Definition

In PHOTO­GRAMMETRY, INDEX MARKS, usual­ly four, which are rigidly connected with the camera lens through the camera body and which form IMAGES on the NEGATIVE and usually define the PRINCIPAL POINT of the PHOTOGRAPH. Also marks, usually four in number, in any instru­ment, which define the axes whose intersection fixes the PRINCIPAL POINT of a PHOTO­GRAPH and fulfills the requirements of INTERIOR ORIENTATION. In SURVEYING, an index line or point. A line or point used as a basis of reference.