Page 49 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

28 Maritime Archaeology: A Technical Handbook, Second Edition
at the same distance. A simple way of checking to see if a transit is good is to move about a meter at right angles to the transit line. If the transit moves out of alignment then it is good. If it hardly moves, then it is poor, and the distance that one has to move in order to make a noticeable difference to the transit gives an idea of its accuracy.
It should be noted that the more measurements that one takes to a point the more reliable the fix is. Take the example shown in Figure 3.3, where a point is resectioned or located by three distance measurements. It is almost certain that the the three measurements will not make a single intersection but create a “cocked-hat” (see upper left of illustration). In this situation it is not possible to determine which measurement is wrong, simply that there is an error in one of the measurements related to the size of the “cocked- hat”. If one make four measurements, it is possible to determine which mea- surement has the largest error. This can be seen in the upper right of the figure. So the more measurements that can be made the more accurate the fix becomes.
A fixed position, such as a wreck site, can often be very easily relocated using two or more transits. Remarkably, wreck site positions are often not
A
CB
Figure 3.3 Resectioning showing the “cocked-hat” effect, where the arcs represent distance measurements from three fixed points A, B, and C. Because at least one measurement is slightly wrong, the result is a cocked hat. If four points were used and only one measurement was wrong, the result would be as shown in the second enlargement.