Last reviewed 16 Sept 2026 · 9 min read
Principle
Plane table surveying is a graphical method in which field observations and plotting are done simultaneously. The map is prepared in the field, so no field book is needed.
Principle — parallelism: the rays drawn on the drawing sheet from plotted stations to objects are parallel to the corresponding lines on the ground, and the plotted positions are geometrically similar to the ground positions (the table must be correctly oriented at every station).
Suitability: small-scale mapping, filling in topographic details (e.g. between triangulation stations), areas where high accuracy is not required, magnetic areas (orientation by back sighting), and places where compass surveys are unreliable.
Plane table and accessories
| Item | Description / use |
|---|---|
| Drawing board (plane table) | Well-seasoned wooden board (commonly about 750 × 600 mm) mounted on a tripod so it can be levelled, rotated and clamped |
| Tripod | Supports the board; types include the Johnson table and coast survey tripod heads |
| Alidade | A straight edge with sighting vanes (plain alidade) or a telescope (telescopic alidade) for sighting objects and drawing rays; the fiducial (bevelled) edge is used for drawing |
| Spirit level | Levels the table |
| Trough compass | Long narrow compass — used to orient the table approximately to magnetic north |
| Plumbing fork (U-frame) with plumb bob | Transfers the plotted station point on the sheet vertically over the ground station (centring) |
| Drawing sheet, pencils, pins, eraser | Plotting |
A telescopic alidade increases the range and accuracy of sighting and permits measurement of vertical angles and stadia distances.
Temporary adjustments
- Setting up and levelling — tripod legs spread firmly; board levelled with a spirit level in two directions.
- Centring — the point on the sheet representing the ground station is brought vertically over the station using the plumbing fork. (Exact centring is less critical on small-scale maps, since a small centring error causes a negligible plotting error.)
- Orientation — the table is rotated so that lines on the sheet are parallel to the corresponding ground lines. It is essential for accurate work.
Methods of orientation
- By trough compass — the direction of magnetic north is marked on the sheet at the first station; at subsequent stations the table is rotated until the compass needle again lies along that line. Quick, but affected by local attraction.
- By back sighting — the alidade is placed along a line already drawn from a previous station to the present station, and the table is rotated until the line of sight bisects the previous station; then clamped. Most accurate method.
- By resection — when the station occupied is not yet plotted (see below).
Methods of plane tabling
| Method | Procedure | Use |
|---|---|---|
| Radiation | Rays are drawn from a single station to all objects; distances are measured and plotted along the rays to scale | Small areas visible from one station; details close to a station |
| Intersection (graphical triangulation) | From two plotted stations (base line measured and plotted), rays are drawn to objects; the objects are located at the intersections of rays; distances to objects are not measured | Inaccessible points (e.g. across rivers), broken boundaries, locating distant details |
| Traversing | Similar to compass or theodolite traversing — at each station, the table is oriented by back sighting, a ray drawn to the next station and the distance measured and plotted | Surveying roads, rivers, and running a framework |
| Resection | Locating the position of the station occupied by the table on the plan by sighting to known (already plotted) points | Filling in details; establishing new stations quickly |
Resection methods
Resection after orientation by compass or back sighting
- By compass: orient the table with the trough compass; draw back rays from two known plotted points through their alidade sightings; the intersection gives the station.
- By back sighting: if a line from a known station towards the new station has been drawn earlier, orient by back sighting along it and then draw a resector from another known point; the intersection locates the station.
Two-point problem
The station occupied is located by sighting two well-defined points already plotted, when the table cannot be oriented directly. An auxiliary station is chosen, and the procedure involves setting up at the auxiliary station, drawing rays, and correcting orientation by comparison of a constructed line with the known line. It requires more work and is less accurate; it is used only when the three-point problem cannot be applied.
Three-point problem
The station occupied is located by sighting three well-defined points already plotted on the sheet. Solutions:
- Mechanical (tracing paper) method — rays to the three points are drawn on a tracing paper fixed on the board; the tracing is moved until the three rays pass through the plotted points; the station is pricked through.
- Graphical methods — e.g. Bessel's method (construction of an auxiliary point by drawing rays, which then gives the correct orientation).
- Trial and error method (Lehmann's method) — the table is oriented approximately, back rays are drawn from the three points, forming a small triangle of error; the true position is estimated by Lehmann's rules and the orientation improved until the triangle reduces to a point.
Lehmann's rules:
- The point sought lies on the same side of each of the three rays (either to the right of all or to the left of all, when looking towards the respective point).
- Its distance from each ray is proportional to the distance of the corresponding known point from the station.
- If the station is inside the great triangle (formed by the three known points), the point lies inside the triangle of error; if outside, it lies outside the triangle of error.
Indeterminate case: if the station lies on the circle passing through the three known points (the great circle), the problem has no unique solution — another point must be chosen.