Alignment of map segments

A computer implemented method of aligning a plurality of map segments having a local reference frame to a reference map having a global reference frame, each map segment overlapping a portion of the area represented by the reference map. The method comprises for each map segment independently generating one or more candidate alignments, aligning the map segments and for each candidate alignment evaluating a candidate alignment cost function representing the likelihood that the candidate alignment is correct. Based on the candidate alignments and associated candidate alignment cost functions one or more candidate solutions are generated with each candidate solution comprising a single candidate alignment for each map segment of the plurality of map segments and for each candidate solution evaluating a candidate solution cost function based on at least the candidate alignment cost functions for each candidate alignment included in the candidate solution then, based on the evaluation of the candidate solution cost functions, determining the alignment of the plurality of map segments to the reference map.