A hand-held probe touches a specific point of an object, and when the user presses a button the touch probe transmits a signal to a receiver. The receiver calculates the 3D co-ordinates of the ‘touched’ point by synchronously receiving information from two other, fixed points (3-point measurement) trough either magnetism (magnetic digitizer) or sound (sonic digitizer).
The method is very robust and errors can be corrected instantaneously. However, the method is very time-consuming and the resulting 3D image is only very schematic.
A 3D skeleton of a wheat plant (left) obtained through touch probe digitising (right). Variables like leaf curvature and azimuth can be extracted from this type of data.