Gravel Size Analysis from Photographs

Gravel sizes measured from photographs of river beds are biased estimates of size when compared to standard sieve analysis. Mean apparent short axis size (for gravel coarser than 8 mm) from photographs may be converted to sieve-equivalent mean size by subtracting 0.1 phi (if mean size in phi units) or multiplying by 1.07 (if mean size in mm). Percentile of the distribution also differ by the same factors. Measures of grain size standard deviation from photographs are approximately equivalent to sieve values. For sizes in phi units the photograph and sieve size frequency distribution curves have similar shape, but are offset by the same bias as the mean size.