To study the behavior of water flow at interfaces between different soil materials we made computed tomography scans of sand samples using synchrotron light. The samples were prepared with an interface between two sand materials. The contact points between grains at the interface between the sands were identified using a combination of watershed segmentation and a classifier that used the grain-size and -location. The process from a bilevel image to a classified image is described. In the classified image five classes are represented; two for the grains and three for the contact points to represent intra- and inter-class contact points.