Plane table surveying is a practical method of field-based plotting where the observations and measurements are directly drawn onto a sheet affixed to a table top. This technique eliminates the possibility of missing critical measurements, as the plotting is completed on-site. However, the method’s accuracy is somewhat limited, making it ideal for filling in details between survey stations established by other, more precise methods.
This article provides an in-depth look at the components and equipment used in plane table surveying, its applications, and its limitations.
Components of a Plane Table
The plane table is the primary instrument in this surveying method. It comprises a well-seasoned wooden table top mounted on a tripod, allowing it to rotate freely about a vertical axis. The table can also be clamped in a fixed orientation when necessary. To ensure accurate plotting, the table is leveled by adjusting the tripod legs, which is critical for reliable results.
Essential Equipment for Plane Table Surveying
To carry out plane table surveying effectively, the following accessories are required:
1. Alidade
The alidade is a straight-edge ruler equipped with a sighting device, used to draw lines of sight on the plotting sheet. There are two main types of alidades:
- Plain Alidade: This simple instrument features sight vanes at each end of the ruler. The narrow slit of the eye vane aligns with a wide slit and a thin wire on the object vane to create a line of sight. Although functional, the plain alidade is limited in hilly terrains due to its inability to handle inclined sights effectively.
- Telescopic Alidade: This advanced version includes a telescope mounted on a ruler, providing greater accuracy and range. The telescope is equipped with a level tube and vertical graduation arc, making it suitable for both horizontal and inclined sights. The telescopic alidade significantly enhances the precision of the survey.
2. Plumbing Fork and Plumb Bob
The plumbing fork, a U-shaped metal frame, is used to transfer ground points to the drawing sheet. The upper horizontal arm has a pointed edge, while the lower inclined arm suspends the plumb bob. Together, these tools ensure accurate vertical alignment during surveying.
3. Spirit Level
The spirit level is used to level the plane table. A flat-based instrument, it ensures that the table is perfectly horizontal by checking the bubble’s position in two perpendicular directions. Proper leveling is essential for accurate plotting.
4. Trough Compass
This compass consists of a long, narrow box with a freely suspended needle at its center. The needle aligns with magnetic north when it stabilizes, and the edges of the box indicate the north-south direction. The trough compass is particularly useful for orienting the table during surveying.
5. Drawing Sheet and Accessories for Drawing
High-quality, seasoned drawing sheets are essential for accurate plotting. For critical surveys, fiberglass sheets or aluminum-backed paper may be used. Accessories like clips, adhesive tapes, sharp pencils, erasers, and sandpaper ensure smooth plotting. A protective plastic sheet is often used to shield the drawing sheet from environmental factors like rain and dust.
Applications and Limitations of Plane Table Surveying
Plane table surveying is primarily used for filling in details between survey stations established through more precise methods. Its on-site plotting capability reduces the risk of errors caused by omitted measurements. However, this method is less accurate than other surveying techniques, limiting its use for projects requiring high precision.
Conclusion
Plane table surveying is a practical and straightforward method, especially suited for detailing and intermediate-level surveying tasks. Despite its limitations in accuracy, the technique remains valuable for its real-time plotting capability and ease of use. By employing the right equipment and ensuring proper setup, surveyors can achieve reliable results for specific applications.