GIAA Mailbag: On Carrying a Total Station and the Accuracy of Reflectorless Total Stations
A: Carrying an instrument on a tripod goes back to the days of the open frame transits. They were relatively low accuracy instruments with a vertical axis where the bearings consisted of two nested cones. Deformations of this axis did occur, but were rarely detectable. This is not true with the modern theodolite and its descendent, the electronic total station. Most, if not all, of the manufacturers absolutely do not recommend carrying the instrument on the tripod, particularly if the tripod is held more or less horizontally so that the vertical axis of the instrument is horizontal. This puts a load on the instrument’s bearings that it was not designed to bear. The obvious risk is to the instrument’s ability to deliver accurate horizontal angles. But worse damage can occur with such abuse. They are risking adverse impact on their surveying quality as well as on their wallet.
The reason surveyors have been able to get away with carrying levels on a tripod is that even if the vertical axis is damaged, though the likelihood is somewhat less because of the instrument’s smaller mass, the impact on its ability to generate a horizontal plane only results if the compensator (if it is an automatic level) or the level vial (if it is not an automatic level) is damaged or goes out of adjustment.
Q: I am in the process of buying a reflectorless total station. I have heard that people use them to traverse and to establish control points. I always use targets, tribrachs, etc. How accurate is it if I traverse or establish control with a reflectorless total station?
A: The accuracy of your traverse and your control is a function of how you control, measure and adjust for errors in your work, both random and systematic. A measurement with a three-year-old total station that has not been checked or serviced, mounted on an optical plummet tribrach that has never been checked, to a single prism without a target, mounted on a prism pole that has not had its circular bubble checked or adjusted is likely to have a high probability of suspicion attached to its accuracy. A measurement made with a calibrated instrument, mounted on a tribrach that has been adjusted, to a prism with a target surrounding it mounted on an adjusted tribrach under the same conditions is likely to be more accurate. Similarly, a measurement made with a reflectorless total station to a stake or point on the ground or even a prism pole (with no prism) is likely to be less accurate than if the measurement with the same total station is made to a prism mounted on a tribrach. The reflectorless technology, unless low accuracy traversing is being intentionally done, was not designed to eliminate the place of prisms, targets and tribrachs when doing precise work.
The accuracy of reflectorless EDMs published by most manufacturers is quite high. But if the reflectorless measurement is to a tree, keep in mind that the position of the tree you develop from this measurement is not going to be at the accuracy level given by the manufacturer. Quite apart from assumptions about where the center of the tree lies with respect to its bark, the manufacturer’s statement only applies to the measurement from the instrument to the point(s) on the bark of the tree where the EDM beam struck. Now realize that the bark of the tree is rough. The average distance of the bark “illuminated” by a spot of infrared light projected by the EDM is probably not measured by the EDM. Variations in reflectivity of the bark due to the angle of incidence of the light from the EDM, as well as factors such as moisture content, decay, insect damage and others, may cause a distance to be much more in error than the manufacturer’s standard, which is usually measured, even with reflectorless instruments, to a prism or at the very least, a flat target perpendicular to the line of sight.
The answer to your question is that high accuracy traverses can be done with reflectorless total stations, within the limitations of your manufacturer’s accuracy specifications. But they must be done using precise targeting and positioning devices. This means at the very minimum targets and tribrachs. We recommend prisms also. The spectacular advantages of reflectorless technology in total stations mostly have applicability in non-control measurements, where small sacrifices in accuracy are made up by higher speed of measurement (since you don’t have to wait for the rod person to move to the next point), and reduction in crew size and effort.
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