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2. Centering

The angle desired is the one defined by the ground points. To measure that angle requires an instrument, TSI or sighting mark, be set up at some height above each point. The center of the instrument should be exactly vertically above the ground point but won't be due to random errors. The TSI is oriented using either an optical plummet or plumb bob and, while close, will still have error in it, especially if the optical plummet hasn't been checked. Sighting marks may consist of freely suspended plumb bobs, prism poles with built in bubbles, or tripod/tribrach-mounted target each with its own centering challenge.

Figure E-1 shows the relationship of centering errors on an angle measurement.

Figure E-1
Centering Errors


Because these are random errors, they don't behave in a strictly additive process. More on that in a bit.

How can we minimize centering error?

On a TSI the optical plummet should be checked. If it is off, then it should be adjusted or instrument setup should take that into account. Setting up a TSI with a maladjusted optical plummet was discussed in Chapter C. Set Up; Take Down.

For the sighting mark, it depends on the mark's configuration.

* If the mark tribrach-mounted, then the optical plummet must be checked using a plumb bob.
* If the mark is a prism pole, then the rod bubble must be checked and either adjusted or compensated for.
* The least error-prone, from an equipment viewpoint, is a plumb bob. Plumb bob set up error is a function of its suspension: hung from a tripod, held by a person, etc. The more stable the support, the less centering error.

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