What are curvature and refraction corrections in leveling?

Curvature correction accounts for Earth's spherical shape - horizontal line diverges from level surface. For distance d meters, curvature correction Cc = 0.0785 d squared (mm) where d is in km. Refraction bends the line of sight downward due to atmospheric density variation - typically 1/7 of curvature. Combined correction = Cc - Cr = 0.0673 d squared (mm). For backsight and foresight of equal length, corrections cancel. For long sights (>300m), apply correction to readings. Critical in trigonometric leveling and reciprocal leveling.