For a millennium and a half, the world’s first and tallest lighthouse that stood offshore on the island of Pharos at Alexandria continued to tell seafarers their position by day and night. Guiding them to the entrance of the greatest harbour in antiquity.
Parts of the 135m high structure, admired as the seventh wonder of the ancient world, have now been discovered in the sea off Pharos. Leica GPS equipment is helping the Centre des Études Alexandrines (CEA) to survey and map the exact position of its remains.
Discovery of a lost world
On the sea floor, six to eight metres below the surface, Empereur and his team have found a sunken world: sphinxes that date from various pharaonic periods; decorated stone statuary; parts of royal statues from the Ptolemaic period; and several monoliths weighing fifty to seventy-five tons a piece that must have been part of the vanished lighthouse. The archaeologists and divers have surveyed and mapped most of the over 2000 objects found.
First-ever use of real-time GPS precision surveying
The Leica GPS System 300 and RT-SKI software has now made such difficult, time-consuming work in the sea much simpler. It no longer needs a theodolite set up on shore, because the surveyor can take the boat directly wherever he chooses. Lionel Fadin has fitted a Leica antenna to the boat. This is connected by a plumbing pole to a cable long enough to reach the diver on the sea floor. In the boat itself Fadin has a CR344 controller that automatically measures and records everything on-line in real time.
He has also set up a central Leica GPS System 300 station at a reference point ashore, probably on the very spot where once had stood the furthermost part of the lighthouse. This station also has an SR399 GPS receiver, a CR344 controller, and a radio transmitter to the boat’s moving AT302 antenna. The Leica RT-SKI software automatically computes and optimizes the data from both stations in real time. Reception has proved excellent from the very first time the system was used, and has allowed Fadin to track the signals from eight GPS satellites simultaneously. All data are directly transformed into the local coordinate system.
This not only greatly simplifies the data-acquisition process, it also ensures greater accuracy and saves time. As a result, it may now be possible to make up for the diving time lost so far this year because of particularly rough seas, and to reduce the time that archaeologists have to spend under water.
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