Level and Levelling
Elevation measurements involve measurements in vertical plane. It’s conjointly referred to as levelling. It’s going to be outlined because the art of deciding the elevations of given points on top of or below a data point line or establishing given points of needed heights on top of or below the data point line.
OBJECT AND USES OF LEVELLING
As expressed within the definition of leveling, the article is
(i) To confirm the elevations of given points with relevancy a data point
(ii) To establish the points of needed height on top of or below the data point line.
Uses of levelling area unit
(i) To confirm or to line the pedestal level of a building.
(ii) To decide or set the road, railway, canal or sewerage line alignment.
(iii) To confirm or to line varied levels of dams, towers, etc.
(iv) To confirm the capability of a reservoir.
TERMS employed in LEVELLING
1. Level Surface:
A surface parallel to the mean ellipsoid of revolution of the world is named tier surface and therefore the line drawn on the extent surface is thought as tier line. Thence all points lying on tier surface area unit equal from the middle of the world. Shows a typical level surface.
2. Horizontal Surface:
A surface tangential to level surface at a given purpose is named surface at that time. Thence a horizontal line is at right angles to the cord at that time.
3. Vertical Line:
A vertical line at a degree is that the line connecting the purpose to the middle of the world. It’s the cord at that time. Vertical and horizontal lines at a degree area unit at right angles to every alternative.
The level of a degree or the surface with relevancy that levels of alternative points or planes area unit calculated, is named a data point or data point surface.
5. Mean water level (MSL):
MSL is that the average height of the ocean for all stages of the tides. At any explicit place MSL is established by finding the mean water level (free of tides) when averaging tide heights over a protracted amount of a minimum of nineteen years. In Asian country MSL used is that established at metropolis, presently, in Asian country. All told necessary surveys this can be used as data point.
6. Reduced Levels (RL):
The level of a degree taken as height on top of the data point surface is thought as RL of that time.
A benchmark could be a comparatively permanent reference, the elevation of that acknowledged is understood is thought} (assumed or known w.r.t. MSL). It’s used as a start line for levelling or as a degree upon that to shut for a check.
The following area unit the various kinds of benchmarks employed in surveying:
(a) GTS benchmarks
(b) Permanent benchmarks
(c) Absolute benchmarks and
(d) Temporary benchmarks.
(a) GTS Benchmark:
The long style of GTS benchmark is nice Trigonometrically Survey benchmark. These benchmarks area unit established by national agency. In India, the department of Survey of Asian country is entrusted with such works. GTS benchmarks area unit established everywhere the country with highest exactness survey, the data point being mean water level.
A bronze plate provided on the highest of a concrete pedestal with elevation carven on that is benchmark. It’s well protected with masonry structure engineered around it so its position isn’t disturbed by animals or by any unauthorized person. The position of GTS benchmarks area unit shown within the topo sheets printed.
(b) Permanent Benchmark:
These area unit the benchmarks established by government agencies like PWD. They’re established with relation to GTS benchmarks. They’re typically on the corner of pedestal of public buildings.
(c) Absolute Benchmark:
In several engineering comes the distinction in elevations of neighboring points is a lot of necessary than their reduced level with relevancy mean water level. In such cases a comparatively permanent purpose, like pedestal of a building or corner of a drain, area unit taken as benchmarks, their level assumed randomly like one hundred.0 m, 300.0 m, etc.
(d) Temporary Benchmark:
This sort of benchmark is established at the top of the day’s work, so following day work is also continued from that time. Such purpose ought to get on a permanent object so next day it’s simply known.