The Circumferentor
1824 to 1842: Using the Circumferentor
The circumferentor
Early Queensland surveyors used a circumferentor, or large compass, to define their direction, putting the survey on a magnetic meridian. This instrument was very suitable for working in the heavily timbered areas of the Australian bush.
The magnetic needle of the circumferentor was susceptible to local attractions. Therefore the surveys using this instrument were not very accurate.
The use of the circumferentor was described in the Penny Magazine, August 1840:
Having planted his compass at the commencement of the line he intends to run, and having arranged the sights to the proposed course or particular degree, when the underwood is not thick, it frequently happens that a tolerably clear way may be seen among the trees to the distance of 10 or 20 chains, until some stout tree appears to stand upon the exact line that has to be run, and interrupts the view.
This tree the surveyor particularly notices, for he calls it a sight-tree; and having slung his compass under his arm, pulled up his compass-staff [pole on which the compass stands], and called out 'chain' (as a signal for the chain-men to commence measuring), he sets off a-head of the chain-men, taking the axe-man along with him, who here and there cuts down small saplings that seem in the way of the chain-men, which serves also to mark the line
And having reached the sight-tree, on which he has kept his eye the whole way, he goes to the front side of it, and there re-sets his compass, during which the axe-man is engaged in marking the tree in a particular manner (three, four, or five notches, both in front and rear); while the surrounding trees, particularly the young ones, are scored, with a similar number of notches on the side of each, looking inwards, or towards the sight-tree.
The trees thus marked are called witnesses, and the object in marking them in this manner is, that in case of the sight-tree being cut or blown down hereafter, its place may be nearly ascertained by the position of the witnesses.
When the chain-men have measured up to the tree in question, the distance, as well as the sort of tree, is noted in the surveyor’s field-book.
By this time a new object has been taken, and forward goes the surveyor and the axe-man again.
Circumferentor, circa 1840
1860 to 1880: Measuring direction
By the 1860s, circumferentors were being replaced by theodolites with inbuilt compasses and were known as compass theodolites. These were used to read the angles on a survey and increased the accuracy whilst maintaining a magnetic datum.
Compass theodolite, circa 1860
By the 1880s, the surveyor used the theodolite to read angles and to take astronomical observations on the sun or stars. This put the survey on an astronomical meridian.
Last updated: 17 June 2009