ELEVATION
The elevation of a geographic location is its height above or below a fixed reference point. Commonly,
elevation is measured using the center of the Earth as the reference point.
What
is height of instrument (HI)
In any particular set up of an instrument
height of instrument, which is the elevation of the line of sight, is constant.
The elevation of unknown points can be obtained by subtracting the staff
readings at the desired points from the height of instrument. This is the basic
behind the height of instrument method for reduction of level.
Image
example
With reference to Figure
1 to 6 and Table-1, when the instrument is at I1, the staff reading observed at A is 2.365m. The elevation of the
line of sight i.e., the height of instrument is 102.365m obtained by adding the
elevation of A (100.0m) with the staff reading observed at A (2.365m). The
elevation of S1 (101.130m) is determined by subtracting its foresight reading
(1.235m) from the the height of instrument (102.365m) when the instrument is at
I1 . Next, the instrument is set up at I2. S1 is considered as a point of known
elevation and backsight reading ( 0.685m) is taken . The height of the
instrument (101.815 m) is then calculated by adding backsight reading ( 0.685m)
with the elevation (R.L.) of point S1(101.130m). Foresight is taken at
S2 and its elevation (98.245m) is determined by subtracting the
foresight (3.570m) from the height of the instrument (101.815 m). In this way,
elevation of points are calculated by Height of instrument method.
Table-1 Level book note for Height of instrument
method
Staff Reading
|
Height of Instrument (m)
|
R.L. (m)
|
Remarks
|
||
Points
|
B.S (m)
|
F.S.(m)
|
|||
A
|
2.365
|
102.365
|
100.000
|
B.M.
|
|
S 1
|
0.685
|
1.235
|
101.815
|
101.130
|
T.P.1
|
S2
|
3.570
|
98.245
|
T.P.2
|
||
B
|
2.340
|
97.650
|
|||
