LEGIO SECVNDA AVGVSTA
Roman Living History Society
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Written by: Stan Kitchener.
A line-of-sight instrument to determine the levels (profile) of the land.
The CHOROBATES bench is place over a straight string line produced by a GROMA (see Roman Surveying Techniques -Sheet One), then levelled using the numerous attached PLUMLINES and oil-channels on the chorobates’s top surface, in conjunction with levelling spoil or chocks.
A turf square is then cut adjacent to the Chorobates; this is then filled with cement and marked to identify this `BENCH-MARK’ and its position and `relative’ height. When set, the height (H.) is measured from the cement bench-mark to the top of the bench (Chorobates).
The eye is then used to `sight’, though two sight-holes, along the top surface of the Chorobates bench. A man forty roman feet away along the string line holds an advanced RANGING POLE, who slides a mobile `TARGET’ on the ranging pole (balanced by a counter-weight on the rear of the ranging pole), until the viewer at the Chorobates can see the target on the pole. The ranging pole man then calls out the registered height on the ranging pole; the bench-mark height is then deducted from this to produce the true ground-level difference between the two places.
The man with the ranging pole then retreats a further forty roman feet along the string line and repeats the process. By this method the heights along the string line can be related to each other and thus drawn as a profile, to be used as part of a plan for any construction required.