Comparison of three grain size measuring methods applied to coarse-grained gravel deposits

The size of grains in gravel and conglomerate deposits is most easily measured on photos taken from related outcrops. However, the occlusion of grains by the sedimentary matrix or other grains, and possible distortions of photos, could introduce a bias in such datasets. Here, we explore the uncertainties associated with datasets where the lengths of the grains were measured on photos. To this end, we analysed coarse-grained (>2 mm) fluvial material from a gravel pit (Bern, Switzerland). We compared grain size data collected from digital photos with the results where the same material was measured with a calliper and mechanically sieved. Our analyses reveal that the percentile values such as the D16, D50 and D84 of datasets where the grains' longest visible axes were measured on digital photos best correlate to the corresponding percentile values of data collected through sieving. We also find that the longest visible axes of grains measured on digital photos are c. 17 % smaller than the lengths of the intermediate b-axes of grains measured with a calliper. We therefore suggest to measure the longest visible axes on digital photos, and to correct the data by a corresponding factor such as +17 % for the target grain size percentiles.