Based on analysis of aerial photos, each sample plot (SSU) is allocated to one of three basic categories, reflecting the likelihood of forest or other wooded land (OWL) cover in the plot: Unlikely to contain forest or other wooded land cover, Likely to contain forest, and Likely to contain other wooded land. All plots in the last two categories are inventoried in the field. The indicators were registered by the measurement crews as they were travelling along the edge of the 200 x 200 m grid square of each cluster.