Statewide aerial photo and LiDAR data support engineering and surveying uses across Ohio.

When the state of Ohio decided to replace its decade-old, 1-meter-resolution aerial photos with high-resolution, digital color imagery, Stu Davis, chairman of the Ohio Geographically Referenced Information Program Council, had a dilemma: How to acquire a digital elevation model (DEM) to support orthorectification of the new imagery? Davis considered the following options:

• use the U.S. Geological Survey’s (USGS) National Elevation Dataset (NED); however, at 30 meters between points, these DEMs from the 1970s couldn’t provide the accuracy level needed to render orthophotos;
• compile a more accurate DEM of the entire state using traditional photogrammetric methods, which would be time and cost prohibitive; or
• use Light Detection Imaging and Ranging (LiDAR), which collects highly accurate elevation data via aircraft-mounted laser equipment.

LiDAR, a technology that can quickly and cost-effectively produce elevation data that is accurate within 1 to 2 feet in most terrain, proved to be the best solution. Dayton, Ohio-based Woolpert, Inc., performed both the aerial photography and the LiDAR work.

The collected LiDAR data—literally billions of points, with an average 2-meter spacing between points representing 3-D location/elevation data on terrain, waterways, roadways, buildings, and even tree cover—was used to create new DEMs. The new aerial images were draped over the DEMs to orthorectify the imagery so the scale is uniform and, in effect, becomes a map able to measure true distances. The resulting dataset provided by Woolpert consists of digital aerial imagery/LiDAR points, color digital orthophotography, color infrared digital orthophotography, and a LiDAR DEM. The imagery can be shown in 3-D and offers multiple views simply by rotating the images.

Because the DEM data accurately maps the shape of land surfaces in 3-D, it has many engineering uses beyond its primary purpose. Such uses include hydraulic, hydrologic, and terrain modeling; watershed analysis; landslide prediction and erosion control; flood studies and floodplain mapping; and related emergency planning. Additionally, the data is available for anyone who would like to use it via several delivery mechanisms, including an online map service.

Click here to read the rest of this article and view the LiDAR-generated images.

Bob Brinkman, CP, PLS, project director, is vice president for Woolpert, Inc. He can be contacted at bob.brinkman@woolpert.com. Brian Stevens, CP, project manager, is an associate at Woolpert, Inc. He can be contacted at brian.stevens@woolpert.com.