After fixing the plane table to the tripod, the main operations which are needed at each plane table station are: i) leveling ii) orientation iii) centering
(i), (iii), (ii)
Understanding Plane Table Surveying Operations
Plane table surveying is a graphical method of surveying where the field work and plotting are done simultaneously. After setting up the tripod and fixing the plane table on top, several essential operations must be performed at each station before any sighting or plotting can begin. These operations ensure the accuracy and correctness of the survey work. The main operations are leveling, centering, and orientation.
Key Operations at a Plane Table Station
Let's break down the purpose of each operation:
- Leveling: This operation makes the surface of the plane table horizontal. It is crucial because all the lines drawn on the paper represent horizontal distances in the field. If the table is tilted, the plotted lengths and angles will be inaccurate. Leveling is typically done using a spirit level placed on the table surface.
- Centering: This operation positions the plotted station point on the paper exactly over the corresponding ground station mark. Accurate centering is vital to ensure that all measurements originating from the ground station are correctly transferred to the paper from the precisely located plotted point. Centering is usually done using a plumbing fork and plumb bob.
- Orientation: This operation aligns the plane table such that the lines drawn on the paper are parallel to the corresponding lines on the ground. In essence, it makes the meridian or any other reference line on the paper parallel to the corresponding line on the ground. Orientation can be achieved either by using a trough compass (to align the table with magnetic north) or by back-sighting to a previously plotted station.
Sequence of Plane Table Operations
For accurate surveying with a plane table, these operations must be performed in a specific sequence after the initial rough setup on the tripod. While variations exist depending on the specific method or equipment, a widely followed and logical sequence for achieving accuracy is:
- Leveling (i): The table is first made horizontal. This provides a stable and level base for subsequent operations and plotting.
- Centering (iii): Once the table is level, it is accurately centered over the ground station mark. Leveling before centering ensures that the plumbing fork hangs vertically from a horizontal surface, leading to more accurate centering.
- Orientation (ii): Finally, the table is oriented. This step requires the table to be level and centered correctly so that the orientation accurately reflects the ground conditions relative to the station point.
Thus, the correct sequence of operations after fixing the plane table to the tripod is Leveling, Centering, and then Orientation.
Analyzing the Options
Let's examine the given sequences:
- (ii), (iii), (i): Orientation, Centering, Leveling. Performing orientation before leveling and centering is generally not done as it relies on the table being properly positioned and horizontal.
- (i), (ii), (iii): Leveling, Orientation, Centering. While leveling first is good, orienting before accurate centering doesn't follow the standard workflow where centering establishes the origin point for subsequent bearings/orientations.
- (i), (iii), (ii): Leveling, Centering, Orientation. This sequence (Leveling first for a horizontal base, then Centering accurately over the point, and finally Orientation to align) is a standard and logical procedure for accurate plane table work.
- (iii), (i), (ii): Centering, Leveling, Orientation. Centering is often done roughly first, then leveling, and then centering is adjusted precisely. However, if these are considered distinct steps, leveling first provides a better base for accurate centering using a plumbing fork.
Considering the operations as distinct main steps after the initial rough setup, the sequence (i) Leveling, (iii) Centering, (ii) Orientation is the most appropriate standard procedure.
Conclusion on the Correct Sequence
Based on the standard practices in plane table surveying, the correct sequence of the main operations after setting up the table is leveling to make the surface horizontal, followed by centering to position the table over the ground station, and finally orientation to align the paper with the ground. This corresponds to the sequence (i), (iii), (ii).
| Step No. | Operation | Purpose | Typical Instrument |
|---|---|---|---|
| 1 | Leveling (i) | To make the plane table surface horizontal. | Spirit Level |
| 2 | Centering (iii) | To position the plotted station point directly over the ground station. | Plumbing Fork and Plumb Bob |
| 3 | Orientation (ii) | To align the paper lines parallel to ground lines. | Trough Compass or Alidade (for back-sighting) |
Revision Table: Plane Table Operations
| Operation | Order in Sequence (i), (iii), (ii) | Why this order? |
|---|---|---|
| Leveling | First (i) | Provides a horizontal base essential for accurate measurements and subsequent centering/orientation. |
| Centering | Second (iii) | Accurately positions the plotting origin over the ground station after the table is leveled. |
| Orientation | Third (ii) | Aligns the entire setup relative to the ground after it is level and centered. |
Additional Information on Plane Table Surveying
Plane table surveying methods include radiation, intersection, traversing, and resection. The alidade is a key instrument used for sighting objects and drawing lines on the paper. The accuracy of plane table surveying depends heavily on careful execution of the setting up operations: leveling, centering, and orientation.
Important Questions from Plane Table Survey
Which of the following represents the CORRECT order of setting up a plane table?
In plane tabling the instrument used to measure horizontal and vertical distances directly is known as
What is the purpose of conducting the resection method in the plane table surveying?
The principle of plane table survey is:
Which of the following instrument is used for measurement of sighting objects in a plane table survey?
