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Ohio Uses Lidar and Aerial Photos to Survey State

May 6, 2009 By

Fly over Ohio at 30,000 feet and its cities and counties -- like much of the Midwest -- look like a patchwork quilt.

Until recently, so did imagery and GIS data across the state.

But information that was once "stitched together" is now seamless, thanks to a multiagency effort called the Ohio Statewide Imagery Program (OSIP). The program's data is making government technology professionals' jobs more efficient and their work far more precise.

"It's complete and accurate data that anyone can access for mapping, engineering and planning," said Stu Davis, executive director of the Ohio Office of Information Technology's Ohio Geographically Referenced Information Program (OGRIP) -- the agency that spearheaded OSIP.

In 2006, the state decided to replace its decade-old, 1-meter black-and-white aerial photos with high-resolution digital color imagery. But the imagery would need to be orthorectified (corrected for surface elevation changes) in order to truly be usable as maps depicting accurate distances.

Davis had a problem. He needed a highly accurate digital elevation model (DEM) over which the imagery would be "draped" to orthorectify the imagery. The existing DEM -- the U.S. Geological Survey's (USGS) National Elevation Dataset, spaced points at 30 meters apart -- hardly provided the accuracy needed.

"A lot can vary within 30 meters," Davis said. "The USGS DEMs still have value, but not for 21st-century state and local government requirements. To have used the existing DEM would have created a throwaway product."

One alternative would have been to compile a new DEM of the entire state using traditional methods of surveying and photogrammetry (determining geometric data from photographs). But these methods are often time- and cost-prohibitive.

Woolpert Inc., the Dayton, Ohio-headquartered engineering firm hired to do the aerial flyover, proposed a third alternative: Use lidar (light detection and ranging) to create a new, highly accurate DEM.

Lidar is a method of collecting highly accurate elevation data via an aircraft with mounted laser equipment. The data is accurate within plus or minus 1 foot in most terrain. Billions of lidar-generated points represent 3-D location/elevation data of terrain, waterways, roadways, buildings -- even tree cover.

"Lidar is a best-of-both-worlds technology. It's fast, accurate and the data has multiple uses," said Bob Brinkman, project director and senior vice president of Woolpert. "In terms of cost, it of course is more than free USGS data -- but using that data would have created unusable maps. On the other hand, lidar costs far less than the traditional surveying option."

The total cost for the flyovers and producing a complete data set: $5.5 million. The data set includes digital aerial imagery/ points, color digital orthophotography, color infrared (CIR) digital orthophotography and a lidar DEM.

"The imagery can be shown in 3-D and offers multiple views simply by rotating the images," said Brian Stevens, project manager for Woolpert. "Because the DEM data accurately maps the shape of land surface in three dimensions, it has many engineering uses beyond its primary purpose."

Work began in 2006 and will be completed in 2009. Almost all of Ohio's 88 counties have now been flown, first capturing the aerial photos and then the lidar data. The same equipment has been used each time to ensure consistent data.

Subsequent field-survey checks have yielded results of two- to three-tenths of a foot vertical accuracy.

Ohio is one of the first states to create a seamless and highly accurate imagery data set, a blueprint for a larger goal of a seamless data set for the entire nation.

Image: Lidar data can be used to produce 3-D, photo-like renderings.

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