THE UNDERWATER SITE OF QAITBAY – METHODS AND TECHNIQUES

Direct survey topography

Nelly Martin,

Latest up-date, Cécile Shaalan, February 2004
The Topography Service is involved in the underwater excavations at the Qaitbay site in order to plot the position of the sunken ancient blocks. During the first major campaign of excavations in 1995, a new system was developed by the topographers and divers. A buoy, to which is attached a reflective prism often used for long distance measuring, floats on the surface of the sea. This is in turn connected to an adjustable cord bearing a 35-kilo weight at the end of which is a sort of pointer. Under the water a diver positions the pointer upon the point to be plotted (corner of a block, a fixed metal picket, etc.). He adjusts the cord so that the buoy is vertically above the point. The topographer on shore takes the measurement with an electronic theodolite. Simple but clear and precise hand gestures allowed the team to communicate accurately.

Plotting the ancient blocks on the underwater site at Qaitbayby direct line of sight.
Drawing by Nelly Martin (1997) © CEAlex All rights reserved.

This system is limited by the effects of the swell, which causes the buoy to move on the surface, the current that can stop the cord from being vertical and, lastly, the distance between the theodolite and the buoy. Thus, this method can only be employed close to the shore when the sea is calm.

The map of the ancient sunken blocks off Qaitbay, first created in 1994, as of 1 January 2002, plots no less than 2555 blocks.

Since June 2001 the divers have had at their disposal a new tool for underwater measurements: the D100 aquameter. This acoustic system allows for the registering of the relative co-ordinates between a fixed receiver base and a hand-held pointer-emitter. Never the less, the topography team are still required to plot the position of the base receiver and the reference points within the general co-ordinate system.

At the surface a diver holds the buoy; underwater two divers set the pointer upon a block.

 

Photos - © CEAlex all rights reserved.