The Surveyor's Cross

The surveyor's cross is a simple instrument for making alignments at right angles to each other. It has no movable parts. Its primary feature is vertical slits, hairs, or strings positioned opposite and at right angles to each other—inches or feet apart, depending on the form of the instrument. By lining them up, lines can be projected on the ground for a considerable distance.

The simplicity of the instrument, however, belies its versatility. Besides laying out rectangular plots of ground and street grids with reasonable accuracy, it can be used to make indirect measurements to and determine the altitude of distant or inaccessible points by applying the theory of similar triangles (Elements of Surveying by Charles Davies, 1836, p. 74).

The most advanced version of the cross is an octagonal brass prism. Mine is a little over three inches high and two and a half inches across, with a variety of slits—matching slits—on each face. It also has a small compass affixed to the top, graduated to two degrees. At the bottom, it has a detachable sleeve that allows it to be set over a pin on top of a tripod or staff (Surveying Instruments: Their History by Edmond R. Kiely, p. 148).

A slightly earlier version consists of only four sight vanes screwed to two bars fixed permanently to each other at right angles. The sight vanes are similar to those on compasses, but one of each pair has a vertical slit and the other a hair in a wider opening, called a window (Davies, p. 74).

Both of these crosses are 19th century pieces—19th century AD. The earliest version of the cross, however, dates back at least to the 19th century BC, to the reign of Pharaoh Sesostris II. This is probably the pharaoh whom Joseph served, the Joseph who predicted the seven years of famine and prepared Egypt for them by storing grain during the preceding seven years of plenty (Genesis Chapter 41). The Greek historian Herodotus credits this same pharaoh with originating a canal system and providing a tax adjustment for land lost by the flooding of the Nile. All these initiatives required land measurements (The Roman Land Surveyor by O.A.W. Dilke, 1992, p. 20).

The tools with which the ancient Egyptian surveyors made these measurements (we know from tomb drawings) were a knotted rope to measure distance and a form of the cross to provide alignment. The cross pieces of such a cross, made of palm leaf ribs and tied together with a palm fiber cord, were actually found during an archeological excavation in 1899 and supposedly date to the centuries just before Christ. The cross was suspended from a string held in hand, and it had four plumb lines dangling from the ends of the cross pieces. The plummets were not found (Dilke, p. 27).

In addition to surveying tools by the ancient Egyptians, two crosses of Roman origin have also been found. The lesser of the two consists of two iron strips fused together, with a hole at their intersection and the ends bent down. The ends seem to have held plumb lines, and the cross seems to have been set on a staff. The disadvantage of this arrangement is that the staff obstructs the actual sighting of the two plumb lines opposite each other (Dilke, p. 69; similar to the Greek "Star" illustrated in Kiely, p. 28).

The other Roman surveyor's cross was found in 1912 during the excavation of Pompeii, which was destroyed by the eruption of Vesuvius in 79 AD. Among the ruins was the workshop of a surveyor that contained a portable sundial, a folding ruler, thin iron rods, a bronze cone (use unknown), a stylus (for use on a wax tablet, apparently to take notes and make calculations), an ink bottle (implying that final plans were made on papyrus), two bronze drafting compasses, and the bronze parts and a key to a chest (presumably for keeping his records). The cross that was found there was much improved over earlier versions. Principally, it rested on an arm attached to the top of a staff offset from the line of vision between the plumb lines. The staff was much steadier, and the plumb bobs were of two kinds (Kiely, p. 30; Dilke, p. 69f).

The Roman name for this instrument was groma, which seems to have been derived from the Greek gnoma, the word for a sundial pointer, and may refer to the staff. This is another instance in which the adaptation of a word is lost to history. Regardless, the word should not be used to refer to instruments that the Egyptians and Greeks used. Properly speaking, only the type of cross in which the cross pieces are supported by an offset arm is a groma. But so prevalent was the use of this instrument in Roman times that eventually the agrimensor, literally "land measurer," came to be called gromaticus (Dilke, p. 66).

This instrument was used to lay out not just civilian settlements, both cities and farms, but also military camps. In fact, the central point of a military camp was called gromae locus, the place of the groma. Julius Caesar seems to have been such an advocate of surveying that he is sometimes considered the founder of the profession of surveying, at least of Roman surveying (Dilke, p. 37).

As the Roman empire declined, the groma understandably fell into disuse, and the knowledge of it actually disappeared; there is apparently no mention of it in medieval texts. Its reappearance in modern times and its transformation into a prism (shown for the first time in a German book published in 1547, see Kiely, p.130) are also unexplained. Its use, in any case, was limited. Its only distinctly modern use was in conjunction with the surveying compass to set up offset lines when the lines could not be measured directly (Kiely, p. 149). After the 19th century, the surveyor's cross again passed into oblivion.

About the Author

  • Wilhelm A. Schmidt, PLS
    Wilhelm A. Schmidt, PLS
    Wilhelm Schmidt is the former owner of the surveying firm Bascom and Sieger in Allentown, Pennsylvania. You may contact him at willischmidt@verizon.net.

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