  Visual place recognition is a fundamental capability for the localization of
mobile robots. It places image retrieval in the practical context of physical
agents operating in a physical world. It is an active field of research and
many different approaches have been proposed and evaluated in many different
experiments. In the following, we argue that due to variations of this
practical context and individual design decisions, place recognition
experiments are barely comparable across different papers and that there is a
variety of properties that can change from one experiment to another. We
provide an extensive list of such properties and give examples how they can be
used to setup a place recognition experiment easier or harder. This might be
interesting for different involved parties: (1) people who just want to select
a place recognition approach that is suitable for the properties of their
particular task at hand, (2) researchers that look for open research questions
and are interested in particularly difficult instances, (3) authors that want
to create reproducible papers on this topic, and (4) also reviewers that have
the task to identify potential problems in papers under review.
