The testing procedure is undertaken by an Arborist with training and experience. The PICUS sonic tomograph is a system that measures the speed of sound travelling across timber. Because the speed of sound waves is constant in solid wood, a measurement can be taken from one point to another on the trunk of a tree. If there is decay between the test points then the speed of the sound waves are measured as slower data. The PICUS software calculates all data recorded in a complete test and creates an image called a tomograph. An Arborist can then make an interpretation of the tomograph, taking into account the tree itself and other surrounding features. Previous methods of decay detecting were invasive to the tree, i.e. increment bores created extensive damage to trees tested. The PICUS system is virtually totally non-invasive. The PICUS system only uses small nails to penetrate the bark to make contact with the timber.