Coordinate Reference Systems for Beginners: WGS 84, UTM and Projections

Every location on a map is described using a coordinate reference system (CRS). If two datasets use different systems and the software does not know how to convert between them, layers appear in the wrong place, sometimes by a few metres, sometimes on the wrong continent. Understanding a few basics prevents most of these problems.

Geographic vs projected coordinates

Geographic coordinate systems

These describe positions on a three-dimensional model of the Earth using latitude and longitude, usually in degrees. They are great for storing global data but awkward for measuring distances and areas, because a degree of longitude covers different distances at different latitudes.

Projected coordinate systems

A map projection flattens the curved Earth onto a plane so coordinates can be expressed in linear units such as metres. Every projection distorts something: area, shape, distance or direction. Projections are chosen to keep distortion small for a particular region and purpose.

Datums: the Earth model underneath

A datum defines the shape and position of the Earth model that coordinates refer to. The same latitude and longitude in two different datums can point to locations metres apart, or more. Modern global datums are tied to the Earth's centre of mass; many countries also maintain national datums aligned to their own territory.

Common systems you will meet

• WGS 84 (EPSG:4326): the latitude-longitude system used by GPS and much global data.
• Web Mercator (EPSG:3857): the projection used by most online maps. It is excellent for web map tiles but strongly distorts area towards the poles, so do not use it to measure areas.
• UTM (Universal Transverse Mercator): divides the world into 60 zones, each 6 degrees of longitude wide, with coordinates in metres. It keeps distortion low within each zone and is widely used for survey and engineering projects.
• National grids: many countries have their own projected systems optimised for their territory.

EPSG codes

Coordinate reference systems are identified by codes in the EPSG registry. Saying "EPSG:32643" is unambiguous in a way that "UTM" is not. The site epsg.io lets you search codes, see their area of use and copy their definitions.

Vertical reference systems

Heights also need a reference. GPS receivers measure height above an ellipsoid; most maps and engineering drawings use height above a geoid-based datum that approximates mean sea level. The difference can be tens of metres depending on location. Always state which vertical datum your elevation data uses; see DEM vs DSM vs DTM.

Avoiding misaligned layers

1. Check each dataset's CRS in its metadata or properties before using it.
2. Never just "assign" a CRS to make layers line up. Assigning changes the label; reprojecting converts the coordinates. Assigning the wrong CRS silently corrupts data.
3. Choose a project CRS suited to your area and task, such as the local UTM zone for measurements.
4. Reproject datasets into the project CRS when you need to analyse them together.
5. Record the CRS of every deliverable you produce.

Practise in QGIS

Free software such as QGIS displays each layer's CRS and can reproject data on the fly for display. Our guide to getting started with QGIS walks through a first project.