Coordinate Systems in GIS Explained: Types, Uses, and Examples
Geographic Information Systems (GIS) employ coordinate systems to effectively represent, analyze, and visualize spatial objects on the globe. Understanding coordinate systems is vital in working with data such as satellite images, DEMs, LiDAR, or any vector data, such as shapefiles.
The coordinate system in GIS defines how geographical locations are denoted through numbers. It provides the mathematical basis for establishing the position of geographical objects, measuring distances, areas, and performing spatial operations.
Different types of coordinate systems can be used for different purposes. Some may use latitude and longitude, while others can be based on coordinates expressed in meters or feet. It is exceedingly important to select the right coordinate system for GIS purposes in mapping, surveying, remote sensing, navigation, etc.
What Is a Coordinate System in GIS?
A coordinate system in GIS refers to a framework for storing the coordinates of various geographic features in either two dimensions or three dimensions.
A coordinate system can apply to any aspect of geography, whether it be points, lines, or polygons.
For example, the coordinates of a given region can be laid out as follows:
Latitude: 40.7128° N
Longitude: 74.0060° E
Geographic Coordinate Reference System: WGS84
GIS uses the Coordinate Reference System (CRS) model to relate the coordinates to spatial reference details.
A CRS is made up of a framework for interpreting coordinates, encompassing the datum, coordinate system, reference ellipsoid, map projection, and measurement units.
Types of Coordinate Systems in GIS
There are various types of coordinate systems employed in GIS; they are:
Geographic Coordinate System (GCS)
A Geographic Coordinate System allows you to specify a point on the surface of a spherical object in terms of angular coordinates (latitude and longitude).
It relies on a geodetic datum and reference ellipsoid to assign geographic coordinates.
It has the following two basic coordinates:
Latitude: indicates the angle north/south of the equator;
Longitude: indicates the angle east/west from the prime meridian.
Latitude and longitude are usually expressed in degrees and decimal degrees.
Example: WGS84
One of the most famous global geodetic datums is the World Geodetic System 1984 (WGS84).
The associated geographic CRS of this system is EPSG:4326.
Latitude: 34.0522°
Longitude: -118.2437°
CRS: EPSG:4326
Location: Los Angeles, California
Applications of geographic coordinate systems:
GPS and GNSS positioning
Global navigation systems
Satellite imagery georeferencing
Geographic information transfer
Global environmental monitoring
Geographic applications based on the Web
Geographic coordinate systems serve as good tools for representing position on the globe, but angular coordinates have little implication in practical use since they cannot easily be applied in calculating distance and area on a plane.
Projected Coordinate System (PCS)
A Projected Coordinate System represents locations on a flat, two-dimensional surface using Cartesian coordinates.
Unlike geographic coordinate systems, projected systems typically express coordinates as linear measurements, such as meters or feet.
A projected CRS combines an underlying geographic CRS with a map projection and Cartesian coordinate system.
The mathematical transformation can be represented as:
\[ x=f(\lambda,\phi) \]
\[ y=g(\lambda,\phi) \]
Where:
\(x\) = Projected easting coordinate
\(y\) = Projected northing coordinate
\(\lambda\) = Longitude
\(\phi\) = Latitude
\(f,g\) = Map projection transformation functions
The actual transformation may also depend on ellipsoid parameters and projection-specific constants.
Example: Universal Transverse Mercator (UTM)
UTM divides the Earth into 60 longitudinal zones, each spanning 6 degrees of longitude.
Each zone uses a Transverse Mercator projection to represent geographic coordinates in meters.
For example:
Coordinate Reference System: WGS84 / UTM Zone 33N
EPSG Code: 32633
Coordinate Units: Meters
Projection: Transverse MercatorUTM coordinate systems are commonly used for regional mapping, engineering, land surveying, and environmental analysis.
Uses of planned coordinate systems:
Mapping of land plots
Town planning
Construction of infrastructures
GIS results for a particular region
Measurement of distance and area
Mapping of topography
Projected coordinate systems introduce distortions; however, if the right projection is selected, the distortions for a particular geographical region can be reduced.
Geocentric Coordinate System
A geocentric coordinate system represents locations in three-dimensional Cartesian space using X, Y, and Z coordinates.
Its origin is generally located at or near the Earth's center of mass.
The axes are defined relative to an Earth-centered, Earth-fixed reference framework.
Geocentric coordinates are commonly expressed in meters.
The conversion from geodetic latitude, longitude, and ellipsoidal height to Earth-centered coordinates can be calculated using:
\[ X=(N+h)\cos\phi\cos\lambda \]
\[ Y=(N+h)\cos\phi\sin\lambda \]
\[ Z=[N(1-e^2)+h]\sin\phi \]
Where:
\(X,Y,Z\) = Geocentric Cartesian coordinates
\(N\) = Prime vertical radius of curvature
\(h\) = Ellipsoidal height
\(\phi\) = Geodetic latitude
\(\lambda\) = Longitude
\(e\) = First eccentricity of the reference ellipsoid
Geocentric coordinate systems are used in satellite positioning, geodesy, GNSS processing, and Earth observation applications.
How Coordinate Transformations Work in GIS
Coordinate transformation refers to the process of changing coordinates from one spatial reference system to another.
Often, GIS processes comprise two interrelated processes.
The coordinate conversion changes coordinate values among coordinate systems that have the same datum. For example, transformation from geographic coordinates to projected coordinates can be carried out.
Datum transformation refers to the transformation of coordinates from one geodetic reference frame or datum to another.
The example of changing the coordinates from WGS84 geographic coordinates to WGS84/UTM Zone 33N implies the use of a map projection conversion.
In contrast, NAD27 coordinates may require a datum transformation to NAD83 using suitable transformation parameters or grid files.
Best Practices for Managing Coordinate Systems in GIS
For predictable spatial accuracy, GIS specialists must employ consistent CRS administration techniques:
Confirm the source CRS before importing geospatial datasets.
Use the EPSG codes where possible.
Differentiate between assigning a CRS and changing coordinates.
Choose projection types according to the geographic area and analysis tasks.
Check the compatibility of datums when integrating data from different sources.
Retain horizontal and vertical reference data with 3D datasets.
Keep CRS metadata when saving raster and vector files.
Verify transformed coordinates using known reference points.
Keep a record of coordinate transformation and accuracy procedures.
This set of actions can assist in eliminating spatial errors and enhancing interoperability of GIS software and GIS data sources.
Coordinate systems are pivotal in GIS technology since they reveal the ways through which geographic locations are portrayed, collected, and examined.
Geographic coordinate systems are based on angular coordinates to depict places located on Earth, whereas projected coordinate systems consist of representations that are not suitable for mapping and spatial operations. Geocentric coordinate systems create three-dimensional reference structures to execute geodetic and satellite positioning processes.
Knowing coordinate reference systems, datums, projection methods, EPSG standards, and coordinate transformations allows GIS experts to successfully join geospatial datasets and perform spatial analysis.
Be it satellite images, radar data, aerial surveys, or environmental assessments, the correct choice of a coordinate system is vital for accurate and precise data throughout the GIS process.
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