Evaluation of the Accuracy of Global Positioning System Coordinates for Collision Locations in California

Traffic collision reports typically provide descriptive locations indicating where a collision occurred and referencing the nearest intersection. Global Positioning System (GPS) technology can be used to provide latitude and longitude coordinates in addition to the descriptive location and many states now include GPS coordinates in collision reports. However, research has shown that there is potential for numerous errors when police agencies use GPS to complete traffic collision reports. In California, GPS coordinates have been included in the statewide collision database since 2006, but their overall accuracy has never been evaluated. The objective of this paper was to review the status of GPS coordinates in California collision data from 2009 to 2011 and to categorize types of errors or discrepancies that were exhibited, investigate error trends, and develop recommendations for use of the GPS coordinates. Instead of just classifying a GPS coordinate location as correct or incorrect, eleven categorizations were developed to better assess the breadth of differences between the GPS coordinate and descriptive location. Overall, 43% of GPS coordinates were categorized as correct, 2.5% were unknown, and the other 54.5% exhibited some type of discrepancy with the descriptive location. GPS coordinates located off the roadway were the most frequent error type, comprising nearly 20% of the sample, while systematic GPS errors such as truncated coordinates occurred 7% of the time. Accuracy appears to be improving over time, but it is recommended to thoroughly review the coordinate locations prior to conducting any spatial analyses.