What survey technique is used when the instrument cannot be set up between the backsight and foresight?

Reciprocal leveling is the appropriate survey technique used when the instrument cannot be set up directly between the backsight (the point being observed) and the foresight (the point to which the observation is made). This situation typically arises when there are obstacles or physical limitations, such as changes in terrain or obstructions that prevent a straight line of sight between the two points.

In reciprocal leveling, two instruments are set up at different locations to observe each other. This allows surveyors to measure the same points from different vantage points, effectively compensating for any variations in elevation or possible errors in measurements due to obstruction. By averaging the readings taken from both setups, surveyors can achieve a more accurate determination of the height difference between the two points.

Understanding this technique is critical for surveyors, particularly in complex terrains or urban areas where direct lines of sight may be frequently interrupted. The other methods listed, such as direct line surveying, triangulation, and contour surveying, do not specifically address the issues presented by physical obstructions between backsight and foresight, thus making reciprocal leveling the correct choice in this scenario.