Map Making- Transit's Sight Optics

What could be better than a telescopic sight, as used for an air-rifle?

You need to mount it on something that swings precisely around the exact centre of the base of your transit... but even mounting it is little trouble, given the job for which it was made. I have a hole in the centre of my base. The sight is bolted to a thin strip of wood with a cut off nail pushed through a hole in it. The nail is inserted into the hole in the base, and the sight swings around easily. It is important that the holes be just the right size, so that there is no 'wobble' in the attachment.

When you take readings, the table of the transit should be horizontal, and the crosshair you are using to line up on objects should be vertical. Slight errors in the former will, I believe, make only very slight errors in your results. The problems with a non-vertical cross-hair will be clear if you think about what would happen if you lined the top of it up on the top of a church spire, but the bottom of it up on a low level object.

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