Page 152 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 131 16. Limitations
A least-squares adjustment does not give the true solution, only the most likely solution on average. It is not possible to get the true solution as there are always errors in any measurements made. Using a better measuring instrument will not get the true solution, only a better one. The least-squares method will tend to spread the effects of any blunder around the network. This is why it can be so difficult to identify the measurements that are blun- ders. The position error computed for the points is controlled by error esti- mates in the measurements. You are unlikely to be able to position a point to 20 mm when the error in your tape measurements is 100 mm. The adjust- ment needs the best measurements it can get to be able to compute the best results. The position error computed for each point is affected by the choice of position reference point. This is a side effect of constraining the network. Care in selection of which points to use as reference points and a good network shape will help minimize these effects.
17. Layers
Site Surveyor has the ability to create layers so that different sets of survey work can be placed in different layers. This has the advantage that the complete survey can be held in one file, so that, for example, each day’s survey work can be placed in a separate layer, the common control points maintained in another layer, and by switching the layers on and off adjust- ments can be made without having to needlessly readjust data that have already been fixed.
18. General Conclusions
Site Surveyor is undoubtedly the best underwater archaeological sur- veying program on the market. It has been widely used in various situations and is referred to here on the Pandora survey, Tektash, and its use with the HPASS system (see next section).
VI. ACOUSTIC SURVEYING SYSTEMS
The use and development of the High Precision Acoustic Surveying System (HPASS) has been described previously by Green and Duncan (1999) and Green and Souter (2002). HPASS is discussed here as it illus- trates the potential for acoustic systems for underwater archaeological work, particularly where the site has poor visibility. Where a site has good