UNIT I: FUNDAMENTALS
OF SURVEYING
Surveying is the art or science
of determining the relative positions of point on above or beneath the surface
of earth with the help of direct or indirect measurements of distances,
direction and elevation.
The principle objective of surveying is to
prepare plan or map so
that it will represent the area on a horizontal plane.
Objectives of
surveying:
- To determine the positions of points
with respect to each other or with respect to fixed reference (datum)
above or below the surface of the earth.
- To set out the
positions of proposed work on ground (marking).
- To determine
areas, volumes and other related quantities.
4.
To determine absolute (or relative) directions ofline
with reference to fixed (or arbitrary) direction. Primary Division of
Surveying > Earth seems to be perfect sphere but its actual shape is
oblate spheroid or oblate tri-axial ovaloid.
> Length
ofthe polar axis ofthe earth is 12713.8 km which is lesser than length of its
equatorial axis (12756.57 km) by 42.77 km.
> Earths polar axis is 0.34% less compared to its
diameter.
> The average radius of earth is taken as 6370 km
for all calculations.
> Ifthe
irregularities on the earth surface are neglected the shape ofthe earth surface
is curved, every element of which is normal to plumb line.
> Thus
primary division of surveying is based on whether the curvature of earth is
considered or not. There are two types:
Plane surveying:
> In plane surveying curvature of the earth is not
considered and it is assumed as a plane surface. > All triangles formed by
survey lines are considered as plane triangles.
> All angles (vertical &
horizontal) are planes angles.
>
The direction ofvertical line or plumb line is indicated by the freely
suspended plumb bob and all plumb lines are considered as parallel to each
other.
> Curved line on the earth’s
surface is considered as straight or horizontal.
> Plane surveying is applicable
only when extent of area is less than 200 sq km.
>
Also when the difference between an arc distance of 18.2 km on the earth
surface and corresponding chord distance is not more than 1 cm.
>
And further when the difference between sum ofthe angles in spherical triangle
& plane triangles is only 1 second. For a triangle having an area of 195 sq
km.
Geodetic Surveying:
In geodetic surveying the
curvature ofthe earth is taken into consideration.
All lines on the earth surface
are curved lines and all triangles and there angles are spherical.
Geodetic survey involves
spherical trignometry.
This survey is carried out for
large magnitude work and when high degree ofprecision is required.
The direction ofplumb lines
converges towards the centre of earth.
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