Automated granulometric analysis and grain-shape estimation of beach sediments using object-based image analysis

Urbanski, J. A., Wochna A., Herman A., 2011. Automated granulometric analysis and grain-shape estimation of beach sediments using object-based image analysis. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), – . Szczecin, Poland, ISSN 0749-0208 This work presents an automated, image-analysis based method for determining the grain characteristics of sandy, unconsolidated beach sediments. The main purpose of the study is to develop an efficient and reliable alternative to the traditional, very laborious sieving analysis. Moreover, the method should provide a wide range of features characterizing the sediment, including the grain-size distribution and the main geometric measures of the grains. The method proposed here is based on the Object-Based Image Analysis (OBIA) techniques available in Definiens Developer 7 (DD7). They are applied to high-resolution digital images of sand grains placed on a light box screen. The main stages of the algorithm include a multi-resolution segmentation of the image, separation of the background from the remaining objects and a method of distinguishing between separate grains and their agglomerates, based on the so-called elliptic-fit feature of DD7. In the final, calibrated algorithm, the probability of grain clusters being incorrectly classified as separate grains is less than 3%. The results obtained with OBIA are compared with those obtained by sieving for 22 sand samples. The statistical analysis of the grain-size distributions obtained with both methods showed that in most cases they give very similar results, especially for narrow size distributions with a clearly pronounced peak. For samples with a wider range of grain sizes, and/or with more irregular grain shapes, the differences are larger and depend on the details of the method in which the grain-size distribution is calculated from the data provided by OBIA. ADDITIONAL INDEX WORDS: sand, OBIA, beach sediments, granulometry, grain-size distribution