|Planer-Friedrich, B; Becker, J; Brimer, B; Merkel, BJ: Low‐cost aerial photography for high‐resolution mapping of hydrothermal areas in Yellowstone National Park, International Journal of Remote Sensing, 1, 1-14 (2008)|
A site-specific high-resolution aerial survey was conducted to map a status quo of two rapidly changing hydrothermal areas in Yellowstone National Park. A 2 m3 helium balloon released to 50–80 m altitude served as a low-cost platform easy to launch and retract on a highly flexible operating schedule. Pictures were taken with a digital camera with automatic interval shutter release. Owing to the balloon's sensitivity towards wind and the limited possibility of controlling its accurate positioning, the acquired aerial pictures varied significantly in altitude, tilt, and orientation. Automatic stitching was possible in areas where nearly vertical pictures with sufficient overlap existed. Those aerial pictures with little overlap or taken at a highly oblique angle had to be manually post-processed based on temporary marked ground control points and additional tie points (total time for processing: 10–14 days) to obtain a final mosaic with minimal distortion (0.4 m±0.3 m) and an absolute accuracy of approximately 0.5 m. Ground resolution was better than 2 cm. Vegetation and hydrothermal feature classification, biological and redox zonations, water contents of mud pots, gas activity in hot springs as well as subaquatic sedimentation fans and hydrothermal vents could be outlined, providing a highly detailed thermal inventory for future change detection.