Inclined sight with tilted staff—error expression: If θ is the vertical angle of an inclined sight and δ is the angle of tilt of the staff from the vertical, the resulting error in the staff reading is best represented by which expression?

Difficulty: Medium

• l (1 − cos δ) sec θ
• l sin²(δ/2) sec θ
• l tan θ sin δ
• l (sec θ − sec(θ ± δ))
• none of these