What is reciprocal leveling and when is it used?

Reciprocal leveling determines elevation difference between two points separated by an obstacle (river, valley) where normal leveling is impractical. Procedure: set level near point A, take staff readings on A and B (long sight). Move level near B, take readings on B and A. True difference = average of differences from both setups, eliminating systematic errors from collimation, curvature, and refraction. Formula: true diff = [(reading at A1 - reading at B1) + (reading at A2 - reading at B2)] / 2. Critical for bridge and pipeline surveys.