The combined effects of the earth's curvature and refraction is eliminated in __________. 

Explanation:

1. Reciprocal levelling is a surveying technique used to accurately measure the difference in elevation between two distant points, especially when direct line-of-sight measurements are obstructed or when high precision is needed.

2. This technique involves taking readings at both ends of a line and using the average to eliminate errors caused by earth's curvature and atmospheric refraction.

3. By observing the same points from both directions, the inherent errors (like refraction) are effectively cancelled out. The approach minimizes the impact of errors that might otherwise accumulate over long distances.

Additional InformationLongitudinal Sectioning:

1. Definition: Longitudinal sectioning refers to the vertical cross-section along the length of a structure or terrain.

2. Effect on Earth’s Curvature & Refraction: This method does not directly address the issues of earth's curvature or refraction. It is primarily used to represent a profile along the length, which may involve elevation or slope measurements but does not involve compensating for curvature or refraction.

Profile Levelling:

1. Definition: Profile levelling involves measuring the heights of points along a line, typically following the contour of the ground or a specific path.

2. Effect on Earth’s Curvature & Refraction: While profile levelling is used to measure changes in elevation, it doesn't directly compensate for the earth’s curvature or atmospheric refraction. The measurements are typically taken with a level instrument, but it doesn’t specifically correct for these factors over long distances.

Cross Sectioning:

1. Definition: Cross sectioning involves taking vertical slices or sections across a terrain at specific intervals, usually perpendicular to a central line (e.g., a road or railway line).

2. Effect on Earth’s Curvature & Refraction: Cross sectioning is used to understand the profile of the terrain in relation to a central line but does not address the curvature of the earth or the refraction effect in its standard form. It's a method used to study the shape and gradient of the ground.