Page 59 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

38 Maritime Archaeology: A Technical Handbook, Second Edition D. PHOTOGRAPHIC ANGLE MEASUREMENT
If one can calibrate a camera lens so that the focal length is accurately determined (see Chapter 6), the camera can be used as an angle- measuring device and, thus, used as a surveying instrument. This technique is discussed in detail by Williams (1969), and now there are commercial pro- grams that enable one to do this on a computer (see the Chapter 6, Section V.A). There are two methods to measure photographic angles: direct and photo resection. The direct method involves measuring the distance from the principal point on the photograph to one of the features and dividing this by the effective focal length of the print to give the tangent of the angle. This process is repeated for the other feature and the two angles subtracted to give the included angle between the two points. It should be obvious that if the angle to be measured crosses the principal point then the two angles have to be added together. In this way the camera works like a sextant and the two angles are calculated mathematically (Figure 3.10).
The alternative technique is photo resection, which consists of drawing the angles on tracing paper and then applying the tracing to a map of the area (Figure 3.11) in a similar manner to a Station pointer (see Figure 3.12). However, the ubiquitous GPS has largely replaced this time-consuming system.
Figure 3.10 Photographic angle measurement. In this sequence, the camera has a previously calibrated lens with a focal length of 24.00 mm and the camera has a 36-mm wide format. Two points that are geographical features can be identified on the print (in this case the Port Authority Building and the incinerator chimney). It is possible to calculate the angle between the two points and the location of the camera. If common points on adjacent photographs can be identified, features on the prints can be linked together. In this case the incinerator chimney is common in both, making it possible to link the N Mole Light, incinerator chimney, and the Port Authority Building providing three angles from the two photographs.