Use the calculation routines that use the ephemeris data files of the
Solar System Dynamics Group of the Jet Propulsion Laboratory of NASA
http://ssd.jpl.nasa.gov/.
This sets the buffer size for access to Swiss Ephemeris
or JPL
data files, if you specify an http-URL in swe_set_ephe_path() or via
the SwissEph constructor.
If you want to do calculations relative to the observer on some place
on the earth rather than relative to the center of the earth, you will
want to set the geographic location with this method.