Experiment on accuracy of optical theodolites

Five optical theodolites of different designs and least counts are compared for accuracy in reading horizontal angles. A subtense bar of an accurately known length and a Wild T3 precision theodolite are used for this purpose. Subtended angles are first measured using the Wild T3. These are then remeasured using each of the test instruments. The root-mean-square-error values of angle measurement are then computed as standard deviations. The best accuracy values are obtained with the double-reading micrometer theodolites. However, in almost all cases considered, the optical scale-reading theodolites give accuracy values approximating those obtained with a single-reading micrometer type of instrument