Coordinate systems, datum & map projections

Coordinate Systems, Datum & Map Projections
MASHHOOD ARIF

Contents
 Datum
 Transformation
 Coordinate Systems
 Coordinate System & Datum
 Map Projections
 Distortions
 Coordinate Systems & Map Projections
 Universal Transverse Mercator
 Longitude, Latitude & Meridians
 Conclusion

DATUM
 The reference specifications of a measurement system, usually a system of
coordinate positions on a surface (a horizontal datum) or heights above or
below a surface (a vertical datum)
 Datums are based on specific ellipsoids and sometimes have the same name as
the ellipsoid
 (Ellipsoid: A 3D, closed geometric shape, all planer sections of which are
ellipses or circle)
 Common types of Datum include:
North American Datum 1927 (NAD 27)
North American Datum 1983 (NAD 83)
World Geodetic System 1984 (WGS84)

Transformation
 The process of converting the coordinates of a map or an image from one
system to another, typically by shifting, rotating, scaling, skewing, or projecting
them.
 Geographic Transformation: A systematic conversion of the latitude-longitude
values for a set of points from one geographic coordinate system to equivalent
values in another geographic coordinate system (often called as Datum Shift)

Coordinate System
A reference framework consisting of “a set of points, lines and / or surfaces, and
a set of rules used to define the positions of points in space, in either two or
three dimensions
Major types:
 Geographic Coordinate System
 Projected Coordinate System
 Planer Coordinate System
 Cartesian Coordinate System

Geographic: A reference system that uses latitude and longitude to define the locations of
points on the surface of a sphere or spheroid. It includes a datum, prime meridian and
angular unit
Projected: A reference system used to locate x, y and z positions of point, line and area
features in 2D or 3D. It is defined by geographic coordinate system, a map projection, any
parameters needed by ma projections and a linear unit of measure
Planer: A 2D measurement system that locates features on a plane based on their distance
from origin (0,0) along two perpendicular axis
Cartesian: A 2D, planer coordinate system in which horizontal distance is measured along
an x-axis and vertical distance is measured along a y-axis
Geographic Planer Cartesian

Coordinate Systems & Datum
Geographic & Projected Coordinate Systems are tied to Datums
Datums reflect different ways of measuring the shape of the earth
and impact both Geographic Coordinate Systems using Latitude /
Longitude and Projected Coordinate Systems
Datums can be considered as a set of established reference points
to which coordinate systems are registered
Both are often considered the same, and the terms are sometimes
used interchangeable

Map Projection
 It is the mathematical transformation of locations in the three
dimensional space of earth’s surface onto the two dimensional
space of a map sheet
 The idea of map projections is to preserve the properties of real
world features when they are depicted on a map
 The properties are:
Area
Shape
Distance
Direction

Distortions
Distortion will vary in at least one of each of the properties (area, shape,
direction, distance) depending on the projection used, as well as the scale of the
map, or the spatial extent that is mapped. Whenever one type of distortion is
minimized, there will be corresponding increases in the distortion of one or
more of the other properties.
There are names for the different classes of projections that minimize distortion.
Those that minimize distortion in shape are called conformal
Those that minimize distortion in distance are known as equidistant
Those that minimize distortion in area are known as equal-area
Those minimizing distortion in direction are called true-direction projections

Coordinate System & Map Projection
Both are constructed based upon map projection, but they are not map
projections themselves
Map projection defines how the position on earth’s curved surface is
transformed onto a flat map surface. A coordinate system is then superimposed
on the surface to provide the referencing framework by which positions are
measured & computed
One example of projected coordinate system is:
Universal Transverse Mercator Coordinate System (UTM)

Universal Transverse Mercator (UTM)
In this multiple cylinders are used that
touch the globe at 6 degree interval
meridians resulting into 60 projection
zones each of 6 degree longitude width
To avoid extreme distortions, projection
zones are limited to 84 N & 84 S
It s formed by superimposing a regular
grid on each projection zone such that the
vertical lines are parallel to the central
meridian

Latitude, Longitude and Meridian
 Latitude of a point is the angle between the equatorial
plane & the normal line at the point on the surface of
the earth that intersects it on the plane of a meridian.
 Longitude is the angle in the equatorial plane between
the line that connects the earth’s center with the prime
meridian and the line that connects the center with the
meridian n which the point lies
 Latitude & Longitude are measured in degrees, not in
distance. The actual length of a degree changes over
different parts of the earth
 Meridians are the lines of equal longitudes passing
through the poles, parallels are the lines of equal latitude

Conclusion
 Projections and coordinate systems are important to study because they form
the basis for how a GIS can store, analyze, and display spatial data
 Understanding projections and coordinate systems important knowledge to
have, especially if you deal with many different sets of data that come from
different sources
 Coordinates in the GIS are measured from the origin point
 It is appropriate to choose a projection based on which measurement
properties are most important to your work. For example, if it is very important
to obtain accurate area measurements (e.g., for determining the home range of
an animal species), you will select an equal-area projection

Coordinate systems, datum & map projections

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