Difference between Prismatic and Magnetic Compass

Different important term related with Length and angle measurement during survey.

1. Survey
Dr. Aman Kumar Singh, Dr. B. R. Ambedkar University Delhi
October 13, 2024
Survey and Inspection Tools/ Equipment

2. Important Terms
Leveling
(i) To find the elevations of points with respect to a given or
assumed datum, and
(ii) To establish points at a given elevation or at different elevations
with respect to a given or assumed datum. The first operation is
required to enable the works to be designed while the second
operation is required in the setting out of all kinds of engineering
works. Leveling deals with measurement in a vertical plane.
Surveying
Survey is to horizontal distances.
It is also measures Vertical distances between the points are,
however, shown by contour lines, hachures or some other methods.
Vertical distances are usually represented by means of vertical
sections drawn separately.

3. Division of Survey
(1) Plane Surveying
(2)Geodetic Surveying.
Plane Survey
Plane surveyingis that type of surveying in which the mean
surface of the earth is considered as a plane and the spheroidal
shape is neglected.
The matter of fact that the length of an arc 12 kilometers long
lying in the earth’s surface is only 1 cm greater than the subtended
chord and further that the difference between the sum of the
angles in a plane triangle and the sum of those in a spherical
triangle is only one second for a triangle on earth’s surface cover
194 kilometers.

4. continue
Geodetic surveyingis that type of surveying in which the shape of the
earth is taken into account All lines lying in the surface are curved lines
and the triangles are spherical triangles. It, therefore, involves spherical
trigonometry. All geodetic surveys include work of larger magnitude and
high degree of precision. The object of geodetic survey is to determine
the precise position on the surface of the earth, of a system of widely
distant points which from control stations to which surveys of less
precision may be referred.

5. Type of Survey
(1) Land Surveying
(I) Topographical Survey
(II) Cadastral Survey
Calculation the land Area, fixing the properties line and transfer
the properties.
(III) City Survey Street , Water supply . sewerage systems etc.
(2) Marine or Hydrographic Survey.
(3) Astronomical Survey.

6. CLASSIFICATION BASED ON THE OBJECT OF
SURVEY
1. Military
2. Engineering
3. Geological
4. Archaeological
5. Mine
Instrument based(1) Chain survey (2) Theodolite survey (3)
Traverse survey (4) Triangulation survey (5) Tacheometric survey
(6) Plane table·survey (7)Photogrammetric survey and (8)Aerial
survey.

7. UNITS OF MEASUREMENTS
I. Horizontal distance 2. Vertical distance 3. Horizontal angle, and
4. Vertical angle.
Figure:

8. PLANS AND MAPS
The representation is called a map if the scale is small while it is
called a plan scale 1s large. on a plan, generally, only horizontal
distances and directions are shown. On a topographic map,
however, the vertical distances are also represented by contour
lines. hachures or other systems.
SCALES
Scale is. the fixed ratio thai every distance on the plan bears with
corresponding distance on the ground.
One unit of length on the plan represents some number of same
units of length on the ground, such as , etc. This ratio of map
distance to the corresponding ground distance is independent of
units of measurement and is called representative fraction. scale is
1 cm = 50 m
Figure:

9. Type of Scale
Figure:

10. Figure:

11. Figure:

12. SURVEYING WORK
The work of a surveyor may be divided into three
1. Field work 2. Office work 3. Care and adjustment of the
instruments.
DIRECT MEASUREMENTS
The various methods of measuring the distances directly are as
follows 1. Pacing·2. Measurement with passometer 3.
Measurement with pedometer 4. Measurement by odometer and
speedometer 5. Chaining (a) Instruments for the direct
measurement of directions :
(i) Surveyor’s Compass (il) Prismatic Compass
(b) Instruments for measurements of angles
(I) Sextant·(ii) Theodolite

13. Figure:
Bearing of a line is its direction relative to a given A meridian any
direction·such as (I) True Meridian (2) Magnetic Meridian (3)
Arbitrary Meridian.

14. True MeridianTrue meridian through a point is the line in which
a plane, passing that point and the north and south poles,
intersects with surface of the earth. It, thus,-passes through the
true north and south. The direction of true meridian through a
point can he established by astronomical observations.
True Bearing
line is the horizontal angle which it makes with the true meridian
through one of the extremities of the line. Since the direction of-
true meridian through a point remains fixed, the true bearing of a
line is a constant quantity.
Magnetic MeridianMagnetic meridian through a point is the
direction shown by a freely floating and balanced magnetic needle
free from all other attractive forces. 1ge direction of magnetic
meridian can he established with the help of a magnetic compass.
Magnetic BearingThe magnetic bearing of a line is line
horizontal angle which it makes with the magnetic meridian.
passing through One of the extremities of the line ’ A magnetic
compass is used to measure it.

15. Arbitrary MeridianArbitrary meridian is any convenient direction
towards a permanent and prominent mark or signal, such as a
church spire or top of a chimney. such meridians are used to
determine the relative positions of lines in a small area.
DESIGNATION OF BEARINGS
The common systems of notation of bearings (a) The whole circle
hearing system (W.C.B.) or Azimuthal system. (b) The
Quadrantal hearing (Q.B.) system.

16. Figure:

17. Figure:

18. FORE AND BACK BEARING
Figure:

19. Figure:

20. Figure:

21. DEFINITIONS AND TERMS (1) The vertical axis. The
vertical·axis is the axis about which the instrument can be rotated
in a horizontal plane. This is the axis about which the lower and
upper plates rotate.
(2) The horizontal axis. The horizontal or trunnion axis is the axis
about which the telescope and the vertical circle rotate in vertical
plane. (3) The line of sight or line of collimation. It is the line
passing through the intersection of the horizontal and vertical
cross-hairs and the optical centre of the object glass and its
continuation.
4. Face left observation. If the face of the vertical circle is to the
left of the observer, the observation of the angle (horizontal or
vertical) is known as face left observation.

22. 5. Face right observation. If the face of the vertical circle is to the
right of the observer, )he observation is known as face right
observation.
6. Telescope normal. A telescope is said to he normal or direct
when the face of yhe vertical circle is to the left and the ”bubble
(of the telescope) up”.
7. Telescope inverted. A telescope is said to inverted or reversed
when of the vertical circle is to the right and the ”bubble down”.
8. Changing face. It is an operation of bringing The face of The
telescope from left to right and vice versa.
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