surveying iii (topographic and geodetic surveys) - Modern Prepper

out of adjustment. If the index mark is used, it should be checked each time the instrument istransported, as it may be thrown out of adjustment.(5) In the first method, the presence of the index mark was completely disregarded. Aspreviously explained, once it has been proven that the index mark is not out of adjustment, it may beused to determine the minute value of the reading. This is the faster and easier method of determiningthe minute value. The index mark always refers to the degree of the lower erect image or lower circle.Using the index mark in this way (in relation to only the lower circle), each graduation is equal to 4minutes. When the coincidence is made between the images of both sides of the horizontal circle, or theupper and lower circles as they appear in the circle telescope, the index mark will always be either on agraduation line or halfway between two graduation marks. There are two ways to determine the minutevalue using the index marks on the lower circle. If the index is on a graduation line, the minute value isfour times the number of graduations between the degree mark to the left. If the index is halfwaybetween two marks, add 2 minutes to four times the whole number of graduations between the degreemark to the left and the index mark to obtain the minute value.Example 1: When the true degree value shows in the circle telescope as does 10 in Figure 3-11, thevalue of the direction equals--10° + (1 division x 4') + 2'10° + 4' + 2'10 06'Figure 3-11. Determining the Minute Value Using the Index Mark (Visible True-Degree Value)Example 2: Since the index mark in Figure 3-12 is to the left of the 10° value shown in the lower circle,the minute value must be subtracted from the visible 10 value in order to give the true value of thedirection. Thus, when the true degree value (9) does not show in the circle telescope, as is the case inFigure 3-12, figure the minute value between the index mark and the degree mark to the right andsubtract as follows:= 10° - (3 graduations x 4') + 2'= 10° - (12' +2')= 10° - 14' = 9° 46'EN0593 3-22