Location based applications for mobile computing have become of increasing interest to the research community, and are entering daily usage by the general public. Examples of this range from GPS based navigation systems for vehicles and pedestrians to 'where's my nearest' applications for mobile phones. Emerging applications include gaming and location based entertainment. In this paper we explore the relationship between the position sensing system and the location or target to be recognised. As part of this research we are seeking a method for the objective evaluation of such systems.