1. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology
DIRECTION OFA LINE:
Astronomical Meridian
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.
B.Sc. (Civil Engineering) University of Nairobi

2. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology
Magnetic Meridian
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.
B.Sc. (Civil Engineering) University of Nairobi

3. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology
Assumed Meridian
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.
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.
B.Sc. (Civil Engineering) University of Nairobi

4. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology
Azimuth of a line
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.
B.Sc. (Civil Engineering) University of Nairobi

5. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology
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.
B.Sc. (Civil Engineering) University of Nairobi

6. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology
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.
B.Sc. (Civil Engineering) University of Nairobi

7. FCE 552: Engineering Survey IV Dept. of Geospatial & Space Technology
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.
B.Sc. (Civil Engineering) University of Nairobi