Plane table surveying is a widely used method in fieldwork that requires understanding specific terms for accurate data collection and mapping. Each term plays a critical role in the surveying process, enabling surveyors to correctly set up, align, and record measurements. This article outlines and explains the essential terms used in plane table surveying.
1. Centering
Centering is the initial and critical step in plane table surveying. It involves setting up the plane table so that the plotted position on the drawing sheet, represented by a point ‘o’, is directly above the corresponding ground station ‘O.’ Ensuring proper centering is essential for maintaining the accuracy of the survey and avoiding errors in plotting.
2. Orientation
Orientation refers to aligning the plane table such that all plotted lines on the sheet are parallel to the corresponding ground lines. This alignment ensures that measurements taken from the ground match the drawn lines. Orientation is typically achieved using a trough compass, back sighting, or resection, depending on the situation and surveyor’s preference.
3. Back Sight
Back sight is a method used to orient the plane table by sighting a known station from the current station. For example, when the plane table is set at station A, the alidade is aligned with a previously plotted line from A to B. The table is then rotated until station B is bisected. This ensures that the table is properly oriented for subsequent measurements.
4. Fore Sight
In contrast to back sight, fore sight involves sighting from the current station to a new station whose position has not yet been marked on the drawing sheet. This step helps establish new points and extend the survey.
5. Radiation
Radiation is a method used to locate a point by drawing a radial line from the current plane table station to the target station. The procedure involves setting up and orienting the plane table, then drawing a ray in the direction of the target station using an alidade. The distance to the target is measured and marked on the line using a suitable scale. This method is commonly employed when the target point is within a visible and measurable range.
6. Intersection
Intersection is a technique for locating a point by drawing rays from two different stations and finding their intersection. This method is particularly useful when measuring the radial distance to the point is difficult or obstructed. By sighting the point from two known stations, its location can be accurately determined on the drawing sheet.
7. Resection
Resection is the process of locating the position of the plane table at a station that has not been previously plotted on the drawing sheet. This is achieved by sighting two or three known points, whose positions are already plotted. The intersections of these sight lines determine the current station’s position. Resection is often used in two-point or three-point problems to ensure accuracy.
8. Plane Table Traversing
Plane table traversing involves directly plotting traverse lines on the drawing sheet during the survey. Unlike other surveying methods, no angle measurements are required. Instead, an alidade is used to draw the traverse lines directly. This method simplifies the process and is highly efficient for producing on-site maps.
9. Conclusion
Understanding the key terms in plane table surveying is essential for accurate fieldwork and mapping. Terms like centering, orientation, back sight, fore sight, radiation, intersection, resection, and plane table traversing are fundamental to the procedure. Mastery of these concepts enables surveyors to align the plane table correctly, plot points accurately, and create reliable maps directly in the field. By applying these techniques, surveyors can achieve efficiency and precision in their work.
This comprehensive overview highlights the importance of each term in the surveying process and underscores their role in maintaining the integrity of plane table surveys.