Thursday, August 9, 2012

The Groma

[We had so much fun with the Jacob's Staff (well, I did), that I hope you don't mind talking so soon about another tool for more measurement.]

Roman roads, the grid system for Roman camps, sections of Hadrian's Wall—how did they get their lines so straight? They used a groma. The groma had several parts:

  • The Upright: a long pole with a metal point on the bottom for driving it into the ground.
  • The Rostro: an arm that extended horizontally from the top, then angled upward, designed to swivel where it attached to the Upright.
  • The Groma: a cross of four equal-length arms, with its center at the top-point of the Rostro. From the end of each arm, an equal length plumb line descended.
  • The Marker Peg: a plumb line that descended from the center of the Groma's cross-piece all the way to the ground.
The Upright would be stationed so that the Marker Peg fell to what you wished to be the center of your surveying. From there, you could swivel the Rostro so that each arm extended to the cardinal points of the compass. Once the four plumb lines ceased swaying, you could send your assistant toward the horizon (standard distance was 125 paces). By getting behind the groma and sighting so that two opposite plumb lines were aligned, you could, with a wave of your hand, guide your pole-carrying assistant laterally until his vertical pole aligned with the plumb lines. You now had a straight line of hundreds of feet between two points, exactly along the direction you wanted.

Who used the groma? Land surveyors, or as they were called in Latin, Agrimensores (from ager, "field" and mensura, "measure). Which brings me to the Corpus Agrimensorum Romanorum ("The Work of Roman Surveyors"), considered the earliest scientific or technical manuscript that exists, from a time (the 5th-6th centuries) when most documents were about religion or literature (or politics). The page seen here is from the section on laying out a house as opposed to a settlement. One picture is of the house, and one is of the property lines around it.

A 1554 edition of the Corpus added illustrations to the text with grid-lines to show the accuracy of the process. A few examples of its pages and illustrations can be seen here. And if you are interested in robots, you might get a chuckle from this after today's post.

1 comment:

  1. also, after your first alignment you would rotate the rostro a quarter turn and do it again. Any difference in the alignment would then be split eliminating any error in the instrument and giving an exact 90º
    jes sayin