In a method of and a system for determining inaccuracy information (600) in positioning a data model (1) of virtual information in an augmented reality system, the data model (1) is projected to the image plane of a display device (4) for mixing with visual impressions of a real environment. The inaccuracy information is calculated from parameters (10-0 to 50-2, 40, 50, 60) in a mapping computing process by means of which the data model (1) is projected to the image plane of the display device (4). It is thus possible to indicate relative precise inaccuracy information in positioning the data model in an augmented reality system.