Talk:Groma (surveying)

How does this really work?
Having worked on civil engineering sites, I have set out right angles and other shapes using a variety of techniques and instruments and I just don't believe anything that I have read about the groma.

1. Why bother having 4 plumb lines? A single line will show that your pole is vertical and you could sight between notches cut on top of the instrument - no need to try and look through the strings.

2. Having your pole vertical is not enough. The groma itself (ie the cross which holds the plumb lines) must also be horizontal, since an oblique tilt will cause the plumb lines to be out of square. The plumb lines only assist in ensuring that it is horizontal if you assume that the groma is rigidly and accurately attached to the pole.

3. Why are the sighting marks, ie the plumb lines, so close together? If you are sighting a distant object the further apart the sights are the better. I'm sure the Gromatici were't pushed for space, and if they were, I'm sure they could have developed a fold-up model.

4. There are many more accurate ways to set out a right angle. Probably the most convenient for smaller areas, such as buildings, would be to use a large set-square made out of wood.

I believe that our understanding of the groma comes from Vitruvius. Could it be that he has misunderstood what it was being used for? Or, more likely, that his work has been misunderstood by the translators who have then passed on to us a misleading description?

The Romans were clearly a clever and intelligent people. They simply would not have used an instrument with so many flaws, but would developed something better.

Patan46 (talk) 17:28, 23 May 2011 (UTC)


 * The reason why the sighting marks are so close together because with 2 sighting marks in a row, you are sure that you are looking strait, and then the sights to tell if the road is straight are the 2 plumb lines and another pole a long distance ahead. Also, the groma not only tells whether or not you have a good right angle, but also straight lines

--Mulan15262 (talk) 21:55, 22 September 2012 (UTC)


 * Patan46: (Point 1) 'Why bother having 4 plumb lines?' One answer is to take account of inclination. Roman roads, and other surveying projects would rarely have been consistently and conveniently horizontal, where rigidly placed notches would be ideal. Sighting through vertical strings, combining a fixed azimuth with infinitely variable inclination, overcomes this problem. Secondly, it allows a single instrument to be used for multiple purposes, like sighting either a continuous straight line or one that is perpendicular. Third, they permit checking of the supporting pole for vertical in two planes at once.

The answer to Point 2 is to follow a regime of periodical calibration, and to reject an instrument that shows even the slightest slackness in any of its critical components.
 * I'm not entirely sure about Point 3, since I do not know the precise size of an operational Roman groma if, indeed, there was a single standard size. But it occurs to me that, on physically extended projects, the instrument will be good enough to get a good approximation for the next survey point. Then, at the end of a short march, final adjustment might me made by turning around, and sighting back along the previous two survey points (without using the groma).
 * Regarding Point 4, where greater accuracy was needed, I feel confident the Roman surveyor would have employed additional methods. For many purposes, however, it has been found that (mis-)alignments of 2 or 3 degrees were not uncommon. This was particularly likely where simultaneous forward and backward sighting was not possible, like where a road passed over a broad, round-topped hill.
 * Overall, the versatile groma was more than adequate and fit for purpose. --DStanB (talk) 16:32, 14 April 2016 (UTC)


 * 1) The four plumb lines on the rotating crosspiece allow for alignment and self-calibration and self-proving of a right angle. You sight across opposed lines, and you can pick out fore-sight and back-sight points well above and below the horizon without tilting your instrument.  With the pivot, you can sight a temporary marker at the alleged 90°, rotate the crosspiece 1/4 turn and see how inaccurate the device is, and correct it by 1/2 the error. Self-proving and calibration is important for a survey tool a person would have to carry into the field.  2) As long as the legs are at right angles, the pole doesn't need to be vertical and the groma doesn't need to be completely level--level left and right is sufficient.  The plumb lines, when plumb, will always be parallel. If the arms are square, and lined up along the line of interest, any tilt for & aft would foreshorten the crossarms, but they still remain in the plane perpendicular to the line of interest. Drf5n (talk) 18:25, 10 December 2021 (UTC)

Illustration
A good schematic illustration is available in the M. J. T. Lewis' work cited in our article (page 127). The drawing was actually made by Adolf Schulten in 1912, so it will get into the public domain in 2030. Викидим (talk) 05:08, 30 September 2023 (UTC)