Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:However, there are inevitably errors or uncertainties in spatial data, which are used to represent reality world (Goodchild and Gopal, 1989; Guptill and Morrison, 1995 Burrough and Frank, 1996; Shi and Liu, 2000).
Category: