Civilax4U: Introduction Of Surveying

Surveying is the process of determining relative positions of different objects on the surface of the earth by measuring horizontal distances between them and preparing a map to any suitable scale. Measurements are taken in a horizontal plane.

OBJECTS OF SURVEYING:

(a) Determining relative positions of various points, above or below the surface of earth

(b) To mark the positions of proposed structure on ground.

PRIMARY Divisions OF SURVEYING:

Primary division is based whether the curvature of earth is considered or not.
The actual shape of earth is an oblate spheroid.
The polar axis (12713.168 km) is shorter (than the equatorial axis (12756.602 km) by about 43.434 km (0.34%)
The average radius of earth is taken as 6370 km for all calculations point of view.

PLANE SURVEYING: The surveying in which the curvature of earth is neglected and is assumed to be a flat surface.

Plane survey can safely be used when the extent of area is less than 260 Sq km.
Also when the difference between an arc distance of 18.2 km on the surface of earth and corresponding chord distance is about 10mm.
Also when the difference between the sum of angels in a plane triangle and spherical triangle is only one second(1") for a triangle at the earth’s surface having an area of 195 Sq. km.

GEODETIC SURVEYING: Shape of earth is taken into account.

All lines lying on the surface are curved lines and triangles arc spherical triangles.
Preferred for works of large scale with high degree of precision.
The directions of plumb lines at various Points are converging towards center of earth.

Functional classification of Surveying:

Control Surveying: Establishing the horizontal and vertical positions of widely spaced control points using principles of Geodetic Surveying. ln India, control survey is done by ‘Survey of India”
Topographical Survey : To show natural Features of the country such as rivers, hills, lake, etc.
Cadastral Surveys : Fixing of property lines, to show boundaries of fields, buildings etc. This is to be done by a revenue engineer.
Engineering Surveys : To obtain data for designing any type of project such as roads, railways. water supply systems etc. An engineer is interested in this survey works.
Mine Survey: Under ground works such as mines, shafts, pits, bore holes etc.
Hydro graphic Survey : Surveying under water bodies e.g., determination of channel depth etc.
Astronomic Survey: To determine absolute location of a point on earth by taking latitude, longitude, azimuth, local time etc.

Classification based on instrument:

Chain surveying :This is the simple method of taking the linear measurement using a chain or tape with no angular measurements made.
Compass Surveying : Here horizontal angular measurements are made using the magnetic compass with the linear measurements made using the chain or tape.
Leveling:This is the measurement and mapping of the relative heights of points on the earth’s surface showing them in maps, plane and charts as vertical sections or with conventional symbols.
Plane Table Surveying: This is a quick survey carried out in the field with the measurements and drawings made at the same time using a plane table.
Theodolite Survey: Theodolite survey takes vertical and horizontal angles in order to establish controls.
Techeometric Survey
Photogrammetry
EDM Surveys

PRINCIPALS OF SURVEYING

Working from whole to part: First a system of control points are fixed with higher precision for a large area. It prevents accumulations of errors
Location of a point by measurement from two points of reference.

Scales: Scale is the fixed ratio that every distance on the plan bears with the corresponding distance on ground. A good draftsman can plot a length to accuracy with in ± 0.25 mm

Classification of scales

Large scale : 1cm = 10m
Medium scale: 1cm = 10m to 100m
Small scale: 1cm > 100m

Representing scales: The proportion between the drawing and the object can be represented by two ways as follows:

Scale: 1cm = 1m or 1cm =100cm or 1:100
Representative Fraction: (RF) = 1/100 (less than one) i.e. the ratio between the size of the drawing and the object.

Types of Scales:

Plain Scales: Plain scales read or measure upto two units or a unit and its sub-division, for example centimeters (cm) and millimeters (mm). When measurements are required up to first decimal, for example 2.3 m or 4.6 cm etc.
Diagonal Scales:Diagonal scales are used to read or measure up to three units. For example: decimetres (dm), centimetres (cm) and millimetres (mm) or miles, furlongs and yards etc. This scale is used when very small distances such as 0.1 mm are to be accurately measured or when measurements are required upto second decimal. For example: 2.35 dm or 4.68 km etc.
Use of verniers in scales

A vernier is a device for measuring accurately the fractional part of the smallest divisions on a graduated scale.

Direct Verniers: It is constructed as (n-1) divisions of the main scale is equal to n division of the vernier. In direct vernier, vernier scale moves in same direction of main scale.

nv = (n-1) s

where, s = value of one smallest division of main scale

n = number of division on the vernier

v = value of one smallest division of vernier

Retrograde vernier: It is so constructed that (n + 1) division of main scale is equal to n division of vernier. In retrograde vernier, vernier scale moves in opposite direction of main scale.