Errors due to instrumental imperfections and/or nonadjustment are all systematic, and without exception they can be either eliminated or reduced to a negligible amount. This can be achieved by obtaining the mean of two values one observed before and the other after a reversal of the horizontal plate by plunging the telescope and rotating it about the vertical axis. Further, the error in either horizontal or vertical angle due to inclination of the vertical axis can be eliminated, so far as its systematic character is concerned, by leveling the plate bubbles again in addition to the reversal of the plate. However, for precise work the usual practice would be to make the vertical axis truly vertical by means of the telescope level, and then to proceed in the ordinary manner.

The personal errors are random and hence cannot be eliminated. They form a large part of the resultant error in transit and theodolite work. Of the personal errors, those due to inaccuracies in reading and setting the vernier or reading and setting of the optical micrometer and to not sighting exactly on the point are likely to be of greater magnitude.

Natural errors are generally random, but under certain conditions systematic errors may arise from natural causes. On surveys of very high precision, special attempt is made to establish a procedure which will as nearly as possible eliminate natural systematic errors.

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