A method includes obtaining piece(s) of calendar information indicative of at least one appointment taking place in a space and extracting a set of space identifiers representing space(s) of the venue. The space identifier(s) for one or more spaces are extracted based on an indoor map of the venue. The method also includes determining or triggering determining whether at least a part of the set of space identifiers or one or more spaces of the set of space identifiers match(es) the at least one space as represented by the piece(s) of calendar information; and if a match is found: determining one or more reference location estimates. A respective reference location estimate is indicative of a location of the space that was determined to be a match. A corresponding apparatus, computer program product and system are also provided.