A Foundation to Build On
The Highland County, Ohio, auditor wanted more accurate tax maps and a quicker way to calculate agricultural use value. The county engineer wanted accurate topographic mapping to use as a better basis for planning public works projects. The county commissioners wanted an updated land-use plan to develop a vision for growth in the county. Essentially, the county officials wanted a countywide geographic information system (GIS) created with digital orthophotos for property appraisal.
To meet these needs, Highland County officials did their homework, reading trade publications, attending conferences, picking the brains of neighboring county officials and checking in with the mapping experts at Ohio State University (Columbus). What they decided from their research was to hire professionals who would deliver the very best products and services they could get for their investment. To Highland County officials, continuing to be educated about innovative technologies and new ways of using them was just as important as the various maps, plans and reports they would get.
“When you’re going to do a job, you could just piece it together, but we wanted the most accurate system available today,” says Commissioner Russ Newman. “If we get it started right, by understanding everyone’s needs and planning properly, then in the future, with new technologies, the resulting system is likely to continue to be a good base.”
At the end of 2000, the county selected Woolpert LLP from nearby Dayton, Ohio, as the consultant on the multi-tiered GIS project, and McCarty Associates, a local surveying and engineering firm to assist with ground control. Together, they made four major decisions for technology use and technique that would affect the overall quality of the project: the addition of monumentation as part of the ground control survey, and the application of airborne GPS, LiDAR (Light Detection and Ranging) and CAUV (Current Agricultural Use Valuation).
Highland County is a fitting backdrop for such innovative work relating to cadastre and mapping. The county’s southwestern Ohio location is in Ohio’s historic Virginia Military Survey (VMS) District. This entire portion of Ohio was divided into parcels to be used as payment to large numbers of Virginia’s Revolutionary War veterans in the early 1780s—a measure they had to resort to after they finished parceling out what we regard today as Virginia. Often VMS surveys used creeks, trees and other natural features as landmarks and monuments in property descriptions. To read a description of an individual parcel “from the bend in the river to the big oak tree to the big rock on top of the hill” would not be uncommon.
In time, of course, those features have changed or disappeared, requiring the relocation of important elements of property descriptions to be subject to great difficulty, and in many cases impossible to locate with certainty. County commissioners were determined that future relocation of more modern parcels would be best assured through compilation of a parcel GIS dataset to help document where properties are and take some of the mystery out of the county’s property lines.
Phase 1: Ground Control
From February to June 2001, the Woolpert/McCarty survey team established, targeted and observed with GPS a geodetic control network across the county. The network consists of 32 pairs of monuments located on an approximate six-mile grid. The survey team performed reconnaissance on all proposed site locations and submitted the preliminary locations to the OUPS (Ohio Utility Protection Service) for approval of the locations. The team then set reinforced concrete monuments (NGS Class “C”) with an inscribed bronze disk set flush with the ground marked “Highland County GIS Marker – Engineering Dept.” The preliminary observation plan was sent to the State NGS Advisor, Dave Conner, for advice and approval. A horizontal first-order GPS survey was then done, which met NGS Bluebook Specifications for inclusion into the National Spatial Reference System (NSRS), according to Woolpert’s Jack Kuttrus, PS, who supervised the survey phase of the project. The county chose to monument in pairs for additional accuracy and stability.
The geodetic control surveys were completed using five Trimble 4000 SSI dual-frequency GPS receivers (Trimble, Sunnyvale, Calif.) equipped with fixed height tripods and Trimble compact L1/L2 antennae with ground planes. All sessions consisted of a minimum occupation time of 40 minutes of usable satellite data.
Once the observations were complete, Woolpert performed all the necessary tasks associated with a bluebooking project, including six types of adjustments using NGS’s ADJUST software in accordance with the Standards and Specifications Manual for a First-Order GPS Survey. All field observations and bluebooking tasks were performed in accordance with FGCS (Federal Geodetic Control Subcommittee) specifications for a First-Order GPS Survey.
The decision to bluebook was simple logic, according to County Engineer Dean Otworth, PE, PS. Since the county was in the process of establishing monumentation for ground control anyway, the additional cost to bluebook these monuments was minimal. And once the county’s monuments were bluebooked, the NGS will perform the necessary adjustments and maintain the coordinate information in its database. Seizing the opportunity to improve the standard of surveys done in the county, Otworth expects to begin by encouraging private surveyors to tie in to the county’s existing bluebooked monuments. In the future, however, as monumentation throughout the county is densified, surveyors may be required to do this. He also expects eventually requiring the surveyors to comply with a requirement for digital submission of property survey and construction plans referenced to the State Plane Coordinate System.
Phase 2: Airborne AccuracyIn April 2001, Woolpert began the aerial mapping phase of the project. Airborne GPS made the establishment of high-accuracy survey control network cost-effective. Traditional GPS survey techniques would have required some 300 points on the ground to support 2-foot and 4-foot contour mapping. Using airborne GPS, photogrammetrists were able to work with just 76 ground points to achieve the required mapping accuracies. The horizontal accuracies are ± 2.5 feet for the 1"=100' scale mapping and ±5.0 feet for the 1"=200' scale mapping. The vertical accuracies are ±1 foot for the 2-foot contours and ±2 feet for the 4-foot contours. The mapping work was planned and completed so that both horizontal and vertical accuracies meet the National Map Accuracy Standards (NMAS).
The decision to use LiDAR also indicates the advanced technical level of the county’s preliminary study. Initially, county commissioners were told that LiDAR alone couldn’t meet their accuracy needs. A year or so later, as they continued to study and search for the best solution, Newman recalls, experts at Ohio State told them to revisit the LiDAR decision. By the time Woolpert had been selected to do the project, the technology and operational factors had made significant strides forward, allowing them to combine LiDAR with photogrammetric review to generate accurate contours.
“What we wanted was accurate topographic data,” Otworth says. “LiDAR gives the county what it needs now and that data will still be accurate when it’s time to add floodplain maps that meet FEMA requirements.”
Engineering and planning were foremost in the commissioners’ thoughts when they made the LiDAR decision, Commissioner Newman says. “We wanted to be thorough, and while we were doing the project, we wanted to do it right. We wanted good base mapping. We could have used other systems, but they’re not as comprehensive, which usually affects accuracy as well.”
Newman says the county expects the LiDAR-generated topographic imagery and digital orthophotography to serve as a good basis for future GIS layers. “When we move to new technologies, it will still be a good foundation.”
For the Highland County project, LiDAR data was collected at the same time as aerial photography; then the two were integrated in a photogrammetric quality control process. LiDAR was flown at 5,000 feet above ground level, using a 30-degree scanner field of view, yielding an effective ground coverage for each LiDAR pass of approximately 2,700 feet with a 30 percent sidelap between flights. The ground sample rate was 10,000 measurements per second, resulting in an average ground sample distance of 7 feet along and across flight directions. Then, using proprietary techniques and software, the LiDAR data was processed to remove above-ground features and provide a bare earth surface model. Individual flight lines were verified for completeness of coverage and data accuracy, and overlap between flights was removed to provide homogenous surface model coverage. That coverage was then parsed into stereomodel-sized units for import into a softcopy stereoplotter where the data was supplemented by photogrammetrically compiled breaklines.
Because Highland County is full of dense vegetation, LiDAR was a good choice because when a particular laser pulse actually hit the ground, post-processing could be used to identify it—thus in effect penetrating canopy. By supplementing LiDAR with breaklines along road edges and water bodies, the county was able to eliminate the “puddling” of contours around linear features that often results from LiDAR’s inability to define flat linear surfaces. In addition to the bare earth coverage, the contractor also delivered above-ground LiDAR data as well as the raw, unfiltered data. Though not much use for topographic purposes, the above-ground data can help the county with issues such as forestry inventory, obstruction height analysis, 3D urban modeling and transportation analysis.
Building a GISDigital orthophotos, parcel mapping and a Current Agricultural Use Valuation (CAUV) calculation dataset rounded out the Highland project beginning in November 2001.
“This is one of the biggest services we provide to the public,” Otworth says. “The ability to get information the public needs in a timely manner is paramount to the success of local government and its citizens.”
County Auditor Bill Fawley agrees. “When CAUV first came in during the 1970s, people counted dots. We’ve had lots of (property) splits since then and the splits are averaged rather than redone. So this new overlay will really help determine land values.”
Highland County’s existing maps, created in the 1960s from parcel legal descriptions and survey information, were in pretty good shape, so they were scanned and overlaid onto the new orthophoto base to perform best-fit parcel mapping. In addition to the linework, Parcel Identification Numbers are built into the mapping, serving as a link to the Computer-Aided Mass Appraisal (CAMA) database. Soil data will also be incorporated sooner than expected. The state moved Highland County up on its priority list to receive soil mapping data because of the county’s rapid progress on the GIS project.
Finally, all the data is available quickly because it is organized into a GIS.
“Local government exists to serve its citizens,” says Gregg Gohrband, Woolpert’s GIS phase manager for Highland County. Since so much of the information they need is land-based, the implementation of GIS datasets and software will allow county officials to build ‘what if’ scenarios that efficiently bring together various datasets to support planning and informed decision making. Such tasks include site evaluation and selection of areas for new development and the automation of the county’s CAUV calculation. Eventually, the GIS data will be accessible to the public using kiosks or via the Internet with easy-to-use mapping applications.
Newman believes this data availability will result in better informed decisions for the county. Such decisions will in turn benefit all the citizens.
“Our county is desirable now,” Newman says. “We want to maintain that vision. We can’t keep it rural, but we can maintain the rural flavor. With a good plan, based on good data, we have a basis to work from.”