ArcWatch: Your e-Magazine for GIS News, Views, and Insights

April 2008

Application Ensures Voters Are Assigned to Correct Districts

South Carolina Uses GIS to Verify Voter Registration Data

By Will Roberts, GIS Manager, Digital Cartography Section, Office of Research and Statistics, South Carolina State Budget and Control Board

Since the 1965 Voting Rights Act was enacted, voter registration and election officials have carried the burden of assigning voters to the correct Congressional, House, and Senate districts; county and city councils; and special districts such as fire, water, and school districts.

Recently, in South Carolina, the state's Digital Cartography Section of the Office of Research and Statistics developed a customized, GIS-based application to facilitate this process. The new Voter Registration Accuracy Program (VRAP) application improves the quality of the state's voter registration database and assists county election commissions and voter registration offices with identifying and correcting addresses of voters who have been assigned to incorrect districts or precincts.

In South Carolina, very few counties have GIS in their voter registration offices. Paper maps have typically been used to assign voters to districts. A voter would walk into an office and register to vote, and then it was up to the clerk to decide which district he/she belonged in. The voter would be asked to indicate where he/she lived by pointing to a spot on a wall map. For each type of district, the clerk would walk around to the appropriate map and assign the voter to the district he/she thought the voter lived in. This method was time-consuming and prone to human error. When the Digital Cartography Section developed the GIS-based application with ArcGIS Desktop software to verify that registered voters were assigned to the correct districts, it marked the first time the entire statewide database of registered voters was mapped.

Developing this application required a coordinated effort among the Digital Cartography Section, the South Carolina State Election Commission, county election commissions, voter registration offices, local 911 units, and county GIS offices. The Digital Cartography Section is composed of two main GIS groups: the 911 staff and the Precinct and Demographics group. The former is tasked with maintaining a statewide 911 street centerline database as well as assisting counties with support and training for county 911 GIS. The latter supports the state legislature with mapping needs and provides local voter registration and election offices with official district maps used in elections.

First Run in 2004

To prepare for the 2004 general election, which included a race for seats in the state's House of Representatives, the Digital Cartography Section geocoded more than two million addresses and identified more than 15,000 voters who were assigned to incorrect districts. The voters' list was then sent to the counties for verification and correction. Since the idea for the project was developed just one month before the general election, only contested House districts were checked for errors.

Coordination among all of the parties involved was critical. Without the help of the local 911 offices and county GIS departments, there would not have been an accurate road file to use as the foundational data layer in the application.

Without support from the State Election Commission, there would not have been a statewide voter registration file to geocode. Also, David Morrison, the manager who oversees the 911 section that keeps the state centerline data up-to-date, was vital to the project's success.

Improvements for 2008

All redistricting has to be completed using the Census Bureau's PL94-171 data that is based on the current U.S. Census Bureau Topologically Integrated Geographic Encoding and Referencing (TIGER) system data file. This created one of the hurdles that had to be overcome in 2004—geocoding local data and overlaying that voter data on TIGER data. The spatial accuracy of the TIGER file was not nearly as good as the road centerline files that were received from the counties. This meant the selection of voters who were assigned to an incorrect district had to be corrected manually.

Through the Master Address File (MAF)/TIGER Accuracy Improvement Project (MTAIP), the 911 group of the Digital Cartography Section has been working with the U.S. Census Bureau to update the TIGER files with more spatially accurate road features. The 911 group has submitted local 911 data to the Census Bureau for the TIGER updates. With the MTAIP project, the manual correction process will be eliminated. The MTAIP for South Carolina should be completed in 2008 before the general election.

For all upcoming elections, the Digital Cartography Section will continue to provide counties with error lists to help conduct accurate elections. We would like to thank all the parties involved with the project and the local 911 and GIS offices for their ongoing coordination and support.

For more information, contact Will Roberts, 803-734-8923,, or Timothy M. De Troye, state GIS coordinator, 803-734-3894,

Contact Us | Privacy | Legal | Site Map