OF TIME AND THE SOIL: DETECTING MICROTOPOGRAPHIC CHANGES USING EXTREME CLOSE-RANGE PHOTOGRAMMETRY

Having the ability to detect small changes in hillslope soil movement can provide valuable insight into assessing the effects that land surface disturbing activities can have across the landscape. Land surface elevation changes as small as a few millimeters can have far reaching implications when put into the context of soil loss over hundreds or thousands of acres. Current methods for detecting this level of change over such large areas are cumbersome and cost prohibitive. However, utilizing recent innovations in high-quality digital cameras and 3D measuring and modeling (3DMM) software on selected plots can be a cost effective method for capturing data that is accurate to a sub-centimeter level. This data can be captured, in the field, by simply taking a series of overlapping stereoscopic photographs (an object of known dimension must be placed in the photo overlap to accurately scale the project). The 3DMM software is used to extract 3D information from the 2D images by calibrating and deriving camera information. From this information a digital terrain model consisting of a closely spaced grid of thousands of x, y, z data points is automatically generated. Digital terrain model grid spacing 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 and detecting change.

The Bureau of Land Management is responsible for monitoring activities on public land which could have on-site or off-site resource impacts. The Uncompahgre Field Office in Montrose, CO requested assistance from the National Science and Technology Center in developing monitoring techniques to help determine changes in high use areas. The monitoring sites were photographed in both 2005 and 2006. Highly detailed terrain surfaces of the monitored sites were created and evaluated for elevation changes using extreme close-range photogrammetric techniques. This photogrammetry methodology has promising potential application for assessing impacts from a wide array of ground disturbing activities including off-highway recreational vehicle use, post-fire stabilization monitoring, energy exploration and development, and other soil disturbing activities.