Back to Basics
The leveling system on instruments is simple and easily understood. In its simplest form, a bubble (level vial) is located perpendicular to the vertical axis of the instrument. When the bubble is centered, the instrument is level and ready for angle turning. There are instruments with one or two level vials, and there are now instruments with digital sensors displayed on a screen for leveling. Whatever the device, the procedure is the same; the bubble/sensor is centered by simply moving the leveling screws up or down as needed.
General PrinciplesThe leveling procedure requires that these basic principles be followed to efficiently level the instrument.
The head of the tripod should be horizontal
Start the leveling process with the head of the tripod close to horizontal and over the point. A tripod head that is not horizontal requires more movement of the leveling screws, thus, taking more time to complete the process. Insert the tripod legs farther into the ground or adjust the length of the tripod legs up or down to get the head of the tripod horizontal and over the point.
Left thumb rule
"Both thumbs in, both thumbs out, the bubble follows the left thumb." This rule applies for all types of leveling systems because manufacturers use the same thread direction in their leveling screws. The left thumb rule is very simple; but I repeatedly encounter people who are not aware of it. They use a random method for leveling the instrument, which takes much longer. Both thumbs in, both thumbs out means that the thumbs are moved in different directions. A graphical illustration of the rule is shown in Figure 1.
Level vial positioning For a four-screw leveling head system, position the level vial over opposite leveling screws as shown in Figure 1. Rotate the instrument 90 degrees to get over the other two as the left thumb rule is used to center the bubble. When using the left thumb rule, make sure to move each thumb the same amount, otherwise the leveling screws will get very tight or very loose from the unequal movement. An illustration of the operation of the four-screw system is shown in Figure 2.
A nice feature of the four-screw leveling head system is that it allows the instrument to be re-leveled without changing the height of the instrument. This is possible because the four-screw system involves the pivoting of the instrument around a half ball at the bottom of the spindle. This is very useful when using the dumpy level because the instrument user can re-level between readings as necessary.
For a three-screw leveling head, the bubble should be positioned so that it is over two leveling screws and then turned 90 degrees and placed over the single remaining leveling screw as in Figure 3.
With a three-screw system, I have seen many people make the mistake of rotating the bubble 120 degrees after each centering and then re-position the bubble over two screws again. This has the effect of chasing the bubble around the instrument and prolongs the leveling process considerably. Using two and then one and repeating this procedure is the most effective method of leveling a three-screw system.
Always rotate 180 degrees as a check When leveling an instrument, continually rotating the instrument 180 degrees checks that the instrument is in proper adjustment. That is, by rotating 180 degrees, it can be determined if the axis of the bubble is perpendicular to the vertical axis of the instrument, which is a basic requirement of the geometry of an instrument. (See Figure 4.) If it is found that the bubble drifts off center when rotated, the bubble is in need of adjustment. That is a topic for another article
Turn leveling screws equally(four-screw system)
When using two leveling screws together, rotate them the same amount in opposite directions at the same rate. This is extremely important with the four-screw leveling system. If they are not operated exactly the same, they will become too tight or too loose.
When the leveling screws become too tight on a four-screw leveling system, it is difficult to operate. If the operator continues to operate it incorrectly, it will become tighter and tighter. This will result in not being able to move the leveling screws at all. In an extreme case, the structure of the leveling head itself could be damaged or the threads on the leveling screws could be stripped.
If improper operation causes the leveling screws to become too loose, the instrument will rock on the leveling head. This will make it difficult to keep the instrument level during the operation of turning angles. Again, rotate leveling screws the same amount at the same rate.
This article has reviewed a simple topic, but one which will hopefully be helpful to those who struggle with leveling an instrument. Have fun surveying!