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EPSG Codes in GIS: A Beginner’s Guide to Coordinate Systems

Geographic Information Systems (GIS) require accurate locations. Each point, line, polygon, satellite imagery, drone orthophotomosaic, and spatial database needs to be linked with a coordinate reference system (CRS), which describes how locations on the surface of the Earth should be expressed in coordinates.


EPSG codes represent a universal way of referencing such coordinate reference systems. Knowledge of EPSG codes is crucial for GIS software usage, creation and usage of spatial databases, web maps, GPS data, drone imagery, and remote sensing data.


EPSG Codes in GIS
EPSG Codes in GIS

What is an EPSG Code?


An EPSG code represents a unique number that serves as an identification for a coordinate reference system, coordinate transformation, or any other geodetic datum. Such numbers allow for consistent interpretation of geographic coordinates in GIS software.


Examples of EPSG codes:


  • EPSG:4326 - WGS 84 geographic coordinate system

  • EPSG:3857 - Web Mercator projected coordinate system

  • EPSG:32633 - WGS 84 / UTM Zone 33N

  • EPSG:27700 - British National Grid


The presence of an EPSG code in a dataset allows GIS applications to determine the coordinate reference system of the dataset and perform coordinate transformations.


Why Are EPSG Codes Important in GIS?


In isolation, coordinates do not necessarily give you precise locations. For example, the following coordinates:


40.7128, -74.0060


may be a geographical location if one has the definition of the coordinate system and axis.


GIS applications require data on the datum, units, coordinate axis, map projection, and reference geographies.


The use of EPSG codes provides a standardized reference for the interpretation of the above data by GIS applications.


The importance of EPSG codes includes:


  • Data alignment

  • Coordinate transformations

  • Map projections

  • GPS and GNSS data processing

  • Drone mapping and surveying

  • Satellite imagery analysis

  • Web mapping applications

  • PostGIS databases

  • GIS interoperability


The Difference Between Geographic and Projected Coordinate Systems


Geographic Coordinate System


A geographic coordinate system (GCS) is the location of points on the surface of the Earth using latitude and longitude. Units are usually expressed in degrees.


An example of the common geographic coordinate system is EPSG:4326, which uses the WGS 84 geographic coordinate reference system.


The coordinates may look something like:


Latitude: 38.9072°


Longitude: -77.0369°


Commonly used for:



Projected Coordinate System


A projected coordinate system (PCS) transforms a curved surface into a flat coordinate system. Units are usually expressed in meters or feet.


An example of a commonly used projected coordinate system is the Universal Transverse Mercator system, which divides the Earth into zones each having its own EPSG code.


Commonly used for:


  • Measurement of distance

  • Calculation of area

  • Engineering projects

  • Land surveying

  • Drone mapping

  • Local and regional GIS analysis


Commonly Used EPSG Codes in GIS


EPSG:4326 – WGS 84


This EPSG code is among the most common coordinate reference systems in the world. This EPSG code uses latitude and longitude in degrees to define geographic coordinates.


EPSG:4326 can be used for:


  • GPS

  • GNSS

  • GeoJSON

  • Web APIs

  • Global spatial datasets


EPSG:3857 – Web Mercator


This EPSG code is commonly used by online mapping websites. The Web Mercator projection is commonly used with this EPSG code to project geographic coordinates on maps.


EPSG:3857 is widely used for:


  • Web maps

  • Map tiles

  • Online mapping applications

  • Online basemaps


But this EPSG code causes distortion, especially at the poles. So, this EPSG code should not be used to measure distance or area precisely.


EPSG:326XX and EPSG:327XX – UTM


In the UTM coordinate system, the Earth is divided into 60 longitudinal zones.


Some examples of EPSG codes for the UTM system are:


  • EPSG:32633 – WGS 84 / UTM Zone 33N

  • EPSG:32643 – WGS 84 / UTM Zone 43N

  • EPSG:32733 – WGS 84 / UTM Zone 33S


The common uses of UTM coordinates are:


  • Drone surveying

  • Engineering

  • Environmental analysis

  • Topography

  • GIS projects in regions


EPSG Codes and Coordinate Transformations


There are cases where spatial datasets use different coordinate systems, for example, EPSG:4326 on one side and EPSG:3857 on the other side.


GIS software applications have the capability of transforming datasets from one coordinate system to another based on their EPSG definitions.


Such transformation takes place, for example, as below:


GPS coordinates → EPSG:4326 → Reproject → UTM coordinate system


Common GIS software and libraries supporting EPSG transformations are:



How to Find the EPSG Code of a Dataset


Normally, most GIS software provides the EPSG code of the dataset used.


In QGIS, EPSG code is normally provided by:


Layer properties → Source → Coordinate Reference System.


In the case of raster datasets, there is also the ability to check their spatial references through metadata using GDAL and other software.


Importance of knowing the EPSG code before spatial analysis.


Assigning an EPSG Code vs Reprojecting Data


One of the typical GIS errors that occurs in geospatial analysis is the confusion between assignment of a coordinate reference system (CRS) and reprojection of data.


Assignment of a CRS


Assignment of a CRS informs GIS software about how the existing coordinates must be understood.


It is applied when the dataset contains correct coordinates but lacks CRS information.


Reprojection of a Dataset


Reprojection transforms coordinates from one CRS to another.


Example:


EPSG:4326 → EPSG:3857


In the process of reprojection, the values of coordinates change.


Incorrect application of these operations will lead to major spatial errors and can cause data misplacement.


EPSG Codes in Python GIS


EPSG codes are widely used in Python-based geospatial programming packages.


For example, using GeoPandas:

import geopandas as gpd

gdf = gpd.read_file("data.shp")

print(gdf.crs)

gdf = gdf.to_crs(epsg=3857)

The code above reprojects the GeoDataFrame into EPSG:3857.

EPSG codes are also used in:


How to Choose the Right EPSG Code


Proper EPSG codes depend on the following conditions:


Geography


Local and regional analysis can employ coordinate systems that are tailored to the geographic region under study.


Purpose of Analysis


Appropriate projection is necessary when measuring:


  • Distance

  • Area

  • Altitude-related data

  • Real estate

  • Infrastructure


Data Needs


Many datasets already have coordinate reference systems. Reprojection may be necessary when merging datasets with various coordinate references.


Display of Maps


Web applications often employ EPSG:3857 for displaying basemaps, while data can be in EPSG:4326 or other CRSs.


EPSG codes play a crucial role in modern GIS since they offer standardized names for the coordinate reference systems and geodetic datums. Be it GPS coordinates, web maps, drone photos, satellite information, LiDAR data, or database systems – with the EPSG codes, one can make sure that the geographic data is properly located and transformed.


The best way to begin your GIS education and learn about EPSG codes is to start with the distinction between EPSG:4326, EPSG:3857, and local projected coordinate systems like UTM. With increasing complexity of GIS projects, choosing the appropriate coordinate reference system becomes vital.


Correct usage of EPSG codes allows one to achieve better data compatibility and avoid various projection problems.


For more information or any questions regarding the EPSG Codes, please don't hesitate to contact us at


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