Personal errors are the combined results of carelessness and of the limitations of the human

Figure 13-28.-Exaggerated illustration of error caused when the transit is not centered exactly over the occupied station.

eye in setting up and leveling the instrument and in making observations.

Common causes of personal errors in transit work are as follows:

1. Failure to plumb the vertical axis exactly over the station. Figure 13-28 shows how the result of inaccuracy increases drastically as the sight distance decreases. In that figure, an instrument supposed to be set up at A was actually set up at A, 40 ft away from A. (For demonstration purposes the figure was exaggerated to magnify the error; in actual practice the eccentricity amounts only to a fraction of an inch. Remember that mathematically, 1 in. is the arc of 1 min when the radius is 300 ft.)

In the upper view, you can see that with B located 300 ft from A, the angular error caused by the displacement is about 8. In the lower view, however, with B located only 100 ft from A, the angular error caused by the displacement is about 22.

The practical lesson to be learned from this is that you must plumb the instrument much more carefully for a short sight than for a long one.

2. Failure to center plate level bubbles exactly. The result of this is that the instrument is not leveled exactly. The consequent error is at a minimum for a horizontal sight and increases as a sight becomes inclined. The practical lesson is that you should level the instrument much more carefully for an incline sight than for a horizontal one.

3. Inexact setting or reading of a vernier. The use of a small, powerful pocket magnifying glass is helpful here. Also, when you have determined the vernier graduation that most nearly coincides with a limb graduation, it is a good idea to check your selection by examining the graduations on either side of the one selected. These should fall in coincidence with the limb counterparts by about the same amount.

4. Failure to line up the vertical cross hair with the true vertical axis of the object sighted. The effect is similar to that of not plumbing exactly over the station, which means that the error increases drastically as the length of the sight decreases.

5. Failure to bring the image of the cross hair or that of the object sighted into clear focus (parallax). A fuzzy outline makes exact alignment difficult.

Common mistakes in transit work are the following:

1. Turning the wrong tangent screw. For example, by turning the lower tangent screw AFTER taking a backsight, you will introduce an error in the backsight reading.

2. Forgetting to tighten the clamp(s), or a clamp slipping when it is supposed to be tight.

3. Reading in the wrong direction from the index (zero mark) on a double vernier.

4. Reading the wrong vernier; for example, reading the vernier opposite the one that was set.

5. Reading angles in the wrong direction; that is, reading from the outer row rather than the inner row, or vice versa, on the horizontal scale.

6. Failure to take a full-scale reading before reading the vernier. For example, you may drop 20 to 30 min from the reading, erroneously recording only the number of minutes indicated on the vernier, such as 1518 instead of 1548. Do not be so intent on reading the vernier that you lose track of the full-scale reading of the circle.