# Position measurements using a prism

To measure a position perpendicular to the tunnel profile using a prism:

1. From the tap and hold menu, select Settings.
2. Select the Apply target height perpendicular to profile option.
3. Tap Accept.
4. From the status bar, enter the prism radius as the target height.

You can use the prism on a pole held perpendicular to the tunnel design surface, where the target height is used to project the prism measurement perpendicular onto the tunnel surface.

1

Prism

2

Tunnel surface

3

Design tunnel

4

Target height (prism radius)

5

Overbreak