In this case, control was established in State coordinates to within 3 mm, and the photogrammetric model was accurate to within 7 mm. This model was then used as a baseline for subsequent models constructed based on photogrammetric surveys carried out at regular time intervals. These models were compared against the baseline model or against the previous models to detect movements: for each triangle on the triangulated point cloud in Model i, the distance is measured to the surface of Model i+1. Besides this automatic change-detection algorithm, the high resolution colour texture of the 3D model makes it very easy to: follow a particle in 3D (particle tracking), and to identify rock detachments. A second example of slope monitoring is the slope in shown HERE where change detection is used in order to evaluate the need for road closures necessary to remove or scale dangerous blocks.