Radiation Method: Instruments & Drawing Rays
Discover the principle of the radiation method, including the essential instruments required and steps for selection and setup. Learn how to mark stations on the sheet, draw the rays, and plot effectively for accurate results.
9/20/20267 min read
Plane Table Surveying – Radiation Method
Plane Table Surveying is a graphical method of surveying in which field observation and plotting are carried out simultaneously. Instead of recording all observations in a field book and preparing the drawing later in an office, the surveyor directly plots the details on a drawing sheet fixed to a plane table. This makes the method useful for rapid field mapping, particularly for relatively small areas.
One of the simplest methods of plane table surveying is the Radiation Method. In this method, a single instrument station is selected from which the required objects or points are visible. Rays are drawn from the plotted station toward these objects using an alidade, and the distances from the instrument station to the objects are measured and plotted to scale.
1. Principle of Radiation Method
The basic principle is quite simple. Suppose a survey station on the ground is called P. Several points such as A, B, C, D and E are visible from P.
The plane table is positioned over P, and a corresponding point p is marked on the drawing sheet. The alidade is placed with its fiducial edge passing through p. The surveyor sights point A and draws a ray from p in the direction of A. The same procedure is repeated for B, C, D and E.
After drawing the rays, the horizontal distance from P to each point is measured. These distances are reduced according to the selected scale and plotted along their corresponding rays. The plotted points are
then joined to represent the actual ground features. This procedure is the reason the method is called radiation—the lines appear to radiate outward from one common station.
2. Instruments Required
The following equipment is generally required:
Plane table
Tripod stand
Alidade
Spirit level
Plumbing fork and plumb bob
Trough compass
Drawing sheet
Drawing clips or clamps
Measuring tape or chain
Ranging rods
Pencil and eraser
Laboratory manuals commonly specify a plane table with accessories, measuring tape, and ranging rods for carrying out the radiation method.
3. Selection of Instrument Station
The first and most important step is selecting a suitable instrument station. The station should provide a clear view of all the points that need to be plotted.
For example, if a small site contains the corners of a building, trees, poles, roads or other important details, the surveyor should select a location from which these points can be clearly observed.
The method is particularly suitable when the points are visible and accessible from a single station and the distances can be measured conveniently. (About Civil)
4. Setting Up the Plane Table
After selecting station P, the plane table is mounted on the tripod approximately over the station.
Three important operations are then performed:
Centering
The table is centered so that the plotted station p on the drawing sheet is directly above the actual ground station P.
A plumbing fork and plumb bob can be used to transfer the station position from the ground to the sheet. (About Civil)
Leveling
The table is adjusted until its surface is approximately horizontal. A spirit level is normally used for this operation.
Orientation
The table must be oriented correctly so that the directions represented on the drawing correspond to the directions on the ground. A reference direction, such as magnetic north, may be established when required.
Correct centering, leveling and orientation are essential because errors in these operations can affect the plotted positions.
5. Marking the Station on the Sheet
A convenient point p is selected on the drawing sheet to represent ground station P. The position should be selected so that sufficient space remains on the sheet for plotting all the surrounding details.
The scale of the drawing is selected according to the size and purpose of the survey. For example, if the scale is 1:500, then 1 cm on the drawing represents 500 cm, or 5 m, on the ground.
The scale should be chosen so that the required details can be plotted clearly without overcrowding the sheet.
6. Drawing the Rays
Now the alidade is placed with its fiducial edge passing through point p.
The surveyor sights the first object, say A, through the alidade. Once A is correctly bisected, a pencil line is drawn along the fiducial edge starting from p.
This line represents the direction from P toward A.
The alidade is then rotated around p to sight point B, and another ray is drawn. The same process is repeated for C, D, E and other visible points.
Thus, several rays originate from the same plotted station.
7. Measuring Ground Distances
After the directions have been drawn, the distances from the instrument station to the different points are measured.
For example:
PA, PB, PC, PD and PE
may be measured using a tape or chain.
The measured distance is then converted according to the selected drawing scale. For example, if the ground distance PA is 25 m and the scale is 1:500:
Drawing distance = 25/500 m = 0.05 m = 5 cm
Therefore, point A is marked 5 cm from p along the ray corresponding to A.
The same process is followed for the other points.
8. Plotting the Details
Once the scaled distances have been marked, the corresponding points are labeled a, b, c, d and e.
If the points represent the corners of a building, the plotted points are joined to reproduce the building outline. If they represent a road, boundary, tree or other feature, the appropriate conventional symbols or lines can be added.
The important feature is that both direction and distance are used to determine the position of each point.
The National Programme on Technology Enhanced Learning describes the method in the same basic sequence: draw radial lines after sighting the objects, measure horizontal distances, and scale those distances along the respective rays. (NPTEL)
9. Field Procedure in Simple Steps
The complete radiation method can be summarized as follows:
Select a suitable instrument station from which the required points are visible.
Set up the plane table over the selected station.
Level the table using a spirit level.
Center the table using a plumbing fork and plumb bob.
Fix the drawing sheet securely.
Mark the station point on the sheet.
Establish orientation if required.
Place the alidade over the plotted station.
Sight the first ground point.
Draw a ray toward that point.
Measure the ground distance from the station to the point.
Convert the distance according to the selected scale.
Plot the point along the corresponding ray.
Repeat the process for all required points.
Join the plotted points where necessary.
Add labels and conventional symbols.
This sequence is also reflected in surveying laboratory manuals describing the radiation method. (Aurora Institute)
10. Advantages of Radiation Method
The radiation method has several advantages:
Simple procedure: It is one of the easiest plane-table methods to understand and perform.
Field plotting: Observations and plotting can be completed at the same time.
No angular calculations: The direction is obtained graphically through the alidade rather than by separately measuring and calculating horizontal angles.
Rapid surveying: Several points can be located from one station without shifting the table.
Easy checking: Because the drawing is prepared in the field, the surveyor can compare the plotted plan with the actual ground features.
Useful for small areas: It is particularly convenient where the important details can be seen from one instrument station. (NPTEL)
11. Limitations
The method also has limitations.
The most important limitation is that all required points should generally be visible from the selected station. If a building, tree, hill or other obstruction blocks the line of sight, another method may be necessary.
It is also less convenient when the points are very far away because measuring the distances accurately becomes more difficult.
notes that the method is particularly suitable for objects within a single tape length, while longer distances may require equipment such as a tacheometer or EDM.
The method is therefore better suited to relatively small and compact areas than to large surveys requiring numerous instrument stations.
12. Applications
Radiation method can be used for locating:
Building corners
Trees
Electric poles
Boundary points
Manholes
Small structures
Road details
Fences
Topographical features
For example, a surveyor can establish a plane table at a suitable location in a small site and plot several building corners, trees and other visible features by drawing rays from the same station.
13. Example
Consider a survey station P with four points A, B, C and D around it.
The surveyor first plots p on the sheet. Rays pA, pB, pC and pD are then drawn using the alidade.
Suppose the measured distances are:
PointGround DistanceScale 1:500Drawing DistanceA20 m1:5004 cmB30 m1:5006 cmC25 m1:5005 cmD15 m1:5003 cm
The surveyor marks each distance along its respective ray. Joining the resulting plotted points produces the plan of the surveyed feature.
Conclusion
Plane Table Surveying – Radiation Method is a straightforward graphical surveying technique in which directions are obtained by sighting through an alidade and distances are measured and plotted to scale. The entire process can be performed directly in the field, making it particularly useful for small areas where several details are visible from one instrument station.
The key idea is easy to remember:
One station → Several rays → Measure distances → Plot to scale → Join the points.
For civil engineering students, understanding this method provides a useful foundation for studying other plane-table techniques such as intersection, traversing and resection. Its simplicity also makes it particularly suitable for surveying laboratory exercises and basic field-detail plotting
