News - The tale of a PERFECT three-tripod traversing

Fig. 1 The TotalStation – and Richard and Niels in the background setting up a prism station next to the costal fortifications.

Fig. 2 Eva behind the TotalStation – note the Mounichia Harbour in the background.

Fig. 3 Turning another corner on Koumoundourou Hill – Richard and Eva setting up a prism station

Author: Richard C. Anderson

In 2007 season and in the summer season of 2009, the numerous visible remains of walls and towers of the ancient fortifications on Koumoundourou Hill were surveyed in great detail in three dimensions using the Fradgley-MicroStation surveying system. Each of the individual areas of survey was done on different arbitrary local grids because it was a specialist job to link them together and the required equipment was not then to hand. This necessary tieing-in was done on two beautiful days, 11 and 12 November this year, using a system known as three-tripod traversing.

Three tripods are required and two fine old surveying tripods were kindly lent to the project by the British School at Athens (fig. 1). "Prism-station kits" are also required, these lent by Richard C. Anderson who supervised the work. A "traverse" of 11 "stations" was "carried" around the tree-covered slopes of Koumoundourou Hill beginning and ending with the principal station of the original Mikrolimano survey on the opposite side of the harbour, over 200 m away (fig. 2). The traverse either utilized main control lines from the earlier work or allowed them to be surveyed in passing, on the "global" grid and thus allowed the old work to be moved and rotated into its correct "global" position (in three dimensions). This latter computer work is a fairly simple operation in CAD.

Three-tripod traversing virtually eliminates positional inaccuracy as a traverse is carried through difficult terrain where sight lines had to be carefully threaded through trees and some of the "legs" of the traverse were necessarily short in order to go around the outside of the hill with its summit almost entirely covered by a modern building, the Yacht Club of Greece (fig. 3). With some "tree hugging" required to move offending branches out of the way and shooting (almost)in the dark to some stations in very deep shade plus two very short "legs"on the surveyed polygon, I was not expecting a particularly good "closure" of the traverse. But the muse of surveying was definitely with the excellent team of Niels Bargfeldt, Eva Mortensen and myself. We found the "error of closure"to be less than 0.6 mm on a traverse of 1 km overall. We were a little bit lucky, of course, but it was still a very satisfying moment of truth when the traverse was closed. This work has allowed a great deal of earlier work to fall into its correct, three-dimensional place with considerable confidence.