The use of watershed segmentation and GIS software for textural analysis of thin sections Joseph Barraud ⁎ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom Received 1 July 2004; accepted 30 September 2005 Available online 28 February 2006 Abstract Textural analysis of thin sections of rocks can be performed with a Geographic Information System (GIS) program to improve management and visualisation of data. Automatic detection of grain edges is performed by watershed segmentation on digital pictures of the thin section. After vectorization of the segmented picture, the resulting map of grain boundaries is edited and corrected manually. GIS software allows the user to associate each grain with attributes such as phase name, position, size, aspect ratio, orientation and convexity. The grains can then be classified according to one or several attributes. The spatial distribution of the different classes of grains can be visualised with colour-coded maps or quantified by cluster analysis. A weakly foliated quartzite and an igneous cumulate are taken as examples to show how invisible patterns are made evident with these kinds of maps. The interpretation of these spatial distributions remains problematic, but future development of 2D or 3D numerical models of textural evolution makes this technique promising. © 2006 Elsevier B.V. All rights reserved. Keywords: watershed segmentation; GIS software; textural analysis; crystal size distribution; pattern; foliation 1. Introduction The scientific description of rocks, or petrography, involves observations and measurements at various scales and various levels of details. A general description, including the mode and the texture, is usually enough for the geologist to understand in broad terms the nature and origin of a rock. However, the complexity of the processes involved in the crystalliza- tion and textural evolution of rocks requires more and more quantification in order to test the available models. Textural terms are difficult to quantify, as they are often quite subjective, particularly for igneous rocks. Quan- tification therefore requires objective accurate measure- ments and appropriate statistical analysis. In this contribution a complete method of optical texture determination, from the thin section to the final diagrams, is presented. Textural analysis generally consists in measuring the size, the shape, the orientation and the position of the grains in a rock. Dihedral angle measurements can also provide valuable information (Holness et al., 2005), as can c-axis orientations (e.g., Heilbronner and Pauli, 1993). The simplest and cheapest way to perform textural analysis is to cut thin sections in a rock sample and to study the outlines of the grains in this plane under an optical microscope. X-ray tomography is a promising and impressive technique for direct 3D analysis (Philpotts et al., 1999), but the boundary between two Journal of Volcanology and Geothermal Research 154 (2006) 17 – 33 www.elsevier.com/locate/jvolgeores ⁎ Tel.: +44 1223 333433; fax: +44 1223 333450. E-mail address: jbar02@esc.cam.ac.uk. 0377-0273/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2005.09.017