Digital multi-image photogrammetry combined with oblique aerial photography enables glacier monitoring survey flights below clouds in Alaska

Usually "bad weather" for aerial surveys in mountain terrain means something like normal weather. Only exceptionally good weather conditions can be used successful for vertical aerial photography. In the coastal mountains of Alaska this kind of weather is rare enough to seriously hamper survey activities. Recognizing this problem, a method for survey flights that can deal with far less than ideal weather conditions was developed and is now being tested in a joint venture with the USGS Glaciology section in Fairbanks, Alaska. Our current study is focused on evaluating a new, self contained approach to study high mountain geomorphology using a close range remote sensing method with a low flying light aircraft as a basis for digital oblique aerial photogrammetry. With a special type of camera and digital multiimage photogrammetry system its now possible to extract DEM (Digital Elevation Models) from oblique aerial photography, acquired out of the open window of almost any light aircraft. With this method the aircraft is not bound to a straight flight path and can move freely around and over the target area. This allows it to fly very close to a specific target and get a very high accuracy, depending on target size and distance to it. Even more important, any weather good enough for normal photography and "mountain flying" can be used to get data There are few restrictions on the weather unlike in the case of the usual vertical aerial photography, which requires clear skies, no clouds, and is hampered by strong thermal activity in the air. Especially in a oceanic high mountain weather environment this is a significant advantage, since weather delays are significantly reduced. Depending on target area location, it here is normal to fly low under the clouds within the mountain valleys and approach the photo destination. With this method one flys around the target and takes the measurement pictures from all sides of it, under the clouds. If weather conditions allow, vertical pictures can also be used and built into the system. Usually the accuracy is as good as with vertical aerial standard cameras and operator measured photogrammetry systems. With digital multiimage photogrammetry automated surface measurements are now possible and produce good results. The mating of the camera with a DGPS and a INS unit would create approximate camera positions, that could lead to establishing this method as a survey without the necessity of ground control (GCP's) in the near future