The town had seen better times but one could see that in those better days it must have been a pleasant place to live, surrounded by some breathtaking countryside and a wide, dark river. This forgotten part of dark Africa had not been surveyed for donkey’s ages. Now it was our task to set up a network of Trisponder stations for the horizontal control of bathymetric surveys and dredging of the river and river mouth.
We arrived in the small village of Jamestown on a nice Sunday afternoon and reported to the village chief to inform him of our planned activities. We explained that we would like to base our survey on the available ground-control, a number of old Shell survey points located along various pre-war seismic lines.
Chief
The chief knew exactly what we meant and summoned one of his servants to fetch a local labourer who had been employed in the past by the seismic crews. His task had been to help in cutting lines and making the survey points, consisting of yellow and blue painted 2” piping and blocks of concrete. The messenger left...The local headache drink, kaikai, was brought out for us by the generous chief, and poured into dirty glasses, instantly killing all germs and bacteria. After two glasses of this liquid dynamite the messenger and the man arrived, with some fifty villagers in their wake, all laughing and shouting with enthusiasm. We were forced to drink another glass of local poison, and told the old man that we would like to employ him. The next day we would go into the field and he would show us the survey points. A decent salary was negotiated and the chief also received commission for his co-operation and permission to work in his territory. A thatch-roofed hut was rented, and, after discreetly washing away the foul taste of kaikai with litre bottles of ice-cold Star beer, the survey crew went to sleep: tomorrow would be a busy day...
Help
The next day, at the ungodly hour of 5.30 in the morning (especially ungodly after all these kaikais and litres of beer!), there was a knock on the door and our newly employed assistant stood outside, his face beaming with pride! He explained enthusiastically that, in order to make a good impression, he had gone to the field very early in the morning and found all the survey points for us. And indeed, behind him was a pile of blue and yellow painted pipes with concrete blocks attached: he had neatly dug them out and carried them to the village with the help of some friends.
Tip and Trick
Tip: If you’re working in less developed areas such as in these dark corners of Africa, do not tell anybody what your survey plans are; either your ground control will be taken away or someone will build a house over it!
Trick: With most of the local ground-control gone, we had to find an alternative method of setting up our Trisponder network. No GPS, you know... It was decided, in order to have good coverage over the river, to establish a point (S) on the southern tip of a cluster of mangrove, Parrot Island. From this point ‘S’, constructed on hard sand under a mangrove tree, two survey points (A and B) could be seen on the other side of the river. These points were neither accessible nor visible from each other. And neither is resection to just two points possible. Our solution was to cut a long, narrow path to the northern tip of the island and establish another point (N) from where the two survey points could be seen. Points S and N were visible from one another and the distance S-N was measured accurately using Tellurometer MRA3. By observing all angles from S and N, combined with the measured distance S-N and the known coordinates of A and B, the coordinates of S and N can easily be computed!
Mission Accomplished
Later on, we managed to fit S and N in a much larger network of survey points along the river. But for the time being we had our ground control for the Trisponder ready for the dredger, which was on his way to Cross River. Dredgers don’t like to be kept waiting...
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