An Extreme Close-Range Photogrammetric Method for Monitoring Soil Erosion.
William Ypsilantis, Neffra Matthews, Tom Noble, Victoria Josupait, and Dennis Murphy. USDI, Bureau of Land Management, Denver Federal Center Bldg. 50, Denver, CO 80225-0047
Extreme close-range, less than 150 meters, photogrammetry is an excellent, cost effective method for monitoring soil erosion. The technique developed enables precise measurements to be made from photographs taken with affordable six megapixel or higher single lens reflex or eight megapixel point and shoot digital cameras. The principal requirements to create these measurements are complete plot coverage by overlapping photos for stereoscopic viewing, and adequate x, y, z control for defined points within the overlapping area of the photographs. The camera is calibrated for focal length, format size, principal point, and distortion coefficients using three-dimensional measuring and modeling software. The calibrated camera is then used to capture ground control x, y, z coordinate data in a series of oblique, orientation photographs which are taken of the subject area with circular reference targets, circular coded targets, and an object of known dimension placed within the target layout. A series of stereoscopic photographs are taken of the subject concurrently with the oblique-orientation photographs to facilitate stereoscopic viewing of the subject area. A digital or softcopy photogrammetric workstation recreates, in three dimensions, the geometry of the soil surface for the purpose of compiling data on it. The software creates a digital terrain model that consists of a closely spaced grid of thousands of x, y, z data points. Ditigal terrain model grid spacings of one to two millimeters and positional accuracies of one quarter of a millimeter can be determined for areas up to five square meters. It is possible to make repeat visits to the same location for the purpose of monitoring or change detection. It is necessary to establish monumented points to achieve high levels of accuracy of change detection. The data collected over a time period can be used to precisely document erosion rates or other changes in soil surface features, such as microtopography of biological soil crusts. The purpose of the pilot project for the BLM Montrose, Colorado Field Office was to monitor extreme Off-Highway-Vehicle use in Dry Creek Basin. The three dimensional monitoring and modeling software also has the capability to use historical photographs to detect change from previous time periods. The accuracy for these type projects will be less due to the lack of camera calibration and the lack of an adequate number of oblique photographs to establish camera orientation.