surveying

Surveying is the practice of determining the relative positions of points on the earth’s surface and showing them in a usable form, such as a map or chart. To achieve their goals, surveyors measure distances, horizontal angles, vertical angles and heights and use elements of geometry, engineering, trigonometry, mathematics and physics.

At the time Colonel William Light began surveying South Australia, distances between points were commonly measured with a Gunter’s chain. English mathematician, Edmund Gunter (1581 – 1626), established the practice of using a chain that was 22-yards long. This length soon became known as a ‘chain’ – the length of a cricket pitch. When Light began the survey of Adelaide he called upon one of his men, twenty-year-old John Corney, to ‘undo the chain and if you live to be an old man you can say you measured the first town acre…’. Light and his team used a theodolite to measure angles in the horizontal and vertical planes. Theodolites have a telescope mounted between two perpendicular horizontal axes and the vertical axis. By sighting the telescope on an object, surveyors can measure the angle of each of these axes very accurately. A process known as ‘differential levelling’ is used to measure height, in which a series of measurements is taken between two points. Differentials in height between the measurements are added and subtracted in the series to determine the difference in elevation between the two end points.

In surveying large areas, Light used the ‘triangulation’ method to determine a place’s horizontal location: using the horizontal distance between two places as a starting point and a theodolite, surveyors calculated the height, distance and angular position of a third place, as long as it was visible. Light experienced problems with this method in South Australia, as ‘there were no spires, chimneys or other objects which could be used for triangulation – here they had to rig up their own’. This slowed the rate of survey, which concerned the South Australian Company as it had invested heavily in land, and embroiled Light in controversy. Light’s critics called for the replacement of triangulation with what was known as the ‘running survey method of parallelograms’. Running surveys used landmarks or a minimum number of survey points from which distances were measured with a Gunter’s chain. The system was less accurate than triangulation, but much faster. In the event, a resolution expressing complete confidence in Light and his methods was passed at a public meeting held in Adelaide on 9 July 1838.