Graph showing the relative hardness of wood, from a drilling test carried out by the DmP. Distance into the tree is represented along the horizontal axis, with the outside of the tree on the left. The depth in the tree is shown in millimetres. This graph shows wood of reasonable quality from 1.5 to 13 cm into the tree. Within this area, each peak represents an annual ring, from which historical growth rates, and an estimation of the tree's age can sometimes be made. From 13 cm, there is an almost complete absence of hardness, indicating advanced heartwood decay. By applying a series of tests at different locations on a tree, an accurate picture of the condition of the wood can be determined.