BASIC CIVIL ENGINEERING SURVEYING Part 2
7- Magnetic Meridian:
It is the direction indicated by a freely suspended and balanced
magnetic needle as in a compass, unaffected by local attractive forces. The
magnetic meridians vary with time and location.
8- Azimuths:
An azimuth is the direction of a line as given by an angle measured
clockwise from north end of a meridian. Azimuth range in magnitude from 0o
to 360o
.
9- Bearing:
A bearing is the direction of a line, measured clockwise or
anticlockwise from the north or south end of the meridian. It is always
accompanied by letters that state the quadrant in which the line falls(NE, NW,
SE, SW).
Fig.1.1 Azimuth and Bearing
10- Geographic Reference: (Geographic Coordinates)
The surveying involves in measuring the location of physical
land features relative to one another and relative to the defined reference on
the surface of the earth. The earth’s reference system is composed of the
surface divisions donated by geographic line of Latitude and longitude.
distances from the equator and parallel to it are called line of latitude.
They are used to measure direction of a place on the earth north or south of
the equator. The latitude measures the direction of a place on earth surface, in
degree, at the earth’s center, from the equatorial plane (See figure 1.4). The
equator is at 0o
latitude. The latitude angle is measured 90o max.
Longitude: The imaginary lines that make half circle around the earth sphere
(globe) and meet the geographical north and south poles are called line
meridians or line of longitude.
They are used to measure direction of a place on earth surface, in
degree, at the earth center, east or west from 0o longitude. The 0o longitude is
placed through Greenwich England and called Prime meridian.
The coordinates of a place on the surface of earth are represented as P
(Latitude, Longitude).For example: coordinates of Lahore are as (31.35N,
74.19 E)
The system of geographic coordinates is much used in navigation and
geodesy it is not used in plane surveying.
11- Grid Reference system: (Grid Coordinates)
The states adopted grid system of parallel horizontal and vertical lines.
Which are usually referred to the central meridian of the country and to the
equator so that one common datum for X and Y coordinates in horizontal
plane can be established.
In grid system the meridian of various stations (points) is considered
as parallel (contrary to the fact that they are converge to meet at poles) but the
grid size is limited so that no serious error will accumulate when the curvature
is ignored.
In grid coordinate system the location of a place is represents as
Northing (Y-coordinate) and Easting (X-Coordinate) from the origin of grid
reference. For example; the grid coordinates of a location P are as under;
𝑃 (
𝑁 = 156785𝑚
𝐸 = 25156𝑚
)
12- Vertical Reference: (Datum)
In additions to determining the locations of land features, the
surveying process also demands the altitude (elevation) of that land feature
relative to one another and relative to a reference datum.
