Orientation in plane tabling; reliable method

Orientation is the process of putting the plane table in a fixed direction at every station, so that all the lines on the sheet are parallel to the corresponding lines on the ground. It is needed whenever the table is shifted from one station to another.

Orientation by back-sighting

This is the more reliable method, because it does not depend on the magnetic needle, which is affected by local attraction.

Condition: the table is at station B; the line ba (from the previous station A) is already plotted.

1. Level the table and centre b over station B.
2. Place the alidade along the line ba.
3. Turn the board until the ranging rod at A is bisected by the line of sight.
4. Clamp the board. The table is now oriented, since ba is parallel to BA.
5. Check by sighting another known point; the ray should pass through its plotted position.

   SHEET (table at B)             GROUND
   a o--------o b                 A o ........ o B
     \_ alidade on ba             (rod at A seen
       along the line)             along the line)

The board is turned about its vertical axis, with the alidade kept on the line ba, until the rod at A is bisected.

Why back-sighting is more reliable

• It depends only on the direction of a line already plotted on the sheet, and the line is sighted directly through the alidade.
• The magnetic needle (the other method) is affected by local attraction from iron, steel structures, power lines and by its own errors, so orientation by compass may be wrong by several degrees.
• A long back-sight line gives a more accurate orientation than a short one, and the result can be checked by sighting a third known point.