HICKORY RUN BOULDER FIELD (2): COLLECTING HIGH-RESOLUTION, LOW-ALTITUDE AERIAL PHOTOGRAPHS BY UAV

Recent advances in low-cost digital cameras, batteries, electric motors, and rectification software provide an opportunity to collect inexpensive, high-resolution aerial photographs for geologic research. This study evaluated the merits and challenges of using an Unoccupied Aerial Vehicle (UAV) to acquire aerial photographs of Hickory Run Boulder Field located in Carbon County, PA. Efforts to interpret the geomorphic evolution of the field have focused on patterns of boulders ranging in size from 0.1 to 10 m in length, requiring aerial photographs with sub-decimeter resolution. UAVs have the potential to record aerial photographs at this resolution because they can be operated safely at low (<100 m) altitude.

The digital camera employed by this study had 8.1-megapixel resolution, full-manual capability (ISO, focus, aperture, and shutter speed), a weight of 162 g, and was equipped with a 1-gigabyte memory card. A radio-controlled airplane constructed from foam with a wingspan of 1.6 m was used as the aerial platform. This UAV was powered by a 4.4:1-geared, neodymium brushless motor with a 3-cell lithium polymer battery. The camera was secured to the UAV within a polyethylene container, set to continuous shot mode, and triggered by a thumbscrew. The UAV was controlled using a 4-channel, pulse-code modulated radio. Approximately 325 images were collected during each flight. Selected images were orthorectified with a polynomial algorithm using recognizable, surveyed boulders as control points.

The UAV images revealed boulders and patterns that were indiscernible using standard aerial photographic methods. Typical UAV images taken from an altitude of 100 m covered an area 100 m by 75 m with a resolution of 0.03 m. This resolution was superior to photographs taken from a single-engine plane with a 35 mm camera (0.22 m resolution) or Pennsylvania's PAMAP orthophotos (0.30 m resolution). Image rectification was a daunting challenge given the number of images collected (over 1000), the lack of consistent aircraft altitude and attitude, and difficulty of identifying individual boulders in a field of over 10 million. We conclude that collecting aerial photographs by UAV is a labor-intensive, yet cost-effective approach that can yield outstanding photographs for geologic research.