ArcNews Online

Winter 2005/2006

E-mail to a Friend

Cobb County, Georgia, Creates a Geodatabase for Routing and Other Transportation Services Using GIS

As the corporate home of Home Depot, the world's largest home improvement retailer, it's only fitting that one of Cobb County, Georgia's top industries is construction. New housing additions, shopping centers, and office complexes dot the landscape, and crews have broken ground on a $96 million performing arts venue in an Atlanta business district within the county.

  click to enlarge
A three-dimensional view of the I-75 corridor.

Located in the northwest metropolitan Atlanta area, Cobb County added 22,332 housing units from 2000 to 2004, according to the Atlanta Regional Commission. The agency predicts a population increase of almost 25 percent by 2030, from 607,751 in the 2000 U.S. Census to more than 754,500. Cobb County maintains the second largest county-owned road network in Georgia, and each year approximately 100 miles of road and 7,000 parcels of land are developed, making GIS updates crucial to county operations.

This projected growth prompted the county to invest in an enterprise transportation geodatabase that will enable personnel to more efficiently dispatch emergency responders, track roads in need of repair, and plan trips for public transportation and senior services.

Just a few years ago, three county departments maintained separate GIS centerline databases with varying degrees of accuracy. The GIS dataset used by the Emergency 911 Communications Bureau (E911) had addressing abilities but did not accurately reflect the placement of road centerlines and intersections. The county's Department of Transportation (DOT) system had accurate road lines and DOT-related attributes but did not have addressing capability. The Cobb County Information Services' digital orthophoto basemap had been used to create planimetric mapping. Although the planimetric centerlines in the database were highly accurate spatially, they did not form a connected network and the data had limited attributes.

To remedy the problem, the county created a transportation geodatabase as part of a five-year, $5.2 million project to develop an enterprise GIS. The database program element of the project created four cornerstone GIS databases, one of which was the Cobb County Enterprise Transportation (CobbETRANS) database. It was developed using ArcGIS (ArcView, ArcEditor, ArcInfo), as well as ArcIMS and ArcSDE, and is based on the Unified Network Transportation (UNETRANS) model. The UNETRANS model, funded by Esri and developed by a consortium of ArcGIS users, established a uniform starting point for developing transportation-related GIS databases that could be customized to meet the needs of an organization.

  click to enlarge
Editing the addressing attributes on a road segment.

"We needed to bring E911 and DOT in on our new basemap so we would all be working from the same [spatially accurate] data," says Lynn Biggs, GIS coordinator for the Cobb County DOT. "Before the CobbETRANS project, we converted our paper tax maps to GIS. That really broke ground for county departments to work together on a GIS project."

Cobb County solicited bids for the project and selected ARCADIS, an Esri Business Partner with previous experience on county projects, to develop a prototype model and workflows to convert its transportation database systems to an enterprise geodatabase.

The CobbETRANS geodatabase was designed for the Esri platform so the county could use its existing GIS setup. Cobb County completed the project without investing in new hardware or software. The county holds 20 ArcView licenses, 10 ArcEditor licenses, and 7 ArcInfo licenses, as well as ArcSDE and ArcIMS licenses for its database and Web-mapping needs.

In the database design stage, interviews with employees from many county departments pinpointed what each constituency wanted from the new database. While three departments funded the project, all departments could utilize the new data. In the second phase of the project, ARCADIS updated data on the existing road network by extracting new road segments from builder-submitted CAD files or by digitizing the roads from updated aerial photographs using an updated parcel layer as a reference. Addressing information was then conflated, and the changes were incorporated into the CobbETRANS database.

ARCADIS developed a number of internal workflows that streamlined the process and made the field verification of addressing information seamless. ArcGIS ModelBuilder was used to develop tools to help the county automate the process by following a series of workflows. The customized CobbETRANS toolbar was developed using ArcObjects for use with the workflows, making the update process more systematic and cost-efficient.

One of the main project challenges was creating a true linear referencing system from the centerline road network. Digitized road centerlines had to be "flipped" to conform to the direction of travel along a road network. Customized programmatic tools were developed using ArcObjects to transfer the linear referencing ability onto the traversable link.

  click to enlarge
An overview of the spatially incorrect E911 data overlaid on the spatially correct CobbETRANS database.

Following data migration, routing capabilities were employed to generate address events coincident with the base road network. These events also include city and ZIP Code data, with the potential to house other boundary assignments, such as fire department zones and police precincts and beats. Existing boundary assignments were redrawn to match the new, spatially correct roadway network.

Dynamic segmentation was implemented to account for instances in which one roadway segment ran through two cities or ZIP Codes.

County employees use the CobbETRANS toolbar to edit the existing network and facilitate batch updating. The toolbar collects all necessary actions into one central location and removes multiple steps that inevitably create error. Address transfer is performed via event layers, and address range manipulation is automated to speed the update process.

Six toolsets developed using ModelBuilder cycle through a set of processes that mimic the manual process employed to create the master CobbETRANS geodatabase. The updated data is loaded into a personal geodatabase, then repetitive edits and procedures are executed within the toolsets. The design prevents the user from inadvertently adding data into CobbETRANS or editing existing roadways because edit sessions are limited to the personal geodatabase.

With regular updates, addresses on the roads are integrated into the E911 dispatch system, helping emergency responders reach destinations more quickly and accurately. Biggs says, "CobbETRANS was designed so that it can be expanded in the future for trip planning, and the county may use Esri's Network Analyst to map alternate routes to avoid traffic incidents."

The database is also used for pavement management. When problem spots in the roads are reported, information is entered into the database, which is used to generate work orders.

For more information, contact Lynn Biggs, GIS coordinator, Department of Transportation, Cobb County (tel.: 770-528-1638, e-mail:, or Nishant Gurnani, GIS technical manager/senior GIS analyst-programmer, ARCADIS (tel.: 770-431-8666, e-mail:

Contact Us | Privacy | Legal | Site Map