Page 187 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

166 Maritime Archaeology: A Technical Handbook, Second Edition II. SMALL-SITE SURVEY
The ordinary, graduated measuring rod placed against an object gives a ready reference to overall length. It has the additional advantage of being portable, but its major drawback is that it is one-dimensional. The one- dimensional scale works reasonably well with an object like a cannon, or in the case of an object which is two-dimensional, and the camera is orien- tated at right-angles to the plane of the object. However, as soon as a per- spective view occurs, problems arise because the scale is only correct in a plane at right angles to the optical axis of the camera passing through the scale. It is more useful, therefore, to use a two-dimensional scale like a square grid frame or a cross rod, even though these are more awkward to handle under water (Figure 6.1, note in this case it is a three dimensional grid frame). The square grid placed over or on an object can be used to ori- entate the camera so it is perpendicular to the frame. The grid can also be used to estimate the correct flying height or camera-to-subject distance by working out beforehand what the relative size of the grid square should be in relationship to the frame of the viewfinder. The camera–grid frame dis- tance can then be adjusted until the grid frame is approximately the correct
Figure 6.1 Cross-hairs grid frame used to ensure vertical photograph. The photographer attempts to align the cross-hairs on the grid frame thus ensuring the camera is vertically above the frame.