Which of the following calculations is used to determine the difference in elevations at a survey point?

The foresight and backsight method is a fundamental technique used in leveling surveys to determine the difference in elevations between survey points. In this method, a surveyor takes readings on a leveling instrument using two types of staff readings: foresight and backsight.

The backsight reading is taken on a known reference point with a known elevation, while the foresight reading is taken on the point whose elevation is to be determined. By calculating the difference between these two readings, the surveyor can accurately derive the elevation of the foresight point relative to the backsight reference point. This method is highly effective and commonly used due to its straightforward application and reliable results.

In contrast, the other methods listed serve different purposes. The height of instrument method involves establishing the height of the leveling instrument and is more related to the starting point than directly comparing elevations between two points. Barometric leveling involves changes in atmospheric pressure to estimate elevation differences, which can be influenced by environmental factors and can lead to less accurate results compared to direct measurements. Triangulation is used to determine horizontal and vertical positions based on angles and distances between multiple points and is more complex than simple elevation comparisons.