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Chapter 6: Photogrammetric Techniques 177
survey, because the control scale is usually slightly above the seabed. To scale the site plan, it is necessary either to increase the scale of the photo- mosaic or to reduce the scale of the plan of the coordinates by the deter- mined scaling factor. The less time-consuming option is to reduce the plan, rather than to reprint the photomosaic. Again, computer graphics make much of this discussion redundant unless one is using photographic prints.
In the Ko Si Chang 2 survey, the positions of the stations were first plotted using the calculated coordinates. Station 3 was used as the origin, with the X and Y axes drawn arbitrarily. The plan was then reduced to match the scale of the photomosaic. To determine the scale of the photo- mosaic, the distances from each station (tag) were measured from the pho- tomosaic and divided by the distances measured on the coordinate plan to give the scaling factor.
The scaling factor of 1.05 was applied to the program coordinates and these adjusted station positions were plotted on an overlay. The trans- parent overlay was then placed over the photomosaic and rotated until a “best fit” position was found. The results indicated a very good correlation between the tag positions from the photomosaic and these same positions determined by the least-squares adjustment method. The results suggest that if a photomosaic is laid up using this method and the site has little ver- tical displacement, measurements taken from the photomosaic should have a standard deviation of about 2.5% over long distances.
E. CORRECTING FOR TILT
It should be remembered that the objective of all surveying systems is to maximize efficiency both under water and on land. Therefore, it is impor- tant to consider the merits of making a free swimming photomosaic of a site. This is done taking into account the long period of time spent in the darkroom rectifying and adjusting the magnification of the prints to correct for the variations of tilt and flying height introduced in the field. How do the advantages of reduced diving time weigh against the extensive dark- room time needed with this approach? Would it be preferable to spend a longer time under water using a tower and reduce the time spent in the darkroom afterwards? The choice will depend on a variety of considera- tions: How much time can be afforded for the work? Is time in the field at a premium? Is there a field darkroom?
If a photo tower is used to produce a photomosaic, unless the tower is leveled each photograph will inevitably have some different component of tilt. Although the scale across the grid frame of the tower will be correct, the scale on the seabed may be different. This may cause problems in match- ing adjacent photographs of a photomosaic.




























































































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