New strategies for GIS data collection

March 2009 » Feature Articles
To accommodate new state legislation and data-collection parameters in Florida that now require single-flight data collection every three years, the Tallahassee-Leon County Geographic Information Systems (TLC GIS) is modifying its strategies and scheduling for updating the county’s existing land base maps. To meet its needs, TLC GIS is using LiDAR data at an accuracy specification that supports the generation of 1-foot contours.

To accommodate new state legislation and data-collection parameters in Florida that now require single-flight data collection every three years, the Tallahassee-Leon County Geographic Information Systems (TLC GIS) is modifying its strategies and scheduling for updating the county’s existing land base maps. The maps are under the purview of TLC GIS, an interlocal partnership of the city of Tallahassee, the Leon County Board of County Commissioners, and the Leon County Property Appraiser’s Office that was established in 1990 to create a joint GIS for use by a variety of local public agencies. To effectively and consistently accomplish the mapping update, TLC GIS’ strategy from 2005 to 2008 was to divide the county into three urban and six rural sections and fly one urban and one rural section each year for data collection Thus, all urban areas were updated every three years and all rural areas every six years.

But Florida legislation in June 2008 adjusted the funding processes and impacted TLC GIS’s strategies and scheduling for land base map updating. The Florida Department of Revenue (DOR) had traditionally been responsible for having all Florida counties flown. But with the change in legislation, counties larger than 25,000 population would have to reimburse the DOR or opt out of having the county flown by the state agency. TLC GIS opted out, but was then required to fly the entire county in a single flight every three years, thus prompting the need for a new approach and schedule for acquiring updated data on a regular basis.

Initial data-collection program
Merrick & Company, an employee-owned, multidisciplinary professional services firm headquartered in Aurora, Colo., has been providing services to TLC GIS since 2001 in the form of program management, aerial photography, aerotriangulation, softcopy photogrammetric updating of digital terrain models (DTMs), planimetry, contour generation, and aerial image orthorectification services. What started as a routine project for the agency and the consultant in early 2001 quickly changed following the initial delivery of the first collected data. Now, the approach is changing again to accommodate TLC GIS’ new data acquisition plan and schedule.

The first modification in data acquisition arose when the interlocal agency adopted a change-detection-based update strategy (to leverage as much of the investment in the existing landbase as possible, in contrast to the previous strategy of compiling a new land base every five years). Accompanying that issue were the differing opinions among the interlocal user community about the accuracy of the existing photogrammetrically compiled topographic data.

Thus, in 2002, Merrick worked closely with TLC GIS to evaluate the systems and procedures and identify the optimum solution for providing the agency with the data required to address the mapping needs of all the agencies involved. LiDAR was selected as the best technology to use, making TLC GIS one of the first public agencies in Florida to use LiDAR in the acquisition of mapping data. Since those initial days in 2001/2002, Merrick has provided updates of different portions of the county per the agency’s planned acquisition strategy. Now, under the agency’s adjusted program and the existing contract, Merrick will provide data collection for the entire county. The orthoimagery will meet or exceed the Florida DOR’s requirements.

New data-collection strategy
TLCGIS began this new approach to data acquisition with completion of its initial single flight in January 2009. For that first flight, Merrick provided ortho imagery and LiDAR mapping to accomplish the data collection on a single pass. Approximately 750 square miles were mapped using the remote sensing technology. Collected data provides information on topography, vegetation, structures, water ways, and current drainage patterns. The collected mapping data will be used in support of a variety of programs for the city and county, including public works, police, fire, community development, utilities, sheriff’s office, emergency management, elections, technology, telecommunications, cooperative extensive, county appraiser’s, and planning departments, as well as by the county school board.

A color relief map shows the triangulated irregular network of the classified LiDAR bare-earth points for the entire area of Leon County. Red depicts the higher topography at approximately 200 feet above sea level. Blue depicts the lowest points at or around sea level.

One of the major advantages of using LiDAR in single-flight map updating is the accuracy and completeness of data. "LiDAR provides us with the ease of obtaining accurate land data that is of a much higher quality than what has been produced in the past with traditional technology," says Lee Hartsfield, TLCGIS coordinator. "Beyond that, the land base information we receive from Merrick is the foundation for all our GIS data. TLC GIS maintains over 380 GIS data layers of information which are tied to this base information. The accuracy and completeness in the base layer is everything."

Greg Mauldin, GIS scientist and project manager adds, "Leon County is heavily forested, and it’s difficult to obtain complete topographic data from the standard stereo imagery. Starting in 2009, Merrick’s LiDAR is providing us with a DTM that is more accurate than anything to date. They are collecting the LiDAR data at an accuracy specification that will support the generation of 1-foot contours, in accordance with our specifications. We need that level of accuracy for flood risk analysis in numerous low-lying, relatively flat areas within Leon County.

"We’ve learned we’re also economically benefiting from the use of LiDAR because it offsets some of the field work that is normally needed for engineering projects," Mauldin says. "The existing LiDAR data is accurate enough to minimize the need to deploy surveying teams for the planning phase of capital improvement projects. We already have much of the topographic information we need through the LiDAR collection, thus we’ve eliminated a significant portion of pre-project surveying and saved some dollars. Moreover, the increased accuracy of the new 2009 data is sufficient to support the engineering design phase of many projects."

Cost benefits of LiDAR
As a leader in the use of LiDAR, TLC GIS understands the benefits of the technology. It’s faster, easier, more accurate, more cost effective, more complete, and leads to higher quality derivative mapping from the base information. With the program, TLC GIS has been able to meet its budgets while enhancing the overall quality, accuracy, and timing of the data.

"In today’s economy, every public dollar spent is being evaluated, and the prudent use of those public dollars is a top priority for us," says Hartsfield. "Using LiDAR for data acquisition provides the opportunity to be financially responsible, while providing us with highly accurate data that serves multiple agencies’ multiple needs. It allows us to optimize every dollar we spend."

An increasing number of public agencies are taking advantage of the benefits of using LiDAR for their mapping needs. From flood plain mapping and emergency preparedness to environmental habitat modeling, water management, and public infrastructure planning, LiDAR is growing in use and popularity.

This article was contributed by Merrick & Company, a $75 million geospatial mapping, engineering, architecture, design-build, and surveying firm that serves domestic and international clients in the government, energy, life sciences, infrastructure, and mapping markets.


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