  Multi-tip scanning tunneling microscopy (STM) is a powerful method to perform
charge transport measurements at the nanoscale. With four STM tips positioned
on the surface of a sample, four-point resistance measurements can be performed
in dedicated geometric configurations. Here, we present an alternative to the
most often used scanning electron microscope (SEM) imaging to infer the
corresponding tip positions. After initial coarse positioning monitored by an
optical microscope, STM scanning itself is used to determine the inter-tip
distances. A large STM overview scan serves as a reference map. Recognition of
the same topographic features in the reference map and in small scale images
with the individual tips allows to identify the tip positions with an accuracy
of about 20 nm for a typical tip spacing of ~1 $\mu$m. In order to correct for
effects like the non-linearity of the deflection, creep and hysteresis of the
piezo-electric elements of the STM, a careful calibration has to be performed.
