Common Sources of Error

The accuracy of leveling can be greatly improved if several common errors are controlled:

1. Instrument out of adjustment: Anytime that the instrument is "bumped" or otherwise moved, it must be releveled. It is a good practice to check the leveling bubble both before and after reading the rod. Note: resetting the level may change the instrument height. It must be checked, and if different or all or some, of the measurements will need to be repeated.

2. Rod not plumb: The rod must always be held plumb using a rod level, or rocked back and forth as explained in the section on reading the rod. A good method of plumbing the rod is to stand behind the rod and balance it carefully on the stake by holding your hands lightly on each side.

3. Parallax: If the cross-hairs appear to move over the object as the eye is shifted slightly, parallax exists. When parallax exists, the line of sight of the eye may not be parallel to the line of sight of the instrument. This source of error is eliminated by adjusting the eyepiece until the cross-hairs are the darkest. Because the eyepiece will need to be adjusted for each person, standard practice is for just one person taking all the readings.

4. Sights not equal: When field conditions permit, the distance of backsights and foresights should be as nearly equal as possible. Thus, any errors due to the instrument's being out of adjustment are minimized because they tend to cancel out.

5. Reading the rod incorrectly: The person reading the rod must be very careful to ensure that the correct foot mark is used, and that the target is used correctly.

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