The purpose of the tracking mechanism outlined in Chapter 4 is to build a database of tracked landmarks. Tracked landmarks are, in a sense, the primitives that make up the robot's map of the environment. On-line localisation is performed by matching candidate landmarks from the robot's current view to the tracked landmarks, and interpolating a parameterisation of the set of tracked candidates. This chapter discusses the position estimation procedure assuming that the association between a candidate landmark and a tracked landmark is known. The chapter then presents a method for combining the individual position estimates from several matches to obtain a robust estimate.