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Intersection method and its advantages

Short answer

Plane table surveying is a graphical method where fieldwork and plotting are done simultaneously on a drawing sheet. In intersection, two stations with a measured base line are used; rays drawn from each station to a point intersect to fix its position, requiring no distance measurement to the point. Advantages: it locates inaccessible points, uses only one linear measurement, allows quick plotting of many points, and provides a check with a third ray.

What is plane table surveying? Explain the intersection method and its advantages.

Plane table surveying is a graphical method of surveying in which fieldwork and plotting are done at the same time on a drawing sheet fixed on a board (plane table). The plan is drawn to scale directly in the field.

Intersection method

Two stations A and B (the ends of a base line) are chosen so that the point P to be located is visible from both.

  1. Set the table at A, orient it by compass, and plot a. Sight B and draw the ray; measure AB and plot b to scale.
  2. Sight P from a and draw the ray aP (also rays to the other points).
  3. Shift to B, centre b over B, and orient the table by back-sighting A along the line ba.
  4. Sight P again from b and draw the ray bP. The intersection of the two rays is the plotted point p.
                 p
               / |  \
             /   |    \
           /     |      \
         a ------------- b
       (stn A)  base   (stn B)

Advantages:

  • Used for locating inaccessible points (across rivers, hills, buildings, a far-away church spire, etc.) where distance cannot be measured.
  • Only one linear measurement (the base line) is needed; many points can be fixed quickly.
  • A third ray from another station checks the plotted point.
  • Good intersection angle (about 60 to 120 degrees) is needed for accuracy.

✓ Answer checked by Claude Opus 5.5 (high effort)

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