New Survey Aids

H. Eiteljorg, II

When we returned from Pompeii last summer, I wrote about the use of reflecting tape mounted on the end of a fishing rod. We had used the tape, placed on a plastic card and taped to the fishing rod, as a target for the total station, and it had enabled us to survey points otherwise inaccessible. I promised, in the Newsletter article describing the device (New Total Station - Surveying in Pompeii), that a more sophisticated version would be made for the 1996 season.

The new iteration of the remote target has been completed. It consists of reflecting tape on a clear plastic ruler and a mounting bracket to attach the ruler to the fishing pole. Not only is the bracket removable - and the device therefore reusable - but the presence of the ruler adds some interesting possibilities to the use of the target. The target can be used as the simpler one was last year, simply to position reflective tape in a difficult-to-reach location. In addition, the markings on the ruler allowed us to place marks on the tape at specified distances from the tip of the device. As a result, if the target is positioned with its tip at the point to be surveyed, the operator of the total station can take a reading at a point other than the one to be surveyed (if the survey point could not be sighted, for instance) and know how far away the sighted and surveyed points are from one another. Better yet, the operator can take two sightings, each a known distance from the tip of the target and each on a single line leading to the target. Then, once the data have been transferred to the CAD system, a line can be constructed between the two surveyed points and extended by the known distance from the points to the tip of the target - the survey point. Using this system it would be possible to survey a point accurately without being able to sight it from the position of the total station.

This system would also permit a survey team to take readings of points that cannot be directly sighted, even if such points are within reach of the ground. It is, in principle, the same system used by one of the total station manufacturers, but it lacks the automation of the (very expensive) total station version.

As the target was being prepared, another simple survey aid was also made. In the past it has often been necessary to obtain a survey point from the corner or edge of a stone. However, it is difficult to put a prism pole - even the mini prism bracket - precisely on the edge of a stone, and if the stone has been eroded or abraided, the positioning is impossible. Furthermore, finding the same spot multiple times can be very difficult. Last year in Pompeii a simple piece of wood, with a small hole drilled through and given beveled sides was used to provide a support. The hole could be aligned with the edge or corner of the stone by sighting through the hole; then the survey team member with the prism would stand on the wood to hold it in position and place the tip of the prism pole in the prepared hole. (Using the mini-prism, the survey team member might simply put his hand or knee on the wood to hold it in place.)

In preparation for the 1996 season, a more useful device has been prepared. This one is a piece of transparent acrylic with lines on the surface in a variety of patterns and holes drilled through the material in other patterns. There are also two holes with beveled sides, one the appropriate shape for the prism pole and one the appropriate shape for the mini-prism bracket. The combinations of lines and holes will make it possible to position the acrylic at virtually any place desired on a stone and to know precisely where, relative to the corner and edges of the stone, the prism pole has been placed. This is a simple device, but it should help the survey team to work more quickly and more accurately.