What is double centering used for in surveying?

Double centering is a technique employed in surveying primarily for turning angles or producing straight lines with precision. This method involves using two separate instruments, typically theodolites, to create a setup that allows for more accurate angle measurements and line positioning. The principle behind double centering is to mitigate errors that can occur when relying on a single instrument, such as instrument misalignment or parallax effects when reading angles. By employing double centering, surveyors can ensure that the angles being measured are more precise, thereby enhancing the overall accuracy of the surveying process.

In the context of the other options, creating elevation maps generally relies more on leveling techniques and does not specifically require double centering. Adjusting the horizontal alignment of survey lines can be influenced by angle measurements, but it is not the primary function of double centering. Measuring the curvature of terrain is typically done through other surveying techniques and does not directly relate to the double centering method. The focus of double centering is on achieving precision in angle measurement and alignment rather than these other surveying tasks.