Question
Download Solution PDFWhich of the following is an INCORRECT statement related to the height of the instrument (HI) used in levelling?
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFConcept:
Height of instrument (HI)
- It is the elevation or reduced level of the line of sight with respect to the datum. The height of the instrument is not the height of the line of sight above the ground where the leveling instrument is set up.
- In survey leveling, Height of instrument (HI) = the vertical height of the line of collimation of the instrument over the station above which it is centered, or above a specified datum level.
- In spirit leveling, the vertical distance from the datum to the line of sight of the instrument.
- In stadia leveling, the height of the center of transit above the station stake.
- In differential leveling, the elevation of the line of sight of the telescope when the instrument is leveled. Height of instrument = Back Sight + Elevation of Benchmark
Additional InformationBacksight (BS):
- It is the staff reading taken on the point of the known elevation as on a benchmark (BM) or change point. The backsight reading is the first staff reading taken after the level is set up and leveled at the point. It is also known as Plus sight.
- The value of HI generally changes before recording back sights.
- e.g. If the elevation of any point (P) is known then the staff reading on station P is known as a Back sight-reading.
Foresight (FS):
- It is a staff reading taken on a point whose elevation has to be determined through the leveling process. It is also known as minus sight. The foresight is also taken towards a change point. It is the last reading taken before the instrument is shifted.
- The value of HI never changes while recording fore sights.
- e.g. If the elevation of station Q is to be determined, then the staff reading taken on station Q is called Foresight.
Intermediate sight:
- It is the staff readings taken on staff held at other points whose elevations are to be determined before the foresight is taken. Intermediate sights are all staff readings between the BS and FS.
- The value of HI never changes while recording intermediate sights
Confusion Points In the statements, understanding the meaning of backsight(s) is important.
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