In plane table surveys, the orientation of the table only by back sighting is preferred when _____________.

Explanation:

Orientation of Plane table: 

• Orientation in plane table surveying is the process of putting the plane table into some fixed direction so that line representing a certain direction on the plan is parallel to that direction on the ground.

• This is an essential condition to be fulfilled when more than one instrument station is to be used. 

Methods of Orientation of Plane table:

1. Orientation by trough compass       2. Orientation by mean of back sighting

1. Orientation by trough compass: 

• Orientation by trough compass is less accurate and used for approximate orientation of plane table prior to the final adjustment.
• The plane table can be oriented by compass under the following conditions:

1. ​When speed is more important than accuracy,
2. When there is no second point available for orientation,
3. When the traverse is so long that accumulated errors in carrying the azimuth forward might be greater than the orientation of the compass,

4. For approximate orientation prior to final adjustment.

2. Orientation by mean of back sighting:

• Orientation in plane table surveying can be done precisely by sighting the points already plotted on the sheet. 
• The plane table can be oriented by mean of back sighting the following conditions:

1. When it is possible to set the plane table on the point already plotted on the sheet by way of observation from the previous section.

2. When it is not possible to set the plane table on the point.