Page 53 - Ranger Demo

sunshine while an ex-DOS porter would arrange the passport stamps and clear bags through customs. Some of the aircraft taxi tracks were still composed of wartime pierced steel planking.
The Plan and the Kit
The project called for a connection between the Gambian survey network and the 12th parallel survey. The plan was to use the Bilby towers to traverse along three loops based on the Senegalese part of the 12th Parallel work. Training and equipment calibration began in August and fieldwork started later that month with a connection across the estuary from Banjul to Barra. Progress was hampered initially by the rainy season, the general inexperience with some of the instruments, and particularly the towers. The equipment comprised: Wild T2 and T3 theodolites, Tellurometer MRA3s, a Hewlett Packard 3800B infrared distance meter, Wallace and Tiernan altimeters; beacon lamps and heliographs. DOS supplied all the equipment except for the T3s which were owned by SGN. The instruments were calibrated in grads (400 grads to a full circle). Decimal angles were a novelty to British surveyors but quickly picked up and straightforward for subsequent calculations.
All the main horizontal angles were observed with eight rounds at night to beacon lamps. Measured at night, because of the abysmal visibility during the day, were eight sets of simultaneous reciprocal vertical angles Each line (average length around 18km) was measured twice in each direction with a two-hour gap between measurements. Three groups of pole star azimuths were observed, a fourth was abandoned due to cloud.
Bilby Towers
Many older readers will remember the Bilby tower, which stood at the School of Military Survey in Hermitage for many years. The advent of satellite surveying systems has, of course, removed the need for such equipment in the 21st century.
The towers are named after Jasper Sherman Bilby who started his career in 1884 with the US Coast and Geodetic Survey as a labourer and carpenter. He built wooden survey towers for First Order triangulation surveys. In 1926 he invented and designed the eponymous tower when he realised that re-using steel towers would be more efficient than single-use wooden variety! His design incorporated the established practice of an outside tower to carry the observers and an inner tower to support the instruments. It transformed the speed and efficiency of C&GS triangulation projects from 1926 and into the post-war years. The last Bilby in the US was built in 1985.
When Alan arrived in Banjul one of his first tasks was to sort out the ‘Pika Stick’ jumble of tower parts which had come from Ghana and then train a construction team of local men for the project. Once separated into four colour-coded giant ‘Meccano’ sets a 77 ft tower was successfully erected on the outskirts of the capital as a trial. Although Jasper Bilby had written a comprehensive set of instructions in 1929, we had to make do with just one set of plans and the spanners. Alan’s sapper experience in engineering and man-management was invaluable!
Figure 2 Bilby Tower and Umbrella
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