P10 - Horizontal Angle Measurement

Purpose

The purpose of this exercise is to (1) measure the interior angle of a station in a closed traverse, (2) close the horizon at the traverse station, and (3) adjust the interior angles to the geometrically correct sum.

Equipment

Total station with operators manual, tripod, plumb bobs, field book and pencil.

Field Notes

Your field notes should resemble the sample note form presented at the end of this lab description. Provide a clear, neat, detailed, sketch of your traverse. Be sure to place a north arrow on the sketch. Include any objects such as roads, sidewalks, large trees or buildings to help orient the note reviewer with the traverse. The bottom line is to make sure that all recorded notes are very clear. Field books are due 24 hours after the end of the practicum period.

Procedure

Identify one traverse station for measurement by the group. The first member of the crew will measure the traverse interior angle of the station using the 2DR (twice direct and reverse) method. A second member will subsequently measure the exterior angle of the traverse station using 2DR. Measuring the exterior angle performs a check on the interior angle. The measurement of the exterior angle is often called "closing the horizon," and is a common technique used to check the measurement of a single angle before the instrument is moved. Continue to re-measure the interior and exterior angles of the station until all members have had an opportunity to perform at least one 
2DR measurement of an angle. 

The instrument operator’s manual may provide enough detail to explain the 2DR measurement process specific to your instrument. Refer to Table 10.1 to identify the appropriate sections to read in the manual. Certain manuals assume that the operator understands the repetition process. Therefore, use the operator’s manual in conjunction with the following explanation of the general repetition measurement process.

Measuring Angles by the Directional Method

Measure the horizontal interior angles in a clockwise direction. Traverse in a counter-clockwise direction. Proceed to measure each interior angle. The following steps are very generic. More specific details about the correct measurement procedure may be found in the instrument manual, and in Section 8-9 of your textbook. 

Total Stations with Dual Horizontal Motions

Special attention must be used to the horizontal motion screws when using instruments with upper and lower horizontal motion screws. These are typically older total stations (SOKKISHA SET 4 and TOPCON GTS-3C). The lower motion screw was necessary to mechanically zero the instrument in the days prior to the electronic zero set button. The first instruments that had zero set buttons as a "new" feature retained lower motion screws for some unknown reason. 

The best approach for using these older instruments is to lock the lower horizontal motion screw and not use it. Incorrect use of the lower motion screw during the horizontal angle measurement process will interject a large error in the angle measurement.

The Leica TC600 Total Station

The Leica instruments do not have a hold feature in the horizontal angle measurement mode and therefore, the angle measurement by repetition requires a slightly different approach. The following steps should assist you in the operation of the Leica TC600.

Angle Measurement Face Left

1. Place the scope in the direct (face left) position.
2. Sight the target A (backsight).
3. Lock the horizontal motion and fine-adjust into the target A.
4. Set the horizontal angle display to zero and record the horizontal angle display.
5. Unlock the horizontal motion and turn the clockwise angle to target B (foresight).
6. Lock the horizontal motion and fine-adjust into the target B.
7. Read and record the horizontal angle display.

Angle Measurement Face Right

1. Invert the scope to the plunged (face right) position.
2. Unlock the horizontal motion and re-sight the target B.
3. Lock the horizontal motion and fine-adjust into the target B.
4. Read and record the horizontal angle display.
5. Unlock the horizontal motion and turn the counter-clockwise angle to the target A.
6. Lock the horizontal motion and fine-adjust into the target A.
7. Read and record the horizontal angle display.

Average Angle Computation

1. Subtract the first recorded value of the horizontal display from the second recorded reading. Both of these readings were observed in the face left mode. The result is the value of the direct angle measurement. 
2. Subtract the third recorded value of the horizontal display from the fourth recorded value. Both of these readings were observed in the face right mode. The result is the value of the reverse angle measurement.
3. The average of these two angles is the average angle measurement. Compute and record this angle.

Close the Horizon - Measure the External Angle (See Section 8-10)

1. Repeat the procedure for the angle external to the traverse.

Additional Notes

1. The mean value of the angle measured equals one half the reverse reading. The mean value of the angle measured should agree with the direct reading of the angle to within the least count (horizontal angle accuracy) of the total station. If this is not so, then repeat the 2DR measurement of the angle.
2. To target the end of each line sighted, use a plumb bob held over the point. A short string hold should be used when possible. The instrument person should sight the plumb bob string as the target. 
3. When using a plumb bob as a target, the bob must be able to hang freely (not touch the monument). If the instrument person can sight the monument (aluminum disk) directly, then use a pencil or the end of the plumb bob to locate the monument mark. The instrument person should use the very end of the pencil/plumb bob as the target.
4. Rotate the duties among the party members as the angles are measured around the traverse. Each party member is required to measure at least one interior angle by 2DR.
5. Where groups are composed of four members, and traverses have only three stations, at least one angle in the traverse will be measured 2DR, four times, to accommodate the fourth member.

Computations

The sum of the angles measured at a given station should equal a full circle. For this exercise, the allowable misclosure in the sum of the angles at a station can be computed where "L.C." is the least count (DIN 18723 standard) of the instrument and “n” is the number of angles in the horizon. A misclosure greater than this amount should be corrected after consultation with the instructor. Adjust the measured interior angles to add to exactly the correct geometric sum. Use 1/2 the instrument's least count as the minimum increment of angle adjustment.

1. The sum of the two angles at each station should be 360°. Compute the misclosure and compare with the value of 1.96×DIN at a 95 percent probable error. If it is greater than this value, repeat the entire measurement process for the angle with the largest discrepancies.
2. The sum of the angles measured in the traverse should equal the geometrically correct sum of

S angles = (n - 2)180°

within an allowable misclosure of  where DIN is the least count of the instrument and “n” is the number of interior angles in the traverse. A misclosure greater than this amount should be corrected after consultation with your instructor.

3. Adjust the measured interior angles to add the correct geometric sum using the least count (DIN) as the minimum increment of adjustment.

Report

1. Following the guidelines on the specifications sheet, the report should include
1. Horizon misclosure at each station.
2. Traverse misclosure and allowable misclosure.
3. Adjusted angles, and largest discrepancy.

Sample Note Form