Tunnel working point-text book definition

I have heard of "working point" used to describe the point the instrument is set on. ie/ a synonym of "control point" or "traverse point".

It has been over 30 years but if I remember correctly the WP is the centerline of the tunnel both horizontally and vertically. Imagine the radius point of a pipe for an example.

I found an example;

Thanks for responses.

I'm looking for textbook definition... we can conclude what it is and how to derive NEZ from sections of tunnel in contract drawings, but they don't give the definition.

Thanks.

Look in the specifications of the project you are working on. A work point axis is what we used in the Washington DC Metro, similar to the drawing I posted. It was defined on the plans for each section we worked in. Some tunnels were bored and generally round, some were excavated and ended in a box like configuration. The work point was actually the center of the rails at a specified height above the rails and represented the center of the train car. You may find your answer in the plans for the car.

EDIT: By the way, you may be beating a dead horse if you think the definition will "prove" something to the design engineers.

Sounds like a Request For Information could clear that up.