From Archaeological Methods
Total stations are an efficient and accurate way of measuring angles and distances. These powerful instruments are capable of making calculations during a survey and of producing X, Y, Z coordinates of points shot during the survey. Total stations are an extremely efficient way of obtaining accurate locations of artifacts and datum points in an archaeological survey. These units are able to correct prism contact distances and to notify the user if the unit becomes unleveled during use or if the prism moves out of range. To obtain accurate results and avoid systematic errors, however, the total station must be set up correctly by the user.
II. Parts of a Total Station and Equipment
-Optical Plummet Eyepiece: allows accurate centering over a datum point. The line of sight is in line with the vertical axis of the total station, but will not point directly down or be truly vertical unless the instrument and tripod are leveled properly.
-Horizontal and Vertical tangent screws and locks: these knobs allow for fine adjustment of the cross-hair location through the sight when the screws are locked. If the locks are not set, wider movement of the scope is used to put the cross-hairs in the general location of the prism being shot.
-Collimator: displays an arrow to set on the prism sight when the tangent screws are unlocked. This arrow can be lined up with the range pole operator without keeping one’s eye to the eye piece, which often make it much easier to find the prism, especially when the operator is standing at a distance.
-Leveling Screws: these three screws allow for fine leveling adjustment to be made to the total station towards the end of set up.
-Bull’s Eye Level: this bubble level allows for the tripod to be leveled initially.
-Spirit Level: this glass tube level allows for final leveling adjustments with the leveling screws.
-Keyboard and Display: includes power buttons and readings for points shot.
-Communication Port: this is the port that connects the data logger to the total station (if the total station does not have internal data logging capabilities).
-Battery: needs to be charged before use to collect data or make calculations.
-Tripod: three legged stand to set total station on. The legs are adjustable in length.
-Prism: sits on the range pole and is the target for the eye piece cross hairs in the total station.
-Range Pole: adjustable height pole with a bull’s eye level to keep the prism located directly over the point being shot.
-Data Logger: this unit saves all data collected and holds the screen for the interface that allows the user to interact with the total station.
III. Set Up and Use
Adjust the length of the tripod legs to a height where the total station will be at a comfortable height to look through the eye piece. For stability, the legs of the tripod should have a fairly wide base. Make sure that the legs are set down lightly where the top of the tripod is fairly level and is also centered over the datum point—this can be checked with either a plumb bob or by dropping a small stone from below the center of the tripod top plate. Once this is accomplished, stamp one leg firmly into the ground. This leg will be left alone until later.
Carefully take the total station out of the box. The handles on top of the total station have been known to fall off or break, so it is best to hold the total station by the handle but also with support from beneath it. Set the total station on top of the tripod and tighten the screw to hold it on. Make sure the vertical and horizontal tangent screw locks are on so the unit does not swing around while it is adjusted.
Take a look through the optical plummet to see how close the cross-hairs are to the datum. It is sometimes helpful to put your foot near the datum to get a better idea of how much you will need to adjust the total station’s position. Take the two legs that were not stamped into the ground and with your eye to the optical plummet, move the legs to where the cross-hair is just closer to the stationary leg (it will move when the legs are all stamped in) and the bull’s eye bubble level is centered. You may need to adjust the heights of the legs somewhat. Once the optical plummet is centered and the total station is close to level, the legs can be stamped into the ground as well. Minor height adjustments of the legs will likely be needed at this time to center the bubble.
The optical plummet cross-hair may be slightly off after this, but this should not be a problem as long as it is only off by a centimeter or so. Once the bull’s eye bubble is centered, loosen the screw holding the total station in place and with an eye to the optical plummet, slide it until the cross-hair lines up with the datum. Then retighten the screw holding the total station in place.
Now check the spirit level. Adjust the leveling screws until the bubble is centered. You should always adjust the two leveling screws the spirit level is between at the same time, and always with either both thumbs turning inwards or both thumbs turning outwards. The bubble will follow your left thumb as it moves. This is known as the “Left Thumb Rule.” Then turn the level a quarter turn and use the remaining leveling screw to adjust the bubble again. Repeat between these two positions making finer corrections until the level remains centered when the total station is rotated. Check a couple of other directions to be sure the unit is leveled. Check the optical plummet again and if necessary, slide the unit again to center it over the datum.
Make sure the data logger is connected and that both units are on. Either a new or existing job will need to be used, and before each set up, information on the point and instrument height will need to be reset. Instrument height should be measured from the datum to the dot on the side of the total station at the level of the scope. Be sure to use the same units as your survey is being conducted in. When sighting a back sight with known distance and angle, the circle will need to be solved to since when the unit is turned off, the angle resets to zero. Enter the rod height as well to be sure points are recorded at the correct elevation.
When operating the range pole, be sure to keep the bubble centered while the point is being shot. When operating the total station, be sure to tell the range pole operator when you are finished shooting a point so he does not have to hold the range pole steady and centered for longer than necessary.
Wolf, Paul R. and Charles D. Ghilani. “Total Station Instruments; Angle Measurements.” Chap. 8 in Elementary Surveying: an Introduction to Geomatics. 11th ed. Prentice Hall: 2006.