A transit will not measure angles accurately unless the instrument is in the following condition:

1. The vertical cross hair must be perpendicular to the horizontal axis. If the vertical cross hair is not perpendicular, the measurement of horizontal angles will be inaccurate.

2. The axis of each of the plate levels must be perpendicular to the vertical axis. If they are not, the instrument cannot be accurately leveled. If the instrument is not level, the measurement of both horizontal and vertical angles will be inaccurate.

3. The line of sight through the telescope must be perpendicular to the horizontal axis. If it is not, the line of sight through the telescope inverted will not be 1800 opposite the line of sight through the telescope erect.

4. The horizontal axis of the telescope must be perpendicular to the vertical axis. If it is not, the measurement of both horizontal and vertical angles will be inaccurate.

5. The axis of the telescope level must be parallel to the line of sight through the telescope. If it is not, the telescope cannot be accurately leveled. If the telescope cannot be accurately leveled, vertical angles cannot be accurately measured.

6. The point of intersection of the vertical and horizontal cross hairs must coincide with the true optical axis of the telescope. If it doesnt, measurement of both horizontal and vertical angles will be inaccurate.

NOTE: Any or all of the above conditions may be absent in an instrument that is defective or damaged, or one that needs adjustment or calibration.

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

1. Settlement of the tripod in yielding soil. If the tripod settled evenlythat is, if the tip of each leg settled precisely the same amountthere would be little error in the results of measuring horizontal angles. Settlement is usually uneven, however, which results in the instrument not being level.

2. Refractionbut the effect of this is usually negligible in ordinary precision surveying.

3. Unequal expansion or contraction of instrument parts caused by excessively high or low temperature. For ordinary precision surveying, the effect of this is also usually negligible.

4. High wind may cause plumbing errors when you are plumbing with a plumb bob and cord and may also cause reading errors because of vibration of the instrument.