### 7. Collimation correction

Once the collimation error has been determined, there are three ways to deal with it

#### (1) Adjust the crosshairs

The reticule adjusting screws are located near the eyepiece under a cover. Removing the cover reveals the adjusting screws at the top and possibly bottom of the reticule (there are also side adjusting screws but they aren't necessary for this adjustment), Figure G-10

 Figure G-10 Reticule Adjusting Screws

If there is an upper and lower screw, then one must be first loosened and the other then tightened to move the crosshairs.

If there is only a single screw then only that screw is loosened or tightened.

How far to move the crosshairs?

For the previous example, we know that the total error in the BS from point 2 to A is: (152 ft)x(-0.0000266 ft/ft) = -0.004 ft.

The correct BS reading should be 5.540+0.004 = 5.544 ft

That means to remove the collimation error, we would use the adjusting screws to raise the crosshairs to a 5.544 ft BS reading.

Hmm... trying to see 0.004 ft at 152 ft... not too easy.

Probably best to leave the crosshairs alone and use one of the other two methods.

#### (2) Apply a correction to readings

Whenever the level is used, the correction could be applied to each reading made, Figure G-11.

 Figure G-11 Unbalance Sight Distances

The collimation error is compensated by multiplying the correction by the BS and FS length difference and adding it to the elevation difference.

This requires the length of each sight be captured as the readings are taken. This is the case when three-wire leveling but generally not so when doing simple leveling using only the center hair unless paces are counted and recorded.

#### (3) Balance BS and FS distances

The simplest way to deal with the collimation error is to balance the BS and FS distances so the error cancels itself. No calculations are needed. Plus this method has the advantage of compensating other distance-based errors: refraction and curvature.

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