Page 149 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

128 Maritime Archaeology: A Technical Handbook, Second Edition
depth correction for tide can be calculated. The recording forms should include a sequence of depth measurements and times along with the depth of the depth reference point at the start and end of the sequence of mea- surements. The difference in depth at the start (StartDepth) and end (End- Depth) of the sequence shows how much the tide changed while the measurements were being made (TotalDepth). The difference in time between the start (StartTime) and end (EndTime) of the sequence shows how long it took to make the measurements (TotalTime).
At any time during the sequence of measurements it will be possible to calculate how much the depth changed up to that time (MeasureTime) by using proportions. If the raw measurement was made three quarters of the way along the sequence then the change in tide at that time will be three quarters of the total change.
The total tide height is the difference between the assumed depth of the depth reference (DepthRefDepth) and its depth measured at the start of the sequence (StartDepth) plus the change in tide calculated previously. This is the correction to be applied to the raw depth measurement. The raw measurement and the correction should both be entered on the depth mea- surement dialog.
11. Adjusting
The next step is to adjust the positions of the points using the mea- surements. The adjustment is an iterative process requiring a number of repeated calculations before it comes up with an answer. Once the adjust- ment has stopped and an answer is calculated, it is then possible to inter- pret the results and to work out what to do next.
12. Interpretation
Interpreting the results of an adjustment is the most difficult part of the survey. If the results are good and the network fits together well, then no interpretation is required and the next stage can be carried out. It is more likely that some problems will have occurred so that the network does not fit and thus some additional work is needed to find the causes of these problems and to correct them. It is important to work to the accuracy spec- ified and no better. It is possible to go on fine-tuning the results to get a better result but, in reality, this stage is complete when the answer is good enough.
The RMS residuals (the square root of the mean of the residuals squared) is a quality figure showing how well the measurements fit together.