By Mark S. Schall, CEO, Spatial Data Consultants (www.spatialdc.com), High Point, N.C., and Sean Hawley, project manager, Geographic Technologies Group (www.geotg.com), Goldsboro, N.C.

Flanked by the sparkling waters of the Chesapeake Bay and the Patuxent River, Maryland’s rural Calvert County is fast becoming a popular relocation choice for Washington, Baltimore and Virginia residents hoping to escape congested metropolitan areas. The resulting growth poses challenges for county officials confronting the need for infrastructure improvements, open-space preservation and additional public-safety services. They recognized that accurate 3-D mapping methods could help them effectively manage economic, land and infrastructure development. To get the best value for their capital funding and meet their needs for a fast turnaround, the officials hired Geographic Technologies Group (GTG), consultants specializing in local government geographic information system (GIS) development, to create and implement a new GIS using an accurate image base map.

Increasing Demands for New Mapping Data

Previously, Calvert County relied on mapping data at scales of 1:2,400 and 1:6,000 supplied by state and federal government sources. But the data were outdated because of the county’s fast-paced growth. Additionally, map features—parcel boundaries, water sources, vegetation, structures, driveways, road centerlines and edges, etc.—lacked the positional accuracy achievable with modern mapping technology such as coordinate geometry and planimetrics.
 

Considering Calvert County’s stringent and varied data-use requirements, GTG recommended collecting new data to create orthophotos, digital terrain models, planimetrics and topography maps with two-foot contours. GTG evaluated the county’s existing mapping and property ownership records, along with its immediate and upcoming GIS plans. As the company looked for ways to reorganize data maintenance and recording in a digital environment, it surveyed nine county departments and identified the following high-priority needs:

• Recent residential developments weren’t captured and mapped on existing maps. Parcels need to reflect an up-to-date tax base to have an accurate property GIS.

• Erosion of cliffs that front the county’s bay side and stringent environmental regulations on waterfront land presented a need for highly accurate terrain modeling. Contour coverage would greatly improve the county’s ability to address safety and regulatory environmental concerns.

 • Engineering-grade data are necessary to plan upcoming road construction projects to MD Route 4, the single highway artery that runs through the county. Centerlines and road edges are needed to calculate water run-off. Identifying building and structure locations is vital for construction rerouting.

• Increasing emergency preparedness and response capabilities accompany the recent growth and require enhanced technology to meet the needs of the population served by the county’s Sheriff’s Department, 911 Dispatch Center, Emergency Fire and Rescue Services, and Emergency Operations Center.

• With changing state and local preservation guidelines, land characteristics are required to designate open spaces and identify wetlands.

Combining LiDAR and Aerial Photography

With time in short supply and a tiled set of full-color digital orthophotography required as the first deliverable, a hybrid of conventional photogrammetric base mapping and innovative mapping methodologies that provided aerial photography and digital mapping were used to create 1:1,200-scale base map imagery. The solution combined color aerial photography, Light Detection and Ranging (LiDAR) data and soft-copy photogrammetry services to keep the project within budget and on schedule, providing a complete set of engineering-quality data as the foundation for the GIS.

Aerial photography commenced in early April 2003, when late snows had melted but “leaf-off” conditions still existed. The photography was captured in a one-day, six-hour time frame. The aerial LiDAR survey mission, scheduled to coincide as closely as possible with the aerial photography mission, was flown primarily at night. 

For the aerial photography mission, 21 flight lines totaling 651 frames captured the county’s terrain. The mission was flown at 35 percent sidelap between flight lines and 60 percent overlap between exposures at a height of 4,800 feet. LiDAR collection required 32 flight lines and was flown at a height of 6,600 feet. Airborne Global Positioning System (GPS) technology was used to record location coordinates, with 46 supplemental control points to ensure that precise
 

3-D measurements could be derived for the aerial photography. The final results of the soft-copy aerotriangulation proved the accuracy of the GPS ground control and airborne GPS. The ground control points also provided a crosscheck to the horizontal and vertical accuracy of the LiDAR data.
 

Once LiDAR collection was completed, Spatial Data Consultants (SDC) created and edited a detailed surface model using aerial LiDAR surveys in conjunction with soft-copy stereo photogrammetric techniques. Using Z/I Imaging’s PhotoScan 2001 scanning system, ImageStation soft-copy photogrammetric software modules and SSK workstations to enable an entirely digital processing workflow—from scanning color photography, soft-copy aerotriangulation and LiDAR quality control/quality assurance (QC/QA) to orthophoto production—the group was able to deliver 1,100 color orthophotos at 0.5-foot pixel resolution of the entire 219-square-mile area within four months after the photo mission was flown.

To begin, the negatives were scanned at 14 microns on the PhotoScan 2001 Photogrammetric Image Scanner. The digital files were uploaded into the ImageStation workflow. Files were processed for point extraction and triangulation in the system’s automatic triangulation software to quantify the ground control and airborne GPS; the software also provided a seamless set of georeferenced stereo pairs for LiDAR review, editing and data extraction. Technicians used a variety of software applications within the ImageStation soft-copy workflow to digitally display the LiDAR over the stereoimagery and create triangulated irregular network (TIN) models and contours, which allowed them to dynamically edit the postprocessed LiDAR data. With quality image scans, soft-copy aerotriangulation data and groomed LiDAR DTMs, the technicians used Intergraph’s GeoMedia geospatial software and Z/I Imaging’s OrthoPro application to orthorectify the frames and mosaic them into a seamless and color-balanced orthophoto image covering the entire county.

A New Point of View

Although Calvert County is only in the first year of its four-year GIS implementation, the advantages of 0.5-foot-resolution digital data already are apparent. In commissioner meetings, county officials have used images to show the public the locations and street alignments for highway improvements. Imagery has been used in court to illustrate zoning violations. Even the district attorney’s office has used the imagery to prepare for criminal cases. The data also have increased support for the costly project by enabling staff to envision new, more efficient ways of working.
 

“The clarity and high quality of the data allow our elected officials and constituents to not just mentally picture an area proposed for new construction or zoning changes by looking at a line map, but instead place themselves at the location using visual reference points,” explains Kathleen O’Brien Branch, assistant to the county administrator and project manager. “We are excited that GIS technology can give us tools for better planning and decision making, and significantly improve our ability to provide important services to our citizens.” 
 

Work has begun on the county’s digital planimetric, DTM and topographic GIS layers. The planimetrics, topography and DTM will be delivered with accuracy exceeding National Map Accuracy and American Society for Photogrammetry and Remote Sensing mapping standards. Interactive and automated feature-extraction techniques will be used to develop the planimetric layers. The LiDAR data will be edited further to remove any remaining anomalies caused by man-made features or vegetation to achieve a surface model suitable for any type of earthwork computation, such as contour generation or volumes. Digital planimetric and topographic coverages already are being delivered to the county and should be completed by the end of the county’s fiscal year.
 

Aerial photography and LiDAR met Calvert County’s goal of creating an accurate base map to serve as the foundation for its new GIS. The GTG team combined the efficiency of innovative photogrammetric technologies with time-tested conventional base-mapping techniques, delivering spatial data enhancement quickly and affordably.       

  See  Featured Images
 
  Subscribe to Earth Imaging Journal

  Go to Home Page