Orthophotos Key to Planning Storm Recovery in Superior National Forest

When a freak storm lashed northern Minnesota with 90-mph winds last July 4, what had once been an orderly landscape of white pines, tranquil lakes, hiking trails, and canoe portages in the Superior National Forest had turned into a disaster area. In less than an hour, the winds had blown down an estimated 40 million trees in a 447,000-acre area of the forest.
      The Forest Service responded immediately by taking preemptive steps to locate and mitigate the worst damage that could negatively impact the area for years to come. Rescuing victims stranded in the forest was, of course, the first priority, but soon thereafter the Forest Service set out to map the destruction as one of its initial actions to protect forest resources and future visitors from additional danger.
      These new maps, combined with data in an existing Superior National Forest GIS, would assist in determining the full extent of the damage.
      Within weeks, the Forest Service arranged to have the hardest-hit areas photographed from the air.
      The natural color photos provided a quick look at tree blowdowns and other major wind destruction-information that was valuable in those early phases of clean-up and recovery. But when these rectified photos were compared with the GIS basemap, disparities in accuracy made it clear that orthorectification would be required to adequately map the details of the forest damage using the air photos.
      The Superior National Forest GIS Coordinator, Terry Gokee, knew that orthorectification of the air photos would be a time-consuming process. He also knew that Forest Service crews were in a race against the clock to identify priority areas for clearing or removal of as many fallen trees as soon as possible. Once summer returned, the forest would be visited by both tourists and fire, a potentially deadly mix with so much fresh kindling lying about.
      As long-time users of geographic imaging software from ERDAS Inc. of Atlanta, Ga., Forest Service GIS personnel were aware of a new product called IMAGINE OrthoBASE that could orthorectify large blocks of images quickly, with minimal ground control. Having never used the new package themselves, they contracted LookSee Inc., a remote sensing/GIS consulting firm in nearby Brainerd, Minn., to conduct the orthorectification for them. LookSee had served as an IMAGINE OrthoBASE beta test site and was well versed in its use.
      By early March 2000, the firm had orthorectified more than 50 percent of the air photos and supplied them to the Forest Service for use in inventory mapping. Had the Forest Service used traditional orthorectification techniques, the project would be only 20 percent finished, with little hope of completion by summer.

Recovering from the Blowdown
The Superior National Forest in northeastern Minnesota covers some 3 million acres that include the Boundary Waters Canoe Area Wilderness (BWCAW). With 200,000 vacationers annually, the BWCAW is the most visited designated wilderness area in the United States.
      This means that hundreds or thousands of visitors could potentially lie in the path of danger should a forest fire break out during the summer season. Although removal of trees is forbidden by law in wilderness areas, the Forest Service is working to clear as much of the fallen timber as possible from the surrounding National Forest to reduce the availability of fuel for fires.
      "Forest fires are driven by weather and fuel conditions, and many are started by lightning strikes," said Gokee. "Many smolder and burn out, but others can spread depending on wind and moisture content of the fuels."
      A rapidly spreading fire is by far the most serious danger to life and property posed by fallen timber, but other hazards loom in the forest as well. Trees blown to the ground have dammed streams in many areas, posing risk to canoeists and inducing erosion. And in some areas, spring-loaded or leaning trees have the potential to fall on or hit canoeists.
      The BWCAW surrounds part of the forest known as the Gunflint Trail Corridor, which is of particular concern to the Forest Service because it is a focal point for recreational development outside the wilderness area. Many resorts and campgrounds have been built in this corridor along the borders of the BWCAW, making it a popular tourist destination as well. For safety reasons, the Forest Service has targeted fuel reduction efforts in the Gunflint region.
      The Forest Service's Forest Health Technology Evaluation Team acquired aerial photography over the corridor and adjacent regions on July 26-29, 1999. Flown at an altitude of 7100 feet, 457 air photos at a scale of 1:8000 were taken of large parts of eight USGS 7.5' quadrangles.
      "We rectified the photos, and when we compared them to the orthorectified 1990 aerial photographic basemap in our GIS, we found differences in feature locations ranging from between 10 and 40 meters," said Gokee.
      The rectified photos were sufficient for identifying blowdown zones, but they did not offer the accuracy needed to measure their locations relative to roads and trails, map their boundaries, and calculate their surface areas. This information is crucial for planning recovery efforts such as timber salvaging, prescribing controlled burns, and hand-treating weakened trees.

Orthorectifying by the Block
The Minnesota Department of Natural Resources scanned the air photos and saved them as JPEG files for delivery to LookSee for orthorectification, contrast stretching and mosaicking. LookSee converted the files into the .img format because they would be easier than JPEG files to process in ERDAS IMAGINE and IMAGINE OrthoBASE.
      IMAGINE OrthoBASE is ideally suited for this project because it excels at orthorectifying large blocks of imagery at one time, while requiring minimal ground control. In this project, the orthorectified photos were to be mosaicked into their respective USGS quads, and there were between 20 and 104 photos per quad.
      "Even if we had the time to orthorectify each photo individually, there is no way to mosaic them and expect them to fit together perfectly," said Ben Drake, president of LookSee. "Block orthorectification is the key to mosaicking such a large number of images."
      He added that individual image orthorectification would have required about nine control points per photo, whereas IMAGINE OrthoBASE needs an average of one point per image in the block.
      Picking control points has been the most time-consuming part of the procedure. Drake is using 1991 USGS Digital Ortho Quads to locate X and Y coordinates for points, and USGS 7.5' Digital Elevation Models to extract Z values. Control points are selected manually in IMAGINE OrthoBASE.
      "There were many roads visible on the DOQs that would make good control points. I thought this would be a quick procedure-until I saw the air photos," said Drake. "Many of the 1991 roads had been grown over, or new roads had been built in the intervening time."
      Instead of roads, Drake has primarily used hydrographic features to pick his points, because streams and lakes are plentiful in the forest. But even this has been difficult because many shorelines are obscured by fallen trunks and limbs. He estimates spending about 20 to 30 minutes to locate each point on the DOQ and then find it in the overlap area of two to six photos.
      Once basic information about the camera is entered into the software, and ground control has been established for all photos in a block, IMAGINE OrthoBASE generates tie points in overlapping photos based on pattern-matching techniques. The tie points are examined individually, and the very few that have been located incorrectly are deleted.
      "Instead of having just 20 control points for a 20-image block, you have several hundred tie points which essentially become additional control points," said Drake. "This adjusts the errors you have in tying the photos to the ground, and distributes the errors throughout the whole block to minimize the errors and maximize accuracy."
      Then aerial triangulation is carried out in IMAGINE OrthoBASE using the ground control points and tie points. During the aerial triangulation, bundle block adjustment defines the mathematical relationship between the aerial photos and the ground coordinates.
      "Once that is completed, OrthoBASE performs the orthorectification," said Drake. "And to check it, in the viewer we overlay an orthorectified image on adjacent orthorectified images and also the DOQ to visually inspect how well the same points line up. If the same point is not coincident on the orthorectified images and DOQ, we measure the X and Y offsets to make sure they are within standards."
      LookSee's goal was to achieve a final accuracy equal to 1:24,000 map scale, per Forest Service request.
      Drake then uses image-processing functions within ERDAS IMAGINE to stretch the histogram of the orthorectified images to make them easier to interpret. Finally, the orthorectified air photos are mosaicked in ERDAS IMAGINE. The individual orthorectified air photos and mosaics are delivered on CDs to the Forest Service as both .img and JPEG files.
      "The Forest Service is very satisfied with the accuracy and quality of the orthorectified air photos and mosaics," said Drake, adding that LookSee is also applying other image processing techniques to the orthorectified images and mosaics to highlight the blowdown areas.

Using the Orthophotos
As the orthorectified photos and mosaics for each quad are processed and delivered, the Forest Service is loading them directly into the Superior National Forest GIS for comparison with existing data layers. The most immediate use for the orthomosaics is to create an updated inventory of tree cover and its conditions.
      This information is being used to plan the recovery efforts in the Gunflint Trail Corridor. Some of the analysis has already been included in the recently released Gunflint Environmental Impact Statement. But Forest Service GIS experts say the orthorectified photo mosaics will have usefulness in other operational projects in the forest for years to come.
      "If there is a fire in the forest this summer, we will use the mosaics to produce site-specific maps for use in managing the incident," said Gokee. "In fact, we will use the orthophoto map as a basemap for just about any project that comes along in the future, because it's the best information we have available to us." For more information on the storm recovery and the Superior National Forest, visit www.fs.fed.us/r9/superior/.

About the Author:
Kevin Corbley is the principal in Corbley Communications, Inc. in Littleton, Colo. He may be reached at 303-979-3232, or by email at [email protected].

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