsrid is None) or (self.srid == (-1)) or ((gsrid == (-1)) and (self.srid != (-1)))):
        return self.srid
    else:
        return gsrid
