ACCURACY ASSESSMENT OF AERIAL PHOTOGRAPHS ACQUIRED USING LIGHTER-THAN-AIR BLIMPS: LOW-COST TOOLS FOR MAPPING RIVER CORRIDORS

Monitoring river systems with repeat aerial photography is a powerful tool although the temporal resolution of surveys is rarely performed at anything better than annual time-scales. In recent years, a variety of low-cost aerial platforms for acquiring aerial photography have emerged. While these economical options may facilitate more frequent repeat surveys, the accuracy of imagery needs further consideration. The accuracy of imagery obtained from a Lighter-Than-Air Blimp is investigated through two simple experiments. The first looks at the geospatial error of aerial photographs derived using five geometric transformation models, and the sensitivity of the photo registration quality to various ground control point (GCP) configurations and densities. At high GCP densities, higher order polynomial transformation models provide the highest quality registrations. However, at more modest GCP densities (i.e. 19-28 GCPs Ha(-1)), simple aerotriangulation and 2nd order polynomial transformation models perform modestly, resulting in registration errors at standards equal to or better than obtained with conventional aerial photography (e.g. 0.5-1 m). The quality of image registration is highly dependent oil the configuration of GCPs. In a second experiment, the practical utility of producing a mosaic of blimp acquired imagery is explored over a kilometre long braided reach. Even at relatively low GCPs densities (e.g. 9 GCPs Ha(-1)), a mosaiced aerial of the entire reach can be produced of adequate quality to support bar-scale mapping of patch-scale features. The survey required less than a single day of field work and laboratory processing, and presents a cost-effective alternative to traditionally commissioned flights. Copyright (C) 2008 John Wiley & Sons, Ltd.