When only one point is available for orientation, orientation is done by:
trough compass
Understanding Plane Table Orientation with One Known Point
Plane table surveying is a method of surveying in which plotting is done simultaneously with field observations. Orientation is a crucial step in plane table surveying. It means setting up the plane table at a station such that the lines on the paper are parallel to the corresponding lines on the ground. Correct orientation is essential for accurate plotting of details.
Several methods are used for orientation in plane table surveying:
- Back-sighting
- The two-point problem
- The three-point problem
- Orientation by trough compass
The method used for orientation depends on the available information, particularly the number of known points visible from the current station.
Orientation Methods Analysis
Let's look at the methods presented in the options:
| Method | Description | Requirement for Orientation |
|---|---|---|
| Intersection | A method of locating a point by sighting it from two different stations whose positions are already known and plotted. Lines of sight are drawn, and their intersection gives the location of the point. | Requires at least two known points (the stations from which observations are made). Primarily used for plotting, not typically the primary method for orienting when first setting up at a new station with only one point. |
| Trough Compass | A compass specifically designed for plane table surveying, typically a rectangular box with a magnetic needle pivoting at the center. It is used to draw a line parallel to the magnetic meridian on the drawing paper. | Can be used for approximate orientation when other methods are not feasible, especially when only one known point is available or for the initial setup at a station. It aligns the table with the magnetic meridian. |
| Back-sighting | Orienting the table by sighting back to a station that was previously occupied and plotted. A line is drawn on the paper from the current station back towards the previous one, and the table is rotated until the alidade sighted along this line points exactly at the previous station on the ground. | Requires at least two known points: the current station (whose position is assumed or determined) and a previously occupied station that is visible. Essential for orientation when moving between stations already plotted. |
| Fore-sighting | Sighting forward from the instrument station to a ground point to locate its position on the drawing paper. Lines of sight are drawn towards various points. | Used for plotting details after the table is properly oriented. It is not a method for orienting the table itself, especially when setting up with only one point. |
Identifying the Method for One Point Availability
When setting up the plane table at a station, and only one known point is available and visible, methods like back-sighting, two-point problem, or three-point problem are not directly applicable for precise orientation because they require referencing at least two known points or lines between them.
In such a scenario, especially for initial setups or when other methods are not possible, the trough compass provides a way to orient the plane table approximately. By aligning the magnetic needle within the trough compass with the marked magnetic meridian line on the paper (drawn during the first setup using the compass), the table is brought to an orientation parallel to the magnetic meridian on the ground. This allows surveying to proceed, although orientation by magnetic meridian is generally less accurate than orientation by back-sighting or using plotted points, due to magnetic declination and local attractions. However, it is a viable option when only one known point allows for plotting but not robust geometric orientation.
Conclusion on One Point Orientation
Based on the analysis of the methods, when only one point is available for orientation during plane table surveying, the orientation is typically done using the trough compass. While it provides a less accurate orientation compared to other methods requiring multiple known points, it is the most practical method among the options provided when geometric methods are not possible with only one reference point. Intersection and fore-sighting are primarily plotting techniques, and back-sighting requires sighting back to a previously occupied station, implying at least two known points are involved in the orientation process itself.
Revision Table: Plane Table Orientation
| Method | Best Use Case | Requires |
|---|---|---|
| Back-sighting | Moving between stations already surveyed and plotted. | At least 2 plotted points (current station & back-sight station). |
| Two-point problem | Orienting and locating the current station by sighting two well-defined inaccessible points whose positions are known and plotted. | 2 visible, known points whose locations are plotted. |
| Three-point problem | Orienting and locating the current station by sighting three well-defined points whose positions are known and plotted. (More accurate than two-point). | 3 visible, known points whose locations are plotted. |
| Trough Compass | Initial approximate orientation, or when only one known point is available, or when other methods are not feasible. | Magnetic meridian (established using the compass). Only requires visibility to the single known point for plotting, not for the compass orientation itself. |
Additional Information on Plane Table Surveying Techniques
Plane table surveying is valued for its simplicity and the advantage of plotting in the field, which reduces the chance of omitting necessary observations. However, its accuracy is limited, and it is generally not suitable for large-scale or very accurate surveys.
- The alidade is an essential instrument used with the plane table for sighting objects and drawing lines.
- Leveling the plane table is done using a spirit level or a circular box level before orientation and plotting.
- Centering involves positioning the table exactly over the ground station. A U-frame with a plumb bob is used for this.
- Orientation must be checked periodically during the survey, especially if the table is disturbed.
Understanding the appropriate orientation method based on available control points is key to successful plane table surveying.
Important Questions from Plane Table Survey
What is the function of the plumbing fork in plane table surveying?
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:
