In differential leveling over long distances, what effects must be taken into consideration?

In differential leveling over long distances, the curvature of the earth is a fundamental consideration due to the fact that the earth is not flat but rather an oblate spheroid. When leveling over long distances, the curvature can lead to differences in elevation readings because the line of sight between benchmarks and leveling instruments will not follow a perfectly horizontal line. If these curves are not taken into account, it can result in systematic errors in determining the true relative heights of points.

For leveling projects that span greater distances, this curvature becomes more pronounced, and surveyors must apply corrections to their readings to account for the earth's curvature. This ensures the accuracy and reliability of the elevation data collected, which is critical for effective land development, construction, and other survey-related applications.

The other factors listed—focal length of the instrument, latitude of the observations, and atmospheric refractions—may influence survey measurements in certain contexts, but the curvature of the earth is a more direct and essential concern in the context of long-distance leveling. Properly accounting for it allows surveyors to produce accurate measurements that reflect true differences in elevation between points.