Ottawa National Forest
Ottawa National Forest
Midwest Aerial Demonstrates Benefits of Digital Acquisition in Ottawa National Forest
Just four days after receiving the Notice to Proceed from the U.S. Department of Agriculture, Midwest Aerial Photography had its aircraft onsite and ready to acquire digital imagery of the Ottawa National Forest in Michigan. Six days later, the aerial firm had successfully completed collection of 30-cm image data over the entire one-million acre national forest during an extremely tight acquisition window.
Although the schedule was aggressive, the quick turnaround wasn’t the only thing that impressed the USDA Forest Service about the acquisition project. What also turned heads was Midwest Aerial’s ability to provide digital image products in a format and scale that almost exactly matched the aerial film prints USDA has been acquiring for forest mapping since the 1930s. More importantly, Midwest Aerial completed the digital acquisition and production for less than the cost of a comparable film project.
The RFP had been prepared and released by the USDA Aerial Photography Field Office (APFO) in Salt Lake City on behalf of the Ottawa National Forest on Michigan’s Upper Peninsula. As was the norm for the Forest Service, Ottawa requested near-infrared film photography. Midwest Aerial submitted two proposals, one for film and another for digital acquisition – at a lower cost. The Ohio firm received the best value award based on technical merits and its price, which was among the lowest of all the proposals.
Even as the contract was being awarded, the acquisition window became increasingly important to the applications planned by the national forest officials. They requested acquisition during the approximately 14-day period just prior to leaf-on when trees have budded. That spring, however, had been unusually warm, and the buds were popping early. Without quick acquisition, the project would fail.
“Mobilizing one of our six aircraft on such short notice really wasn’t a big challenge for us,” said Midwest President Ken Scruggs. “Our business focus is aerial imaging and we’ve built our reputation on successfully acquiring data when the client specifies.”
The aircraft carried one of Midwest Aerial’s two Z/I Imaging DMC II digital mapping cameras, which are large-format frame sensors designed specifically to mimic many of the characteristics of film cameras. The DMC II offers the advantage of collecting four-band (R,G,B, NIR) digital data simultaneously from which a wide variety of end products can be generated. For Ottawa, Midwest flew 68 flight lines to collect 2,205 frames at 30-cm GSD which equated a film scale of 1:15,840.
Even with condensation from nearby Lake Superior creating a steady stream of clouds, the airborne segment was completed between April 20 and 26, 2010. Within four weeks, Midwest Aerial had delivered four-band georeferenced image files, photo center data files with metadata, a compressed project mosaic and a set of color infrared inspection prints. Generated from the digital data sets, the inspection prints were produced with the same look and feel of traditional film contact prints.
“The client was impressed with how closely the square-format DMC II image prints emulated the traditional film products,” said Scruggs.
For decades the APFO has been collecting film photography on behalf of the Forest Service to assist in managing its lands. Consistency in both the format and scale of the aerial images is critical to many National Forest management activities, which often involve analyses of multi-temporal images to detect changes in forest conditions and health. According to the project RFP, the Ottawa National Forest planned to use the 2010 images to manage multiple forest resources and perform inventories.
The only minor glitch that occurred in the project was not discovered until after the airborne phase was complete. The RFP had omitted unintentionally the delivery of airborne GPS points among the end products. The National Forest planned to send the delivered four-band digital files from Midwest Aerial out to a third-party photogrammetric services company for orthorectification. This level of processing would have been impossible without precise GPS coordinates of the camera position, but the project budget did not include funds to send ground survey crews out to the site to survey photo-identifiable features.
Fortunately, the DMC II operates with a tightly coupled NovAtel GPS/IMU (Inertial Measurement Unit) that automatically records the precise position of the camera for each exposure during the flight. This feature allowed for a significant reduction in the number of ground control points needed to maintain accuracy for the final orthophotography. Midwest Aerial provided this data set to the photogrammetry firm, preserving the overall project budget and timeline.