Maurizio wrote: > > FLIGHT METADATA IS THE CRITICAL ISSUE: your final results will be as > good as > your flight metadata is. > > Don't ask: I have no idea how that data is actually collected :D
I can tell you a little bit about how it's done on the professional end - the plane's position is recorded with differential GPS, corrected against a base station ideally within about 50 miles. The orientation of the plane is recorded with an IMU (inertial measurement unit). Our contractor explained that the ownership of such units of high levels of accuracy is regulated in the USA, though I don't know the exact specifications or legal restrictions he was referring to. The camera is also mounted in a servo gimbal which not only keeps the camera oriented downwards, but can twist the camera to keep the frame oriented correctly if the plane is "crabbing" due to a sidewind, and also compensates for the forward motion of the plane by moving the camera backwards when the shutter is triggered. They have software for the GPS post-processing and another package which interpolates the position and orientation of the plane to the shutter trigger time, as well as doing the geometric computation to calculate the actual camera lens position (relative to the GPS antenna). As you might expect this is million dollar plus equipment. I'm not an expert on what is available for UAVs, but I would expect you would be able to get GPS down to at submeter with some cleverness. Getting the orientation of the plane will be trickier - at a minimum you can get the approximate heading of the plane from the GPS log, but from what I've seen from the Camp Roberts UAV imagery it's not very reliable. You would probably want a digital compass and some sort of 3d gyro or accelerometer package to try to get the plane orientation. I suspect one of the challenges is not just getting high enough resolution at consumer prices, but also getting a fast enough response time. Most UAVs don't have as much mass as a Cessna and move around a lot more. At the recent Random Hacks of Kindness event I was working with OAM listmember Jeffrey Johnson and Tim Caro-Bruce of Urban Mapping to create a system that would take roughly positioned UAV imagery and then using SIFT image matching would more precisely georeference each frame to existing orthoimagery. I have the code almost finished - if this turns out to be a wet weekend I'll hopefully get it done. Marc _______________________________________________ talk mailing list [email protected] http://openaerialmap.org/mailman/listinfo/talk_openaerialmap.org
