The optical square is hand held but for more accurate centring over a point a plumb bob can be hung from the slot built into the shaft.

Alternatively the optical square can be mounted onto the top of a ranging pole with a special ranging pole adapter.

The prisms mounted one above the other occupy the central part of the housing.

The line of sight through the upper prism is turned 90 degrees to the right and through the lower prism 90 degrees to the left.

 A window between the prisms allows the observer to sight directly through the optical square.

This sighting arrangement ensures exact coincidence of the images of the observed targets (normally ranging poles) and allows the instrument to be used for determining right angles to the left and right and for placing points on line.

The accuracy of a right angle set out with the optical square is approximately ±1cm at about 30m.

Although the double prism optical square is a simple instrument a remarkable amount can be accomplished with it.

Anyone connected with surveying architecture building construction forestry agricultural archaeology mining exhibition work and planning will find it extremely useful.

Used for:

- Placing points on line
- Offset measurements
- Detail plans and area measurement
- Setting out curves
- Determining the horizontal
- Simple height measurements