Photogrammetric monitoring of small streams under a riparian forest canopy

The recent advent of digital photogrammetry has enabled the modeling and monitoring of river beds at relatively high spatial resolution (0.01 to 1 m) through the extraction of digital elevation models (DEMs). The traditional approach to image capture has been to mount a metric camera to an aircraft, although non-metric cameras have been mounted to a variety of novel aerial platforms to acquire river-based imagery (e.g. helicopters, radio-controlled motorized vehicles, tethered blimps and balloons). However, most of these techniques are designed to acquire imagery at flying heights above the riparian tree canopy. In relatively narrow channels (e.g. <20 m bankfull width), streamside trees can obscure the channel and limit continuous photogrammetric data acquisition of both the channel bed and banks, while still providing useful information regarding the riparian canopy and even spot elevations of the channel. This paper presents a technique for the capture and analysis of close-range photogrammetric data acquired from a vertically mounted non-metric camera suspended 10 m above the channel bed by a unipod. The camera is positioned under the riparian forest canopy so that the channel bed can be imaged without obstruction. The system is portable and permits relatively rapid image acquisition over rough terrain and in dense forest. The platform was used to generate DEMs with a nominal ground resolution of 0.03 m. DEMs generated from this platform required post-possessing to either adjust or eliminate erroneous cells introduced by the extraction process, overhanging branches, and by the effects of refraction at the air-water interface for submerged portions of the channel bed. The vertical precision in the post-processed surface generally ranged from +/- 0.01 to 0.1 m depending on the quality of triangulation and the characteristics of the surface being imaged. Copyright (C) 2010 John Wiley & Sons, Ltd.