How is a negative sag correction reflected in distance measurements?

In surveying, when a negative sag correction is applied to distance measurements, it indicates that the measured distance is longer than the true horizontal distance. This typically occurs when surveying equipment, such as a leveling instrument, is affected by the curvature of the earth or atmospheric refraction, especially over long distances.

When you apply a negative sag correction, you are essentially adjusting the measured value to account for these effects. In practical terms, this means that the measurement taken without correction overestimates the horizontal distance. To derive the true horizontal measurement, a sag correction is subtracted from the measured value. Therefore, recognizing how negative sag corrections influence distance measurements is vital for achieving accurate horizontal observations in surveying practices.