Tallahassee-Leon County, Florida Topographic Partnering Group/LIDAR Project (2003-Single Process)

Article excerpt

System Summary

In 2001, when the Tallahassee--Leon County GIS (TLCGIS) Interlocal Project was implementing the 2001 Flyover Project to update orthophotography and planimetric data sets previously gathered in 1996, concerns were voiced about the accuracy of contour data initially captured in 1996, especially in those areas of the county that had heavy tree canopy. Some users preferred contour data created in 1988 rather than the 1996 data. Users were concerned that if the same photogrammetric methods used in 1996 to collect data were used again in 2001, the accuracy of the new data would not meet their needs.

Rather than simply moving forward with the 2001 Flyover Project, the TLCGIS staff called together all the potential users of the data and formed the Topographic Partnering Group to discuss accuracy issues, and the pros and cons of available methods used to collect data and create contour data sets. The Partnering Group decided to have a prototype study of LIDAR-based terrain mapping conducted in four pilot test areas. The Tallahassee--Leon County GIS Interlocal Project diverted funds from the 2001 Flyover Project to complete the prototype study. Thus, a Digital Terrain Model (DTM), vector contour data, and vector spot elevation data were delivered and evaluated. Based on these results, the LIDAR DTM data and derived contour data had accuracy levels of +/-1.5 feet in heavily wooded areas, in comparison to 1996 contour data accuracy levels of +/-30 feet in heavily wooded areas. The group determined that the prototype study was a success and recommended LIDAR data be acquired countywide.

The Topographic Partnering Group/LIDAR Project is exemplary for several reasons. First, it brought together critical users from multiple agencies--many of whom had never worked together before--to discuss their concerns and needs about data accuracy. Second, users learned about LIDAR, a cutting-edge technique for mapping terrain data in heavily vegetated areas with high accuracy. Third, the Tallahassee--Leon County GIS Interlocal Project diverted funds from an existing project to acquire LIDAR data. Finally, because some members of the Partnering Group required a turnaround time of one year for finished data sets, members assisted in securing funding for the project, and, therefore, ensured that more accurate data would be developed so that every user's needs would be met.

Motivation for System Development

The Tallahassee-Leon County GIS is an Interlocal Project funded by three local governments: the city of Tallahassee, Leon County, and the Leon County Property Appraiser's Office. One of the duties of the Interlocal GIS is to maintain GIS data, including orthophotos and planimetric data, for use by staff in these three governments. As such, every five years, the Interlocal GIS arranges for a flyover of the entire county, to acquire orthophotos from which data sets, including contours, are derived using photogrammetric techniques.

In the course of the 2001 Flyover Project, however, it came to the attention of project personnel that some of the critical users of the data from city and county local governments, especially engineers who rely on data for infrastructure projects, had concerns about the accuracy of contour data initially captured in 1996, especially in those areas of the county that had heavy tree canopy. Because Leon County has such a dense tree canopy--more than 60 percent of the county is covered by a heavy tree canopy--traditional photogrammetric techniques cannot provide accurate elevation information for the ground underneath the tree canopy. This often resulted in inaccurate elevation data in those obscured areas. Additionally, the finished data required extensive field surveys to verify elevation to facilitate preplanning for large community improvement projects. Based on these factors, users were concerned that if the same photogrammetric methods used in 1996 to collect data were again used in 2001, then the accuracy of the new data would not meet their needs. …