State researchers examining Kenai River bank erosion want to harness a tool they say will allow them to peer back in time to determine how the river has changed to become what it is today.
The Alaska Department of Fish and Game recently completed a study to test the use of orthorectified photographs in conjunction with aerial photogrammetry to examine the Kenai River’s banks and landscape within 200 feet of the river’s banks.
This two-part tool allows both historical and current aerial photos to be used to create accurate landscape maps that reach into the past and compare it to the present, by correcting distortions found in photographs taken from the air.
Fish and Game would like to use orthorectification and aerial photogrammetry to correct aerial photo sets it has of the Kenai River dating back to 1975 and compare them to contemporary orthorectified aerial photo sets.
Although to the naked eye photographs may appear to accurately depict what we see, the image seen in a photograph is actually distorted. The degree of distortion is greatest at the edge of the photograph and varies depending on how big of an area the photograph was used to capture.
These distortions are typically too small to bother the casual observer, but render photographs almost useless to researchers trying to create precise maps.
“If you want to use it for measurement ... that distortion can be huge,” said Mary King, a Fish and Game fisheries biologist. “It’s one thing if you are taking a picture of a 12-by-12 surface, (but) if you’re taking a picture of a landscape from an airplane, you’re magnifying that error potential hugely.”
In orthorectified aerial photographs, however, distortions have been corrected, transforming the photographs into valuable tools that researchers can use to examine how development, recreational activities and restoration projects have influenced changes on the Kenai River.
“It also tells you what people may have done to give back as well. It’s not just a picture of loss. There is some return,” she said. “That’s the power of the tool, that you can look at all of the permeations of change.”
Fish and Game first started looking into using orthorectified photographs and aerial photogrammetry in 1998 and decided to test the tool to determine if it would be accurate enough to answer the questions it had about changes occurring on the river. Now that the department has completed their test run of the tool, King said they are satisfied with the results.
When used to measure the river’s banks, the tool was accurate to within a foot and when used to measure “cover class” areas on the river’s surrounding landscape, it had an error rate of less than five percent.
“Even if we had a five percent error, you still have a tool that is measuring change enough for land managers to use to determine if it’s an acceptable change,” King said.
Now that Fish and Game has finished its test run of the tool, they are now seeking funding to support a project that would examine changes along a 50-mile stretch of the Kenai River using photo sets taken in 1975, 1984 and 1998.
The project would be costly, $350,000, but would provide a comprehensive look at how the river responds to changes in development, recreation and restoration projects in a way that a project that does not use orthorectified photographs and aerial photogrammetry cannot, King said.
This two-part tool has already been widely recognized and embraced for it’s usefulness in the Lower 48, she said.
“It’s becoming more and more utilized for all sorts of projects in the Lower 48. Alaska just hasn’t done a lot with it,” she said.
Patrice Kohl can be reached at firstname.lastname@example.org.
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