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Reliable orientation; intersection over radiation

Short answer

The reliable orientation is by back-sighting: at a new station, the alidade is placed along the plotted line to the previous station, and the table turned to bisect that station's rod. Intersection is preferred over radiation for inaccessible points where distances cannot be measured, when many distant points are to be plotted quickly, and when a check on location is needed, since only a base line is measured.

Describe the reliable method of orientation of plane tabling and the circumstances in which the intersection method of plane tabling is more preferable than radiation.

Reliable method of orientation

Orientation by back-sighting is the more reliable method. It is not affected by local attraction, unlike the magnetic needle.

  1. At the new station B (line ba already plotted from the previous station A), level the table and centre b over B.
  2. Place the alidade along the line ba.
  3. Turn the board until the ranging rod at A is bisected, then clamp it.
  4. The sheet is now parallel to its first position. Check by sighting a third known point; the ray must pass through its plotted position.

When intersection is preferred over radiation

IntersectionRadiation
Used when the point is inaccessible (across river, valley, swamp) so distance cannot be measuredNeeds the distance to the point to be measured with tape/chain
Only the base line is measuredEvery ray needs a measured length
Many distant points can be plotted quicklySuitable for points near the station
Third ray gives a checkNo check on the plotted point
Used for large areas and long distancesUsed for small areas

So intersection is preferred when the distance to the point cannot be measured, when the points are far, when many points are to be located quickly, and when a check on the position is required.

✓ Answer checked by Claude Opus 5.5 (high effort)

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