between the elevation of the target and the elevation of the marker and the instrument bottom plate, such as when the target is in a deep hole, or on a Bilby tower, the target will appear to move laterally by a large distance in the optical plummet as the tribrach level is adjusted. Tribrachs are precision instruments and should be checked regularly to ensure they do not become a source of error. References Gerard L'E. Turner, Nineteenth Century Scientific Instruments, Sotheby Publications, 1983, Gerard L'E. Turner, Antique Scientific Instruments, Blandford Press Ltd. 1980, External links Tribrachs and other accessories to Wild theodolites. Surveying instruments