What value minus the height of instrument (HI) equals the elevation?

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Multiple Choice

What value minus the height of instrument (HI) equals the elevation?

In leveling, the height of the instrument (HI) serves as a reference line from which all staff readings are measured. When you take a backsight on a point of known elevation, that reading tells you how far the staff is below or above the instrument’s line of sight, and you use it to tie the instrument height to a known elevation. The essential relationship is that the known elevation equals HI minus the backsight: EL_known = HI − BS. This is why the backsight is the value involved in deriving the elevation from the instrument height—the backsight is subtracted from HI to yield the elevation of the known point. Once you’ve established HI using the backsight, you can find the elevation of a new point by subtracting the fore sight from HI (EL_new = HI − FS). The other terms listed (station number, distance) aren’t part of these height-elevation relationships, and the fore sight is the reading subtracted from HI to get the elevation of a new point, not the known point.

In leveling, the height of the instrument (HI) serves as a reference line from which all staff readings are measured. When you take a backsight on a point of known elevation, that reading tells you how far the staff is below or above the instrument’s line of sight, and you use it to tie the instrument height to a known elevation. The essential relationship is that the known elevation equals HI minus the backsight: EL_known = HI − BS. This is why the backsight is the value involved in deriving the elevation from the instrument height—the backsight is subtracted from HI to yield the elevation of the known point. Once you’ve established HI using the backsight, you can find the elevation of a new point by subtracting the fore sight from HI (EL_new = HI − FS). The other terms listed (station number, distance) aren’t part of these height-elevation relationships, and the fore sight is the reading subtracted from HI to get the elevation of a new point, not the known point.

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