Reading оf horizontal angles

The usual methods of reading the angles are:

1. By direct reading.

2. By azimuth.

3. By repetition.

I. By direct reading. In this method the telescope is set on the back target with the plates at zero using the lower clamp, then without plunging the telescope is turned to the forward target using the upper clamp and the angle is read. The instrument should be graduated from 0° to 360°, preferably to the right, so that angles are measured at the clockwise direction.

The direct reading is the most natural way to measure the angle. It is used for numerous purposes but it gives only a single reading without any check except by repeating it.

2. By azimuth. In this method the telescope is reversed and set on the back target with the plates set on the azimuth of the last course as read from the previous station using the lower clamp. Then the telescope is plunged and is turned to the forward target using the upper clamp and the azimuth of the forward course is read from the plates. The instrument should be graduated from 0° to 360° to the right or clockwise. After taking the foresight the plates are left clamped and are used in this position without resetting unless they have slipped in taking the backsight at the next station.

This method is a rapid one for traverse work, especially when combined with stadia measurements for the distances. The instrument retains the fractions of the minutes which are too small to be read. This gives a traverse whose angles close exactly. With the same care in doing the instrument work and in the measurement of the distances, the results by this method are more accurate than by any of the preceding ones but not so accurate as by the repetion method. There is no check on the angle readings as they are taken only once, and if repeated the plates have to be moved and the accumulation of the fractions of the minutes is lost.

3. By repetition. In this method the telescope in the normal position is set on the back target with plates set at zero using the lower clamp. Then the telescope without plunging is turned to the forward target using the upper clamp and the angle is read from the plates and recorded. This is the first reading and the method so far is the same as that by direct reading. Then without resetting the plates or plunging the telescope a second reading is taken consisting of a sight on the forward target using the upper clamp. The plates will then show twice the original angle but this need not be read. The angle is repeated, as on the second reading, as many times as desired. The vernier is read at the end of the last measurement and this angle divided by the number of repetitions gives the angle between courses. The last angle should agree closely with the first one of the set, and the comparison of the two will detect errors in the work. By taking the first half of each set with the telescope in one position and the second half with it in the reversed position many instrumental errors are balanced and so eliminated. The instrument should be graduated from 0° to 360° to the right.

The method is always used for tnangulation work and should be used on other important work. It is recommended for traverse work that a set of four repetitions be used, two readings being taken with the telescope normal and two being taken with it reversed.

Дата добавления: 2015-07-19