a) Using a Total Station as a Level Instrument in Differential Leveling:
A Total Station is primarily used for measuring angles and distances, but it can also be used as a level instrument in differential leveling. To use a Total Station for leveling, you would follow these steps:
1. Set up the Total Station at a known benchmark or reference point.
2. Level the instrument using the built-in leveling screws and bubble level.
3. Measure the vertical angle to a target point (staff) using the Total Station's vertical angle measurement feature.
4. Record the staff reading at the target point.
5. Move the Total Station to the next setup point and repeat the process, measuring the vertical angle and recording the staff reading.
6. Calculate the height difference between the two points by subtracting the initial staff reading from the final staff reading.
b) Use of hm in Trigonometric Leveling Equation for Distances >= 100m:
In trigonometric leveling, hm (horizontal distance) is used in the leveling equation when the horizontal distance between the instrument and the target point is equal to or greater than 100 meters. This is because for longer distances, the curvature of the Earth becomes significant and needs to be taken into account. The hm value is used to calculate the correction needed to account for the curvature of the Earth in the leveling equation.
c) Difference between Height of the Instrument in Differential and Trigonometric Leveling:
In differential leveling, the height of the instrument (HI) remains constant throughout the leveling process. The instrument is set up at a known elevation and all height differences are measured relative to this fixed point.
In trigonometric leveling, the height of the instrument (HI) changes as the distance to the target point changes. The instrument is set up at each new location and the height is determined based on the trigonometric leveling equation, which takes into account the horizontal distance (hm) and the vertical angle.
d) Three-Wire Leveling:
Three-wire leveling is typically used when extremely high precision is required in leveling measurements. It involves the use of two additional leveling rods called staves, in addition to the standard leveling staff. By measuring the difference in readings between the three rods simultaneously, errors due to rod calibration and temperature effects can be minimized.
e) Field Procedure for Measuring Elevation using a Laser Rotating Level:
1. Set up the laser rotating level at a known benchmark or reference point.
2. Level the instrument using the built-in leveling screws and bubble level.
3. Place the leveling staff at the first point whose elevation is to be measured.
4. Adjust the height of the staff until the laser beam from the rotating level hits the target on the staff.
5. Record the staff reading at this point.
6. Move the leveling staff to the second point and repeat steps 4 and 5.
7. Calculate the elevation difference between the two points by subtracting the initial staff reading from the final staff reading.
f) Calculation of Refraction, Curvature, and Combined Corrections:
To calculate the corrections needed for a height measurement at a distance of 3 km, the following formulas can be used:
Refraction Correction:
Refraction Correction = 0.13 * (tan^2 Z) / (1000 * D)
Refraction Correction = 0.13 * (tan^2 Z) / (1000 * 3000)
Refraction Correction = 0.0000121
Curvature Correction:
Curvature Correction = -0.0785 * (D^2)
Curvature Correction = -0.0785 * (3000^2)
Curvature Correction = -706.5
Combined Correction:
Combined Correction = Refraction Correction + Curvature Correction
Combined Correction = 0.0000121 - 706.5
g) Refraction Correction:
b. Adds to the curvature effect.
h) Sights taken on a "turning point":
b. Backsight only.
i) Errors neutralized by setting the level midway between two stations:
c. Error due to both curvature and refraction.