  The Pierre Auger Observatory is being used to study cosmic rays with energies
larger than 10 EeV. An essential quantity that must be deduced from data is the
lateral distribution function (LDF). Knowledge of the LDF is important for the
reconstruction of the shower core and the shower direction. Here we describe
how the LDF is measured using the large sample of events recorded with the
surface detector (SD) array and with a small sample observed with the
fluorescence detectors (FD). For hybrid events, in which SD and FD measurements
of the same shower are available, the core position is much better constrained
than for SD-only events, thus providing an important cross-check on the LDF
determined from SD measurements alone.
