Groma
is the principal tool used by the Roman surveyors to trace on the
ground simple and orthogonal alignments, necessary to the construction
of roads, city, temples and agricultural lands subdivision.
According to the linguists the groma term derives from the correspondent Greek term gnoma, but there is who makes it derive from a-grumus (field without heaps), that is the plan where they used the groma.
Description
Groma composition
This topographic tool is composed by three fundamental parts:
- stelletta,
it is formed by two equal arms, cross and forming right angle, made in
hollow metal with reinforcement of a soul in wood; from the four
extremity, called curnicula, four threads (plumb lines) fall suddenly down with cone-shape or pear-shape coupled weights;
- rostrum,
is a strong pole with the cylindrical extremities: one to receive the
swivel pivot of the stelletta, the other for the grafting in the
vertical pole; the distance among the two cylinders measure exactly a
foot (0,2963 m);
- ferramento, it is a pole in
hollow metal able to support the preceding parts, equipped in the
inferior extremity with metallic cone-shape point, for the fixing.
The
groma was positioned with the ferramento in vertical position, using
the plumb lines, to verify the parallelism. Through a fifth plumb line
they centered the stelletta with the station point.
The point could be hammered in the ground or ad lapidem, a particular stone that the surveyors brought with themselves.
In this way the groma was ready for use.
The plumb lines falling from the cornicula constitute two
copies of finishing lines, the use of the groma consisted of
contemplating the various points of the ground through these virtual
lines.
Tracing of a simple alignment among the points A and B
Groma was fixed in station in A point, in the B point was planted the meta. Then they rotated the stelletta
so as to reach, through a couple of finishing lines, the destination in
B. They planted along the direction of this view different metae, getting the layout of the alignment. This was the basic procedure for all the operations with the groma.
Simple alignment
Tracing of a orthogonal alignment
Fixed the first alignment A-B, the groma was set in station in
the intermediary C point, intersection of the two directions that will
be traced.
Then rotating the stelletta they established the direction on
the metae in A and in B through the first couple of finishing lines.
After having fixed the stelletta, they contemplated, through the second
couples of finishing lines the other two points, D and E follow the
basic procedure. This was the most frequent case in the use of the
groma; it concerns the tracing of the ecumani of the cities or the
camps and for the assignment of the lands to the inhabitants of the
cities and the colonies.
Orthogonal alignment
Finding the distance of an inaccesible point on the ground from station point
They
positioned at first the groma in the A station point and they centered
the inaccessible point B with the first couple of finishing lines. Then
they established an orthogonal alignment to the line AB, through the
other couple of finishing lines, determining the point C to a certain
distance from A. With groma in C they established C-D, an orthogonal
alignment to A-C. They divided the length A-C in two equal parts and
they settled the groma in the middle point E. Prolonging the alignment
B-E up to the meeting with C-D line, the intersection gives F. The
found distance (C-F) is equal to the looked for one (A-B).
Distance of an inaccesible point on the ground from station point
This procedure was applied for calculating the width of the
rivers, the extension of the marshes and the swamps, the distance of
the ships in approach to the harbors.
Besides with the groma they noticed the data for draw the
ground form, a topographical draft, directed with the help of the
portable gnomone (reduction of the great solar clocks).
A model
At
the following link, it is possible to see a reconstructed and working
Groma, made by Stilarte for Opera Laboratori Fiorentini, 1999. Link: POMPEII - SCIENCE
Sources
Changelog
- (11/03/2006) First version
