Positional accuracy and geographic bias of four methods of geocoding in epidemiologic research.
OBJECTIVE
We 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.


METHODS
We 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.


RESULTS
Coordinates 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.


CONCLUSIONS
We 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.
Schootman
Mario
M
Division of Health Behavior Research, Department of Medicine, Washington University School of Medicine, St. Louis, MO 63108, USA. mschootm@im.wustl.edu
Sterling
David A
DA
Struthers
James
J
Yan
Yan
Y
Laboube
Ted
T
Emo
Brett
B
Higgs
Gary
G
D001249:Asthma / Q000453:epidemiology; D015982:Bias; D002648:Child; D004812:Epidemiologic Methods; D040362:Geographic Information Systems; D006801:Humans; D011204:Poverty Areas; D015203:Reproducibility of Results; D017062:Small-Area Analysis; D019371:Topography, Medical / Q000379:methods*