Using Outcrop-Scale Digital Images for Size Distribution of Boulders and Blocks

Digital images are used commonly during site characterization. Placing range features in view permits dimensions to be estimated. Terrestrial photogrammetry developed for rock structure analyses can be used for reasonably precise measurements of boulders and blocks exposed at the ground surface. Simple image analysis provides a means of inexpensive, rapid estimates of parameters needed for block dimensions and size distributions. Images collected with calibrated cameras are needed for terrestrial photogrammetry; the same images or images taken with uncalibrated cameras can be used with public-domain software for simple collection of size information. ShapeMetriX3D (3G Software and Measurement) and ImageJ (National Institutes of Health) software are used to demonstrate the utility of these approaches for colluvial deposits in a California road cut, a boulder-bar on an Oregon stream, and a rocky headcut in a Utah channel. Parallax distortion is corrected with photogrammetry, but not with image analysis. Sources of error in image analysis need to be considered with the variability in earth materials and how well the distribution of rock blocks or boulders is represented by the visible exposure. Dimensions determined with photogrammetry can be used as image-analysis scale references.