Coordinate systems define locations on Earth and enable datasets to integrate spatially. There are two main types: geographic coordinate systems use latitude and longitude, while projected coordinate systems define planar coordinates like x and y distances to allow for measurement. When data in different coordinate systems is viewed together in GIS, on-the-fly projection converts between systems to align the data spatially. Geographic transformations define the mathematical operations for converting coordinate values between geographic coordinate systems.

2. •Introduction:
Many of new concepts and terms will be found once you introduced to
ArcGIS and geographic information system (GIS), especially when you
are new to this approach.
So, one of the most important topics that needs to be covered and
understood is coordinates system, which is divided into types which are
Geographic coordinate system, Projected coordinate system.
Also, walking through, we will learn more about how the system will
behave if the projection wasn’t set,(Unknown projection),and
Geographic transformation.
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3. •Key-Term(Definition):
1) Coordinate system:
Coordinate systems enable geographic datasets to use common locations for
integration. A coordinate system is a reference system used to represent the locations of
geographic features, imagery, and observations, such as Global Positioning System
(GPS) locations, within a common geographic framework.
Each coordinate system is defined by the following:
 Its measurement framework, which is either geographic (in which spherical
coordinates are measured from the earth's center) or planimetric (in which the earth's
coordinates are projected onto a two-dimensional planar surface).
Units of measurement (typically feet or meters for projected coordinate systems or
decimal degrees for latitude-longitude).
The definition of the map projection for projected coordinate systems
Other measurement system properties such as a spheroid of reference, a datum, one
or more standard parallels, a central meridian, and possible shifts in the x- and y-
directions.
Several hundred geographic coordinate systems and a few thousand projected
coordinate systems are available for use. In addition, you can define a custom
coordinate system.
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4. 2) Longitude and latitude:
•Latitude measures how far north or south of the equator a
place is located. The equator is situated at 0°, the North Pole
at 90° north (or 90°, because a positive latitude implies
north), and the South Pole at 90° south (or –90°). Latitude
measurements range from 0° to (+/–)90°.
•Longitude measures how far east or west of the prime
meridian a place is located. The prime meridian runs through
Greenwich, England. Longitude measurements range from
0° to (+/–)180°.
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5. 3) Datum:
A datum defines the position of the spheroid relative to the center of the
earth. A datum provides a frame of reference for measuring locations on
the surface of the earth. It defines the origin and orientation of latitude
and longitude lines.
4) Spheroid:
[Euclidean geometry] A three-dimensional shape obtained by rotating
an ellipse about its minor axis, resulting in an oblate spheroid, or about
its major axis, resulting in a prolate spheroid.
[geodesy] When used to represent the earth, a three-dimensional
shape obtained by rotating an ellipse about its minor axis, with
dimensions that either approximate the earth as a whole, or with a part
that approximates the corresponding portion of the geoid.
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6. 5) Projection:
A method by which the curved surface of
the earth is portrayed on a flat surface. This
generally requires a systematic
mathematical transformation of the earth's
graticule of lines of longitude and latitude
onto a plane. Some projections can be
visualized as a transparent globe with a
light bulb at its center (though not all
projections emanate from the globe's
center) casting lines of latitude and
longitude onto a sheet of paper. Generally,
the paper is either flat and placed tangent to
the globe (a planar or azimuthal projection)
or formed into a cone or cylinder and
placed over the globe (cylindrical and
conical projections). Every map projection
distorts distance, area, shape, direction, or
some combination thereof.
To find more projection, visit:
map-projections 6

7. •Types of coordinate systems:
Geographic coordinate systems:
The main aim of coordinate system is to define
their location on earth. The coordinates in GIS
dataset can be stored in a geographic, projected,
or local coordinate system. This system uses
angles of longitude and latitude to define
locations on earth, on a three-dimensional model
of the earth. The terms of geographic coordinate
system (GCS) and datum are often used
interchangeably, a datum is only one part of a
(GCS).Besides the datum and a spheroid, a
geographic coordinate system includes the units
for longitude and latitude, and the prime meridian
that has been used for measuring longitude.
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8. 8
•Types of coordinate systems – Cont.:
Projected coordinate systems:
Geographic coordinate systems are good for storing data but not
very good for measuring distances. This is because the angles of
latitude and longitude can locate exact positions on the surface
of the earth, but angles don’t provide consistent distance
measurements. Assume that it :30 degrees of longitude along the
equator and 30 degrees of longitude close to the north pole
represent much different distances.
To measure distances, we use projected coordinate
systems(PCS). Because they define locations on the earth in a
flat (Planar), two-dimensional surface. The coordinates in a
projected coordinate system, also known as a Cartesian
coordinate system, represent distances from the origin(0,0 point)
along two perpendicular axes: a horizontal x-axis representing
east-west, and vertical y-axis representing north-south.

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•Types of coordinate systems – Cont.:
Projected coordinate systems:
In order to apply a projected coordinate system, we need to convert the three-
dimensional surface of the GCS into a two-dimensional one. This mathematical
operation is commonly referred to as a map projection. When a dataset is projected, the
latitude and longitude values are converted into (x , y) values in meters or feet.
When datasets covering large geographic areas are projected, four properties of the data
are distorted: shape, area, direction, and distance. When datasets covering small areas
such as a neighborhood or a city are projected, these distortions are often negligible.
Projections are categorized according to the spatial properties they preserve: some
projections preserve shape (conformal projections), while others preserve area(equal
area projections), distance(equidistant projections), or direction(true direction or
azimuthal projections).Only one of these spatial properties can be preserved with a
given projection, the other ones will be distorted. Some projections preserve a second
spatial property reasonably as well. For example, conformal projections preserve shape,
but they also preserve direction fairly well. Equal area projections preserve area but can
also preserve distance reasonably well. Alternatively, some projections distort all four
of these spatial properties.

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•Projection on the fly:
Geographic and projected coordinate systems aren’t only a property of a dataset
but also a property of an ArcMap data frame. There is no default coordinate
system for a data frame in ArcMap. When you open a new map without any
layers, the data frame has no coordinate system. To set a coordinate system for the
data frame, you can search for the desired coordinate system in the data frame
properties, narrow down your search using a spatial filter, and select the
appropriate one from a list of geographic and projected coordinate system. Also,
you can add a layer with the desired coordinate system as the first layer to the data
frame. The first layer that you add to a data frame defines its coordinate system.
ArcMap displays all layers in a data frame in the coordinate system that has been
set for that data frame.
This process is called projection on the fly. Projection on the fly makes it possible
to align data from different coordinate systems in the same data frame. When you
add a layer to a data frame that has a different coordinate system than the layer, the
layer is projected on the fly into the coordinate system of the data frame.

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•Geographic transformations:
A geographic transformation define the mathematical operation for
converting the coordinates of a dataset from one geographic coordinate
system to another. There maybe more than one transformation method
available for converting between any two geographic coordinate
systems. Each transformation method is designed for a particular area
and each has a different accuracy. e.g., there are eight transformation
methods for converting between the NAD 1983 and WGS 1984
geographic coordinate system.
When choosing a transformation method for your datasets, it’s important
that you are consistent and use the same transformation method every
time.