Leveling - Part 01 (Surveying)
LEVELING
Important terminology
1. 1. Reduced level (RL)
2. 2. Mean sea level (MSL)
3. 3. Bench mark (BM)
a. Permanent bench mark: - established by GOI, Indian railway, CPWD.
b. Temporary bench mark: - e.g. change point
Measurements in leveling work
1. Back sight reading
2. Fore sight reading
3. Intermediate sight reading
Change point: - it is station or point after which instrument location is changed. At changed point first fore sight reading is taken from previous instrument and then back sight reading is taken from next instrument.
Methods to find Reduced Level
1. Height of instrument (HI) method: - HI = RL of BM + BS, RL of any point = HI – IS/FS, check ∑BS - ∑FS = last RL – first RL
2. Rise and fall method: - rise/fall = previous reading – this reading, +ve – rise, -ve – fall, RL of any point = RL of previous point + rise/-fall, check ∑rise - ∑fall = last RL – first RL
Leveling staff is inverted in leveling work just to find RL of base of roof.
Inverted staff reading are represented with –ve sign.
Geodetic leveling work:
1. Horizontal line (HL) – Tangent to earth’s surface.
2. Level line (LL) – Line Parallel to Mean Sea Level Equi-reduced level line – line of equal HI.
A. Correction due to earth’s curvature: - this correction is always –ve. CC = -0.0785d2 in meters, d taken in Km.
B. Correction due to refraction: - this correction is always +ve. CR = 0.0112d2 in meters, d taken in Km.
C. Combined correction due to earth’s curvature and refraction: - C = CC + CR = -0.0673d2 in meters, d taken in Km. if staff reading is h then, corrected staff reading = h – 0.0673d2.
Distance of visible horizon
Maximum distance visible on earth’s surface from any height h. Dip of horizon (θ).
θ = Cos-1[R/(R+h)]. R = 6400Km radius of earth.
Case –I: - when only one point is considered at height.
d = 3.855√h - distance of visible horizon.
h = 0.0673d2 – height of object in meter, d in Km & h in m.
Case II: - when both points are considered at height.
D = d1 +d2 = 3.855 [√h1 + √h2]
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