A system and method determining an error radius reflecting the accuracy of a calculated position of a processing device is provided. A data structure includes an error radius mapped to a scaled geographic area or “tile” comprising an area in which a calculated position may be determined. The data structure may include a plurality of first fields identifying a scaled geographic area based on a global projection reference system, and a plurality of second fields identifying, for each of the first fields, a position error radius associated with a scaled geographic area and level. For any calculation of an inferred position based on beacon observations, a rapid lookup of the corresponding scaled area including the new inferred position in the data structure returns an error radius for the new inferred position.