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Roman Surveying Technique 2 - The Groma Written by: Stan Kitchener. A line-of-sight surveying instrument, for straight roads or right angle construction. The DATUM-PEG is placed in the ground as the chosen starting point. The GROMA is planted in the ground adjacent to the datum peg such that the DATUM-STRING `B.’ of the groma hangs vertically directly above the datum-peg. The top cross of the groma is then point towards the desired direction of the road/wall/trench-line: viewing line `A.’, through `B.’ to `C.’. A man with a RANGING POLE is directed, at a distance, to be in-line with the groma lines A., B. & C., and a STRINGLINE is then run along the ground between them. Then, without disturbing the groma top-cross, walk around to groma-line `D.’ and view line `D.’, through `B.’, to `E’. A man with a RANGING POLE is directed, at a distance, to be in-line with the groma lines D., B. & E., and a STRINGLINE is then run along the ground between them. This produces a RIGHT-ANGLE on the ground between the string lines.

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