equipment calibration

chapter ten

10+356

When sighting onto a level rod (story pole), usually the center part of the horizontal crosshair next to the vertical hair is used. However, any part of the horizontal crosshair should be able to be used to read the rod. If the crosshair isn’t oriented properly, it may cause error in the rod reading.

Objective

The horizontal crosshair should lie in a plane perpendicular to the vertical axis.

Test

Level the instrument and sight the horizontal crosshair towards a well-defined point. A very small pencil mark on a power pole is acceptable.

Placing the crosshair on the point, slowly rotate the instrument about its vertical axis, using the tangent screw. The horizontal crosshair should stay on the point as the in-

strument is rotated.

Repeat the test several times. If the crosshair moves off of the point, the crosshair reticule is in need of adjustment.

Adjustment

This adjustment cannot be made in the field on many instruments. Take the instrument to a repair facility.

If Not Adjusting at this Time

Make it a point to always use the center part of the horizontal crosshair. If this is consistently used, any reading error will be negated.

Test Three - Two-Peg Or Quick-Peg Test

Testing to ensure that the line of sight is horizontal has always been called the two-peg. It requires reading a rod, and adding and subtracting the rod readings to determine if the line of sight is horizontal. It works well.

An alternative method of performing the two-peg is what has appropriately been labeled the Quick-Peg. The same theory applies in setting up in the middle between two points, and then setting up close to one of the points. However, rather than taking readings and having to add and subtract them, NO recorded readings are necessary. Field engineers can even set up this test so it can be performed by just one person in less than five minutes!

Even when the instrument bubble is level (Dumpy) or the compensator is working (Automatic) there is the possibility the line of sight is not truly horizontal. The crosshairs may be positioned with the line of sight inclined slightly causing faulty readings.

Objective

The line of sight should be parallel to a horizontal line represented by the axis of the level bubble in a Dumpy Level, or the axis through the compensator in an Automatic Level.

Procedure For Quick-Peg