Positional Accuracy and Geographic Bias of Four Methods of Geocoding in Epidemiologic Research

PurposeWe examined the geographic bias of four methods of geocoding addresses using ArcGIS, commercial firm, SAS/GIS, and aerial photography. We compared “point-in-polygon” (ArcGIS, commercial firm, and aerial photography) and the “look-up table” method (SAS/GIS) to allocate addresses to census geography, particularly as it relates to census-based poverty rates.MethodsWe randomly selected 299 addresses of children treated for asthma at an urban emergency department (1999–2001). The coordinates of the building address side door were obtained by constant offset based on ArcGIS and a commercial firm and true ground location based on aerial photography.ResultsCoordinates were available for 261 addresses across all methods. For 24% to 30% of geocoded road/door coordinates the positional error was 51 meters or greater, which was similar across geocoding methods. The mean bearing was −26.8 degrees for the vector of coordinates based on aerial photography and ArcGIS and 8.5 degrees for the vector based on aerial photography and the commercial firm (p < 0.0001). ArcGIS and the commercial firm performed very well relative to SAS/GIS in terms of allocation to census geography. For 20%, the door location based on aerial photography was assigned to a different block group compared to SAS/GIS. The block group poverty rate varied at least two standard deviations for 6% to 7% of addresses.ConclusionWe found important differences in distance and bearing between geocoding relative to aerial photography. Allocation of locations based on aerial photography to census-based geographic areas could lead to substantial errors.