Fce 552 part2

A plane passing through a point on the

surface of the Earth and containing the Earth’s

axis of rotation defines the astronomical

meridian at the point.

The direction of this plane may be

established by observing the position of the sun,

or a star, or by observing a planet.

By popular usage the intersection of this

meridian plane with the surface of the Earth is

known as the true meridian.

DIRECTION OFA LINE:

Dept. of Geospatial & Space Technology

University of NairobiB.Sc. (Civil Engineering)

Astronomical Meridian

FCE 552: Engineering Survey IV

The Earth acts very much like a bar magnet with a

north magnetic pole located considerably south of the

north pole defined by the Earth’s rotational axis.

The magnetic pole is not fixed in position, but

rather changes its position continually.

A magnetised needle freely suspended on a pivot

will come to rest in a position parallel to the magnetic line

of force acting in the vicinity of the needle.

The direction of the magnetised needle defines the

magnetic meridian at the specific time.

Unlike the true meridian, whose direction is fixed,

the magnetic meridian varies in direction.


University of Nairobi

Magnetic Meridian


B.Sc. (Civil Engineering)

For convenience in a survey of limited extent, any

line of the survey may be assumed to be a meridian or a

line of reference. An assumed meridian is usually taken to

be in the general direction of the true meridian.



Assumed Meridian

Convergence of Meridians True meridians on the surface of the Earth are lines

of geographic longitude, and they converge toward each

other as the distance from the equator towards either of

the poles increases.

The amount of convergence between two meridians

in a given vicinity depends on i) its distance north or

south of the equator and

ii) the difference between the longitudes of the two

meridians.



The azimuth of a line on the ground is the horizontal

angle measured from the plane of the meridian to the

vertical plane containing the line.

Azimuth gives the direction of the line w.r.t. the

meridian.

It is usually measured in a clockwise direction w.r.t.

either the north meridian or the south meridian.

In astronomical and geodetic work azimuths are

measured from the south meridian. In plane surveying

azimuths are generally measured from north.

A line may have an azimuth of between 0 – 360.



Azimuth of a line



Azimuths are called true azimuths when measured

from the true meridian, magnetic azimuths when

measured from the magnetic meridian, assumed azimuths

when referred to an arbitrary north-south line, and grid

azimuths when referred to the central meridian in a grid

system.

The magnetic poles do not coincide with the poles

defined by the Earth’s rotational axis, and certain

irregularities in the Earth’s magnetic field cause local and

regional variations in the position of the magnetic needle.

The amount and direction by which the magnetic needle is

off the true meridian is called the magnetic declination.

Two azimuths differ from magnetic azimuths by the

magnitude of the magnetic declination at the time.





Traverses, triangulation and trilateration particularly,

require azimuth control. The direction of a survey line

w.r.t. true north – the azimuth of the line – is

determined from theodolite observations to the stars

or the sun, which itself a star, presents special

problems when observed from the Earth. The

quantities observed are:

1. The horizontal angle between the star (sun) and a

reference object (RO) placed at some suitable point of

the survey, and either

2. The vertical angle to the star (sun) at the same instant

as the horizontal angle; or

3. The accurate time of the horizontal observation.





The position of the star (sun) at the instant of

horizontal/vertical angle observation is

extracted from the star almanac.

The accuracy with which these quantities are

observed depends on the accuracy required of

the final azimuth, the particular method used,

and the position of the observer on the Earth.





Fce 552 part2

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