Plane Table Surveying is a graphical method of surveying in which field observations and plotting are done simultaneously on a drawing board mounted on a tripod. Unlike other surveying methods that rely on separate field books and office drafting, this technique allows the surveyor to prepare maps and plans directly in the field using a plane table, alidade, and drawing sheet. The result is a rapid, cost-effective approach that is especially useful for small-scale mapping and topographic surveys. Because plotting occurs on site, omissions and errors become immediately visible and can be corrected before leaving the station, making the process both efficient and reliable.
Equipment and Temporary Adjustments for Plane Table Surveying
The plane table itself is a well-seasoned drawing board made of soft pine or seasoned wood, mounted on a heavy tripod that allows leveling and rotation. A high-quality drawing sheet is fixed to the board using paper clips or thumb pins. The alidade is the most important accessory: a straight ruler with a sighting device (either a plain vane or a telescopic attachment) used to draw rays and direction lines. A compass is needed for orienting the table to magnetic north, and a plumbing fork with a plumb bob ensures accurate centering over the ground station. Spirit levels help with leveling, and a chain or tape is used for distance measurement. Each item serves a specific purpose in ensuring the field map is accurate. For a deeper look at the pros and cons of this method, refer to Plane Table Surveying Advantages And Disadvantages.
Before beginning any survey, the plane table must undergo three temporary adjustments at each setup station:
- Centering – The process of ensuring that the point on the ground is exactly represented at the corresponding point on the drawing sheet. This is done using a plumbing fork: the pointed end rests on the paper at the plotted station while the plumb bob hangs freely over the ground mark.
- Leveling – The drawing board must be perfectly horizontal. Leveling is achieved through the ball-and-socket arrangement of the tripod head, by tilting the board manually, or by adjusting the tripod legs until the spirit level confirms the board is level in all directions.
- Orientation – Keeping the board position parallel at each successive survey station. If the table is not oriented correctly, rays drawn from the new station will not align with those from the previous one, causing angular errors that accumulate across the traverse.
Orientation can be performed by two methods: back-sighting, where a previously drawn line from the old station is aligned from the new station, or by magnetic needle, where the compass is used to maintain a consistent north direction. Back-sighting is more reliable because it references a physical survey line rather than the magnetic meridian, which can vary locally.
Methods of Plane Table Surveying
Four primary methods are used in plane table surveying, each suited to specific terrain conditions and survey objectives. Understanding the distinction between Plane Surveying Vs Geodetic Surveying helps clarify when these graphical techniques are appropriate: plane surveying treats the earth as a flat surface and works well for areas under 250 square kilometers, while geodetic surveying accounts for curvature for larger projects.
Radiation Method
In the radiation method, a single station is selected as the central point from which all other points are sighted. The surveyor sets up the plane table at station O, then sights points A, B, C, D, and E using the alidade. Rays are drawn from O toward each target, and the distances are measured and plotted to scale along these rays. This method works well for small, open areas where all points are visible from one central station. It is straightforward but limited to surveys where no obstruction blocks the line of sight from the central point.
Intersection Method
The intersection method is preferred in mountainous or broken terrain where distances cannot be measured directly. The surveyor establishes two known stations, P and Q, and sets up the plane table at each one in turn. From station P, rays are drawn toward the unknown points A and B. The table is then moved to station Q, oriented, and rays are again drawn toward A and B. The intersection of the rays from P and Q gives the exact location of each point on the drawing sheet. This method requires no distance measurement between the plane table and the target points, making it invaluable for rugged landscapes.
Traversing Method
Traversing is used to run survey lines between stations that have been fixed by other methods. It begins by setting the table at station A and sighting station B. The distance AB is measured and plotted. The table moves to station B where a back sight is taken to A, and the distance BA is measured again. The average of AB and BA is plotted to scale. This process repeats for all subsequent stations (C, D, etc.) with regular checks at intervals. The following table summarizes the key differences among the three primary methods:
| Method | Best Terrain | Distance Measurement | Number of Setups |
|---|---|---|---|
| Radiation | Flat, open areas | Required (direct) | One central station |
| Intersection | Mountainous, rough terrain | Not required | Two stations minimum |
| Traversing | Linear corridors, roads | Required (measured twice) | Multiple stations in sequence |
Advanced Resection Techniques
Resection is a method used to determine the location of a new station by sighting known points whose positions have already been plotted on the drawing sheet. This is especially useful when the surveyor cannot occupy the original control stations. Two forms of resection are commonly practiced: the two-point problem and the three-point problem. Additional details on this technique can be found in the discussion on Plane Table Surveying 2.
Two-Point Problem
In the two-point problem, two known stations (A and B) are visible from the unknown station C. The plane table is first set at station B and oriented by sighting A. A ray is drawn from B toward C. The table then moves to station C and is oriented by back-sighting B. With the alidade over point a on the sheet, the surveyor sights A and draws a ray that intersects the previously drawn ray from B, giving the plotted position of C. If the controlling stations cannot be physically occupied, an auxiliary point D is taken near C. The table is oriented at D parallel to line AB, and observations are carried out to transfer the location back to C through successive ray intersections.
Three-Point Problem
When three known points are visible from the unknown station, the three-point problem offers greater accuracy. The surveyor sets up the plane table at station P, from which three plotted stations A, B, and C are visible. The most common field procedure is the tracing paper method: a sheet of tracing paper is placed over the drawing board, and rays are drawn from P toward A, B, and C on the tracing sheet. The tracing paper is then moved across the underlying drawing sheet until each ray passes through its corresponding plotted point. The position of P is then marked onto the drawing sheet by pricking through the tracing paper. Alternative approaches include the mechanical (graphical) method and analytical methods using angle and distance calculations. The Plane Table Surveying 3 resource provides an extended overview of resection workflows.
Advantages, Disadvantages, and Precautions
Plane table surveying offers several benefits that make it attractive for certain types of work. It is fast and rapid compared to other methods because plotting happens at the same time as observation, eliminating the need for separate office work. The omission of measurements is avoided because the surveyor can see immediately what has and has not been plotted. It works well in magnetic areas where compass surveying is unreliable, and it produces an accurate visual representation of contours and irregular objects. The equipment is relatively cheap and does not require highly skilled operators. For further reading on potential inaccuracies, the Errors In Plane Table Surveying page details common sources of error and their remedies.
However, the method has notable limitations:
- Weather dependent – cannot be carried out during rain, strong wind, or foggy conditions, as the drawing sheet is exposed.
- Equipment is heavy and bulky, making transport and handling tedious, especially in remote areas.
- Setting up the table at every station is time-consuming and physically demanding over large sites.
- Accuracy is lower than theodolite or total station surveys, making it unsuitable for high-precision engineering works.
- Re-plotting at a different scale is difficult because there is no field book with raw measurements to refer back to.
- For large area surveys, frequent sheet changes are required, increasing the risk of misalignment between adjacent sheets.
To achieve good results, several precautions must be observed. The plane table must be properly oriented at each station, or the drawn lines will not match the field lines. The Plane Table Surveying Radiation Method is one technique where careful orientation is especially critical. Rays should be drawn with a sharp pencil using a straight ruler only. Paper clips must hold the drawing sheet firmly to prevent contraction or expansion from humidity changes. The alidade must be centered accurately over the station point during each sighting. Regular checks should be performed by re-sighting previously located points to confirm that the table has not shifted.
Conclusion
Plane table surveying remains a valuable technique in the surveyor’s toolkit, particularly for small-scale topographic mapping, reconnaissance surveys, and projects in magnetic areas where compass methods fail. Its strength lies in the direct field-to-map workflow that eliminates transcription errors and provides immediate visual feedback. While modern electronic total stations and GPS have replaced plane table methods for large-scale precision work, the graphical nature of plane table surveying offers an unmatched intuitive understanding of site geometry. Engineers and surveyors learning the trade benefit from mastering this technique because it builds a strong spatial awareness that transfers to more advanced methods. For a quick reference to the essential terminology, the Key Terms In Plane Table Surveying guide summarizes the vocabulary every field practitioner should know.