What is the function of the plumbing fork in plane table surveying?
Used for centering of plane table
Understanding the Plumbing Fork in Plane Table Surveying
Plane table surveying is a graphical method of surveying where the field work and plotting are done simultaneously. It requires several instruments, and the plumbing fork is one of the essential tools used during the setup process.
Function of the Plumbing Fork
The main function of the plumbing fork in plane table surveying is precise centering of the plane table over a specific ground station. When a plane table is set up at a station, it needs to be positioned exactly above the survey marker on the ground. This is crucial for maintaining accuracy in the plotted measurements. The plumbing fork helps achieve this.
How the Plumbing Fork Works for Centering
The plumbing fork is a U-shaped metal frame. It has a pointed end at the top and a hook at the bottom. A plumb bob is attached to the hook at the bottom. The pointed end of the fork is placed on the drawing sheet, directly above the station point marked on the paper. The plumb bob hanging from the bottom hook will then indicate the corresponding point on the ground below the table. By carefully shifting the table horizontally, the surveyor can make the plumb bob hang exactly over the ground station marker. This ensures that the point on the paper is vertically above the point on the ground, achieving accurate centering.
| Instrument | Primary Function in Plane Table Surveying |
|---|---|
| Plane Table | Drawing surface and base for instruments |
| Alidade | Sighting objects and drawing lines of sight |
| Spirit Level | Leveling the plane table |
| Trough Compass | Orienting the table to magnetic north |
| Plumbing Fork and Plumb Bob | Centering the table over a ground station |
Analyzing the Options
- Option 1: Used for centering of plane table - As explained, this is the primary function of the plumbing fork. It ensures the station marked on the paper is vertically above the ground station.
- Option 2: Used for leveling the plane table - Leveling is done using a spirit level, which is placed on the table in different positions to make the surface horizontal. The plumbing fork is not used for this purpose.
- Option 3: Used for orientation of plane table - Orientation involves setting the table in such a way that the lines plotted on the paper are parallel to the corresponding lines on the ground. This is usually done using a trough compass (to orient to magnetic north) or by back sighting a previously occupied station. The plumbing fork is not involved in this step.
- Option 4: Used for sighting the object - Sighting objects and drawing lines of sight from the station on the paper is done using an alidade. The alidade has vanes or a telescope for sighting and a straight edge for drawing. The plumbing fork is not used for sighting.
Therefore, the correct function of the plumbing fork in plane table surveying is for centering.
Revision Table: Plane Table Surveying Tools
| Tool | Purpose | Related Concept |
|---|---|---|
| Plane Table | Provides a stable drawing surface | Field plotting |
| Alidade | Sighting and drawing lines | Line of sight, Ray method |
| Spirit Level | Ensures the table surface is horizontal | Leveling |
| Trough Compass | Aligns the table with magnetic north | Orientation |
| Plumbing Fork with Plumb Bob | Positions the table precisely over the ground station | Centering |
Additional Information on Plane Table Centering
Accurate centering is vital in plane table surveying because all measurements (directions and distances) are taken relative to the station point plotted on the paper. If the table is not correctly centered over the ground station, all subsequent sightings and plotted points will be displaced, leading to errors in the final map.
The process typically involves setting up the tripod, placing the plane table on it, approximately centering the table by eye over the ground station, attaching the paper, performing approximate leveling, and then using the plumbing fork and plumb bob for precise centering. After centering, final precise leveling is done, and then the table is oriented.
Important Questions from Plane Table Survey
When only one point is available for orientation, orientation is done by:
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
The correct sequence of these operations is-The process of determining the plotted position of the station occupied by the plane table, by means of sights taken towards known points is termed as ________.
The size of plane table is
In the plane table survey, 'three point problem' and 'two point problem' are orientation methods connected to:
