from: John Rocque.doc dated 11/07/2008
last revised: July 22, 2008 7:38 PM
John Rocque
John Rocque and other surveyors used measuring equipment which had been developed over the centuries. Here he is seen at work at the top of a tower. From here he could see all the buildings and roads in the neighbourhood and he was preparing to mark out his map by using a theodolite.
John Rocque, surveyor and map maker, at work on a tower.
From Rocque's "Environs of London Maps"
The theodolite stands firmly on its tripod and because it is on three legs it cannot wobble. The sextant telescope swings round and the angle it turns can be noted down. He is measuring the angle of all the buildings, street crossings, etc. he can see round the tower and marking them on a drawing. When he has turned a full circle he will have a drawing looking like starburst, with a name of a church or street corner on each line and the tower he is standing on in the centre. When this is done he will climb down the tower and start measuring along each line with a measuring wheel.
A Measuring Wheel
The surveyor sets the dial to zero and walks in a straight line to one of the buildings on his starburst. Each complete turn moved the dial one division and the final distance can then be marked on the line. He might have had an assistant to do that to save time, but he could have marked it himself on the plan he is carrying in his pocket.
Then he would go back to his office and draw it all on his map. This would have given him one set of streets and buildings on his map. Next he would have gone to the top of another tower, which he had already mapped, and do it all again. Early sea maps of the Mediterranean are covered with these networks if radiating lines, measured I suspect in average sailing times from one port to another.
Modern surveyors use the same methods but their theodolites measure both angles and distances. They record the angle and also send out a beam which bounces back from the building. The time it takes for the beam to hit the building and return is calculated and gives the distance an accuracy of 3 cms. It saves a lot of walking.
Ships first used these echo sounders to calculate the depths of water below them. This saved many ships from running aground and so saved lives. It also revealed the sea ridges and plate edges which confirmed the whole theory of Plate Tectonics. This moved science a complete step forward, and the whole theory is now accepted.