By means of the relative measurement system a CNC-controlled machine can determine it's position.

These machines have mainly optically working measurement devices at their mobile axes; these devices measure the path relatively to a start point.

In the period of introducing the first CNC-controlled wood processing machines, relative measurement systems were used for cost reasons nearly without exception. In the 1990s, a new and economical absolute measuring system was used for the first time on beamsaws. Since then, the significance of relative measurement system has decreased. However they are still required for many applications.

Advantages:

• Relative measurement systems don't need much space and can be properly protected from damage.
• If position data got lost - and the system is not damaged mecanically - the operator can easily re-reference the machine referencing and continue the processing.

Desadvantages:

• Before each operation start, the machine must go to it's calibration postition for referencing the measurement system, which takes several minutes.
• Due to the fact that the device only measures distances, a measure error ("position error") will occur even if with a minimum tolerance. That means that high precision needs demand frequent referencing.