Page 171 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

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Maritime Archaeology: A Technical Handbook, Second Edition
Table I
The relationship between the number of points and the number of measurements required if every inter point distances to be measured.
No points
4 5 6 7 8 9
10 11 12 13 —
100
101
Number of measurements
3+2+1=6
4+3+2+1
5+4+3+2
6+5+4+3
7+6+5+4
8+7+6+5
9+8+7+6 10+9+8+7+6+5+4+3+2+1=55 11+10+9+8+7+6+5+4+3+2+1=66 12+11+10+9+8+7+6+5+4+3+2+1=78 etc.
4950
5050
= 10
+ 1 = 15
+ 2 + 1 = 21
+ 3 + 2 + 1 = 28
+ 4 + 3 + 2 + 1 = 36
+ 5 + 4 + 3 + 2 + 1 = 45
number of control points, therefore involving far fewer measurements than the interpoint system. The main issue with this type of survey is the ques- tion of how many control points should be used. Obviously, to obtain a three-dimensional coordinate the very minimum number is three (there are also issues relating to vertical accuracy which will be discussed later). With three measurements, one cannot estimate error or reliability. If one makes four measurements it is possible to estimate the accuracy of the point or fix and determine if one measurement is incorrect. Five measurements increases the reliability of the fix and the error estimation. So there is usually a compromise between the number of measurements to any one point (three is inadequate; four is just adequate, but usually requires the fix to be re-measured; five is good but is starting to become a major “produc- tion,” etc.).
One of the major considerations in any surveying technique is the esti- mation of errors (see Holt, 2003). There are a number of different types of errors that occur in survey work, but for standard tape-measurement systems the errors are relatively simple. They include:
1. Errors due to misreading the tape; these can range from very large (gross errors) to very small. The gross errors are usually caused either by a transcription error or a serious error in the tape caused by snag- ging. Readings with these sorts of errors can be simply disregarded.