When reading one-position horizontal angles, your first reading is a a reverse reading to the rear station.

b. direct reading to the rear station. c. direct reading to the forward station. d. reverse reading to the forward station.

MCI Course 0813B 4-22 Study Unit 4, Lesson 1 Exercise

Lesson 1 Exercise,

Directions for Item 3 Through Item 6

The figure below pertains to items 3 through 6. Select the appropriate answer based on the recorded field notes.

3. What is the mean reading to the rear station? ____________ 4. What is the mean reading to the forward station? ____________ 5. What is the mean horizontal angle? ____________ 6. What is the mean vertical angle? ____________

Answers The table below provides the correct answers to the exercise items. If you have any questions, refer to the reference page listed for each item.

Item Number Answer Reference Page

1 d 4-4 2 b 4-8 3 0000.142 4-15 4 1093.771 4-16 5 1093.649 4-17 6 -15.944 4-18

MCI Course 0813B 4-24 Study Unit 4, Lesson 1 Exercise (This page intentionally left blank.)

Introduction

Scope This lesson will introduce you to the procedures required to measure two- position angles during survey operations.

Learning Objectives

After completing this lesson, you should be able to

• Identify the procedures required to measure a two-position angle with the theodolite.

• Given a partially completed page of the Field Record Book, determine the mean of two-position angles.

In This Lesson This lesson contains the following topics:

Topic See Page

Introduction 4-25

Measure Two-Position Angles 4-26

MCI Course 0813B 4-26 Study Unit 4, Lesson 2

Measure Two-Position Angles

Introduction It may be necessary for you to measure two-position angles (each angle is measured twice), as in fourth order triangulation or fourth order intersection procedures. You measure the second position in the same manner as the first position, except that you measure the second position with the telescope in the reverse position for the initial pointing on each station. The procedures for measuring and recording the second position and multiple angles are

discussed in this lesson.

Procedures After completing the measurements and recording the data for the first position, wait for confirmation from the recorder that the first position met survey closure requirements, and then use the following procedures to complete the second measurement as shown in the table below:

Step Action

1 Set an initial circle setting of 4800.150 (± 0.100) mils on the horizontal scale. This is your reverse reading to the rear station. 2 Announce the reading to the recorder. In the illustration below, the

reading is 4800.138.

Procedures, continued

Step Action

3 Record the two-position measurements in the same manner you record the one-position measurements, except you do not circle the separate mean horizontal angles of the first and second position, you place them in parentheses. You record the second position on a separate page of the field notebook.

4 Loosen the horizontal clamping screw and rotate the instrument (clockwise) toward the forward station.

5 Tighten the horizontal clamping screw and use the horizontal and vertical tangent screws to center the cross lines on the forward station at the lowest visible point. Announce the reading to the recorder. The illustration below shows a reading of 5285.488 mils.

Note: There is no requirement to take vertical reading in the second position.

6 Plunge the telescope to the direct position and sight back on the forward station. Announce the reading 2085.467 to the recorder.

MCI Course 0813B 4-28 Study Unit 4, Lesson 2

Measure Two-Position Angles,

Procedures, continued

Step Action

7 Rotate the instrument (clockwise) back to the rear station. Announce the scale reading 1600.107 to the recorder. The

illustration below shows the second position direct reading to the rear and forward station recorded in the field recorder notebook.

Mean Angles To determine the mean reading of the two pointings to each station for the second position measurements, follow the steps in the table below:

Step Action

1 Add 3200 mils to the direct reading. 2 Add the result to the reverse reading. 3 Divide the sum by 2.

Example: Rear Station R 4800.138 4800.138 D 1600.107 + 3200.000 = 4800.107 (4800.138 + 4800.107) ÷ 2 = 4800.122 Forward Station R 5285.488 5285.488 D 2085.467 + 3200.000 = 5285.467 (5285.488 + 5285.467) ÷ 2 = 5285.478

Mean Angles, continued

Step Action

4 Place the reading in the recorder’s notebook. The illustration below shows the mean reading to the forward station, second position.

5 Determine the horizontal angle by subtracting the mean reading to the rear station from the mean reading to the forward station. If the mean reading to the forward station is less than 4800, add 6400 to the mean reading to the forward station, then subtract the mean reading to the rear station.

Example:

Mean reading to the forward station 5285.478 Mean reading to the rear station - 4800.122 Horizontal station angle 0485.356

MCI Course 0813B 4-30 Study Unit 4, Lesson 2

Measure Two-Position Angles,

Mean Angles, continued

Step Action

6 Place the second position mean horizontal angle in the recorder’s notebook. The illustration below shows the second position mean horizontal angle recorded properly in the recorder’s notebook.

7 Compare the mean horizontal angle of the second position to the mean horizontal angle of the first position to see if they agree within 0.05 mils. You do this by subtracting the smaller angle from the larger angle. The second position page of the field record book has a column labeled MEAN HORIZ . This is where you enter the first and second position angles and determine the mean. Example:

Second position mean angle 0485.356 First position mean angle - 0485.336 Difference .020

Mean Angles, continued

Step Action

8 If they agree within 0.05 mils, add the two angles and divide by 2. Enter the mean of the first and second position in the field record book and circle it. The illustration below shows the mean

horizontal angle of the first and second position recorded in the notebook.

MCI Course 0813B 4-32 Study Unit 4, Lesson 2 Exercise

Lesson 2 Exercise

Directions Complete exercise items 1 through 4 by performing the action required. Check your answers against those listed at the end of this lesson.

Item 1 As part of a fourth order survey, it may be necessary to measure two