What is Plane Table Surveying?
Plane table surveying is a method of surveying which involves the use of a table top, similar to a drawing board, that is mounted onto a tripod. A drawing sheet is placed on the table top, and observations are made to the objects. The distances are then scaled down and the objects are plotted in the field itself, allowing for no chance of omitting any necessary measurements. This type of surveying is used for filling in details between survey stations previously fixed by other methods, however, the accuracy achieved is less.
What Does a Plane Table Consist Of?
A plane table consists of a well-seasoned wooden table top mounted on a tripod. The table top is able to rotate about a vertical axis freely, and can be clamped in the desired orientation when necessary. Additionally, the table can be levelled by adjusting the tripod legs.
What Are the Benefits of Plane Table Surveying?
One of the main benefits of plane table surveying is that it ensures that no necessary measurements are omitted since the plotting is done in the field itself. Additionally, this type of surveying is used for filling in details between survey stations previously fixed by other methods.
Fig: Plane table with tripod stand
Equipments for Plane Table Surveying
The following accessories are required to carry out plane table survey:
- Alidade
- Plumbing fork with plumb bob
- Spirit level
- Trough compass
- Drawing sheets and accessories for drawing.
1. Alidade
What is an Alidade?
An alidade is a straight edge ruler that features a sighting device. It has one beveled edge that is graduated, and is used for drawing a line of sight.
Types of Alidade
There are two types of alidade: plain and telescopic. Plain alidade has a basic line of sight, while telescopic alidade has a more precise line of sight.
Plain Alidade:
Figure 2: Plain Alidade
Overview:
Figure 2 shows a typical plain alidade which consists of two sight vanes, one with a narrow slit serving as an eye vane and the other with a wide slit having a thin wire at its centre serving as an object vane. Both vanes are provided with hinges at the ends of the ruler so that they can be folded when not in use.
Limitations:
Plain alidade is not suitable in surveying hilly areas as the inclination of line of sight in this case is limited.
Fig: 2 – Plane Alidade
Telescopic Alidade:
INTRODUCTION
A ruler with a telescope mounted on it is known as a telescope rule. It is used to measure lines of sight with high accuracy and range. It has a beveled edge, a level tube, and a vertical graduation arc to ensure accurate measurements.
FEATURES OF THE TELESCOPE RULE
The telescope rule consists of a telescope mounted on a column that is fixed to the ruler. The column is beveled on the edge, and the line of sight through the telescope is kept parallel to the beveled edge. The telescope is equipped with a level tube and a vertical graduation arc to measure the inclination of the line of sight.
BENEFITS OF THE TELESCOPE RULE
Using the telescope rule, users can accurately measure lines of sight. The range and accuracy of the line of sight is also increased due to the telescope being mounted on the ruler. The level tube and vertical graduation arc help in measuring the inclination of the line of sight.
Fig. 3: Telescopic alidade
2. Plumbing Fork and Plumb Bob
Plumbing Fork: Tool for Aligning Ground Points
A plumbing fork is a U-shaped metal frame that consists of an upper horizontal arm and a lower inclined arm. The end of the upper arm is fitted with a pointer, while the lower arm is equipped with a hook to suspend the plumb bob. This tool is used to align ground points between the plane table and the drawing sheet.
Using the Plumb Bob for Accurate Measurements
When the plumbing fork is placed on the plane table, the vertical line of the plumb bob passes through the pointed edge of the upper arm. The plumb bob helps in transferring the ground point from the plane table to the drawing sheet and vice versa. This ensures accurate measurements are taken and recorded.
Fig. 4: Plumbing fork and plumb bob
3. Spirit Level
Using a Spirit Level for Table Leveling
A spirit level is an essential tool used for leveling a plane table during surveying. This tool consists of a bubble tube, which should show a central position when checked in any two of the mutually perpendicular directions. The accuracy of the leveled plane table depends on the precision of the spirit level.
Fig. 5: Spirit level
4. Trough Compass
WHAT IS A MAGNETIC NEEDLE?
A Magnetic Needle is a device used to determine the direction of the Earth’s magnetic field. It consists of a box that is 80 to 150 mm long and 30 mm wide, with a freely suspended needle at its center. The box is provided with a glass top to prevent oscillation of the needle due to wind. The needle is marked with graduations from 0 to 5 degrees on either side of the center. When the needle is centered (reading 0-0), the direction of the needle is parallel to the edge of the box, indicating the magnetic north-south direction.
FEATURES OF A MAGNETIC NEEDLE
The Magnetic Needle is a simple yet efficient tool to determine the direction of the Earth’s magnetic field. It consists of a box that is 80 to 150 mm long and 30 mm wide, with a freely suspended needle at its center. The box is provided with a glass top to prevent oscillation of the needle due to wind. The needle is marked with graduations from 0 to 5 degrees on either side of the center. When the needle is centered (reading 0-0), the direction of the needle is parallel to the edge of the box, indicating the magnetic north-south direction.
BENEFITS OF A MAGNETIC NEEDLE
The Magnetic Needle is a useful tool for navigation and direction determination. It is designed to be simple yet efficient, with a box measuring 80 to 150 mm long and 30 mm wide, and a freely suspended needle at its center. The box is provided with a glass top to prevent oscillation of the needle due to wind. The needle is marked with graduations from 0 to 5 degrees on either side of the center. When the needle is centered (reading 0-0), the direction of the needle is parallel to the edge of the box, indicating the magnetic north-south direction. The Magnetic Needle is an essential tool for sailors, hikers, and anyone who needs to determine direction.
Fig. 6: Trough compass
5. Drawing Sheet and Accessories for Drawing
- Necessary Materials for Plane Table Surveying
Plane table surveying requires quality materials for the best results. It is recommended to use a seasoned drawing sheet, which should never be folded but can be rolled when not in use. For important projects, fiberglass sheets or paper backed with thin aluminum sheets may be used. Other necessary materials include clips, clamps, adhesive tapes for fixing the drawing sheet to the plane table; a sharp, hard pencil; a good quality eraser; a pencil cutter; and sand paper to keep the pencil point sharp. If needed, a plastic sheet may be used to cover the drawing sheet from rain and dust.
- Advantages of Using Quality Materials
Using quality materials for plane table surveying has many advantages. Quality drawing sheets are more durable, allowing them to withstand more wear and tear. They also provide a more stable platform for drawing, ensuring that the lines are precise and accurate. The use of fiberglass sheets or paper with thin aluminum backing further enhances the durability and stability of the drawing. Additionally, the use of other necessary materials, such as clips, clamps, and adhesive tapes, helps to ensure that the drawing sheet is securely attached to the plane table.
- Benefits of Sharp Pencils and Erasers
Using a sharp, hard pencil is essential for plane table surveying. This ensures that the lines are clear, precise, and accurate. A good quality eraser is also necessary in order to make any necessary corrections without damaging the drawing sheet. A pencil cutter and sand paper can also be used to keep the pencil point sharp. This helps to ensure that the lines are consistently sharp and precise throughout the entire drawing.