Tomography provides a representational image of the cross-sectional plane of a tree stem. Since the required measuring pins penetrate only into the outermost annual rings of the tree, these methods are considered to be injury-free.

Tomography is used for detailed tree assessment if, after a visual assessment, there is uncertainty about the proportion of load-bearing wood within the stem. The trunk level that is generally examined, is where the greatest impact of the damage observed on the safety of the tree is expected.

There are two tomography techniques that measure different physical properties: Sonic Tomography (SoT) and Electric Resistance Tomography (ERT). By combining both methods, an appropriate interpretation of the measurements can be achieved and the reliability of the information provided can be increased (Weihs et al. 2007).