Back to Basics
A construction company manager once showed me some of the projects his company had built. He discussed a building that was erected out of plumb. It seems they plumbed the steel by using a transit. Unfortunately, the instrument was out of adjustment, and they didn’t use standard practices by sighting direct and reverse, or change technology by using a simple plumb bob or four-foot level to check plumbness before the steel was welded. That simple mistake in procedure cost them quite a bit of money in redesign of the curtain wall system of the building to fit the steel.
Using standard practices and occasionally testing angle-measuring instruments in the field cannot be overemphasized. Most problems stemming from instruments being out of calibration can be eliminated by using the direct and reverse process for critical-angle measuring or alignment work. However, since proper techniques are sometimes neglected, it is very important that the instrument be in proper adjustment for times when only direct angles are turned. Perform the following tests to be sure your instrument will perform as expected.
Review of Instrument GeometryThe principal lines on a theodolite are shown on an old engineer’s transit for clarity. An optical theodolite, digital theodolite or total station all have the following geometric lines and relationships (see Figure 1).
The following are geometric relationships and test objectives for a transit, theodolite or total station.
- The vertical axis should be perpendicular to the axis of the plate levels.
- The line of sight should be perpendicular to the horizontal axis.
- The horizontal axis should be perpendicular to the vertical axis.
The Plate Level Bubble TestWhen leveling an instrument, a bubble that moves off-center when the instrument is rotated 180 degrees indicates the vertical axis is not truly vertical—a fundamental requirement when turning angles. The vertical axis should be perpendicular to the plate level axis (see Figure 2). The objective of this test is to make the plate bubbles center when the vertical axis is vertical.
- Level the instrument so the bubbles are exactly centered when over two opposite leveling screws.
- Rotate the instrument 180 degrees about the vertical axis.
- The bubbles should remain exactly centered. If they are, the objective has been met.
- If the bubble does not remain centered, perform the following adjustment.
- Note the amount the bubble moved from the center. This amount is double the total bubble error.
- With the proper tool, rotate the capstan screw to move the bubble one half the amount of error towards the center.
- Re-level the instrument, rotate 180 degrees and re-check the bubble.
Check adjustmentRepeat test and adjustment steps until the bubble remains centered in all locations.
If not adjusting at this timeUsing the instrument without making the adjustment is possible if the observed total error is balanced. That is, bring the error halfway back to center with the leveling screws. It will appear the instrument is out of level, but it is not. Split the difference!
The Line of Sight TestWhen prolonging a line, the line of sight after each plunge should hit in the same place on the hub. If instead two points result, then the instrument needs adjusting. The objective is to determine if the line of sight is perpendicular to the horizontal axis (see Figure 3).
- Set up and level the instrument over the center point, “B.”
- Sight precisely onto point “A” with both horizontal motions clamped and invert (plunge) the telescope toward point “C.”
- Set a hub at point “C” and very precisely mark the line of sight “#1” on it.
- Release the lower motion and turn the instrument 180 degrees back to sight onto point “A.”
- Invert the scope again and sight to the hub “C.” If the line of sight “2” hits on the hub, precisely mark it. If the line of sight hits off of the hub, nail a short board to the top of the hub in order to make the mark (see Figure 4).
- If both line of sight marks, “1” and “2” fall exactly in the same place, the objective has been met.
- If two separate marks result after performing the test three times, the instrument needs to be sent in for adjustment.
AdjustmentAlternatively loosen and tighten opposite crosshair capstan screws to move the cross-hair in the direction needed. This is best done by an equipment technician at an approved repair facility.
If not adjusting at this timeThe true point is always halfway between “1” and “2.” Split the difference to set the actual point (see Figure 5).
The Horizontal Axis TestWhen plumbing a structure, this test is extremely important. If the horizontal axis isn’t truly horizontal, a control line projected upward will not be plumb.
ObjectiveThe horizontal axis should be perpendicular to the vertical axis (see Figure 6).
- Set up the instrument to within 200 feet of a building with a high, well-defined point, such as a church steeple. Be sure the instrument is level (see Figure 7 on page 56).
- Sight onto the high point “A,” locking both horizontal motions.
- Lower the scope to set a hub at the base of the object sighted on. Place a hub or locate a spot on something solid where a mark can be made. With a sharp pencil, mark the line of sight “B.”
- Invert the telescope, release the horizontal motion, rotate the instrument 180 degrees and sight onto the high point “A” again.
- Lower the scope and mark the line of sight “C” on the top of the hub.
- If the two marks “B” and “C” coincide, the objective has been met. If there is a deviation, repeat the test two more times to confirm the results. Then, if two separate marks result, the instrument is in need of adjustment (see Figure 8).