Some Typical Theodolite Diaphragms

This is roughly what the diaphragm in the Zeiss 015B theodolite looks like

This is FACE LEFT This is FACE RIGHT

This is the sort of target that we have fixed to the wall outside. We need to superimpose the theodolite diaphragm over the target

It is better to pick a well defined point such as the top point. Going for the centre point is difficult particularly if the central line of the target is not vertical

Because of the “hole” in the lines of the diaphragm this is still not good practice. I am not sure that I am lined up on the top point.

This is much better. I can repeat this alignment with a fair degree of certainty. This is how we should sight a target for horizontal angle measurements.

This is how we should sight a target for vertical angle measurements.

Sometimes we use the single lines and sometimes it is better to use the double lines.

An example might be sighting onto a distant spire.

The single line covers the spire and I am not sure that I am accurately lined up on it.

By lining up the double lines I can use my judgement to ensure that the spire is central to the two lines.