Projections and coordinate reference systems (CRS) are essential concepts when working with geographic data in QGIS. Incorrect handling of projections can cause significant misalignment between layers.
When working with geographic data in QGIS, projections and coordinate reference systems (CRS) are unavoidable topics. Many users encounter problems when layers do not appear in the correct location or when multiple datasets seem not to match.
In most cases, these problems are related to incorrect projection management.
In this article, we will explain what a projection is, how QGIS handles coordinate reference systems, and how to solve the most common issues.
1. What Is a Coordinate Reference System?
The Earth is a curved surface, but maps are generally represented on a flat plane. To transform a spherical surface into a flat map, we use a map projection.
Earth (curved surface)
🌍
│
│ geodetic system
│ (for example WGS84)
▼
Map projection
(mathematical transformation)
│
▼
Flat map
used in QGIS
A projection transforms the surface of the Earth into a flat map.
A Coordinate Reference System (CRS) defines:
- how the Earth is represented
- the coordinate system used
- the map projection.
Each CRS is usually identified by an EPSG code.
Examples:
| CRS | Description |
|---|---|
| EPSG:4326 | WGS84 – latitude / longitude |
| EPSG:3857 | Web Mercator projection used in web maps |
| EPSG:2154 | Lambert 93 (France) |
2. Two Important Projections in QGIS
Two coordinate systems are very common.
WGS84 (EPSG:4326)
This system is used by:
- GPS devices
- many geographic databases
- latitude / longitude coordinates.
Example:
latitude : -20.3484
longitude : 57.5522
Web Mercator (EPSG:3857)
This projection is used by most web maps:
- Google Maps
- OpenStreetMap
- Leaflet
- OpenLayers.
Coordinates are expressed in meters.
Example: EPSG:4326 vs EPSG:3857
Let’s take a point with the following coordinates:
Latitude : -20.3484
Longitude : 57.5522
These coordinates are expressed in:
EPSG:4326 (WGS84)
This system is used by:
- GPS devices
- many geographic databases
- latitude / longitude coordinates.
If we project this point into:
EPSG:3857 (Web Mercator)
the coordinates might become:
X : 6404700
Y : -2309000
The coordinates are now expressed in meters.
This system is used by:
- OpenStreetMap
- Google Maps
- web mapping libraries such as Leaflet and OpenLayers.
3. The Project CRS in QGIS
In QGIS, every project has a project CRS.
This CRS defines how layers are displayed on the map.
To view the project CRS:
- check the status bar at the bottom of QGIS
- or go to:
Project → Properties → CRS
QGIS can reproject layers on the fly so they display correctly.
4. Reprojecting a Layer
Sometimes it is necessary to reproject a layer into another CRS.
In QGIS:
- Right-click on the layer
- Select Export → Save Features As…
- Choose a new CRS.
QGIS will create a new reprojected layer.
5. FAQ – Common Projection Problems
Layers do not align
This usually means that:
- the layers use different CRS
- or one CRS is incorrectly defined.
Data appears in the wrong location
For example:
- points appear off the coast of Africa
- data is shifted by thousands of kilometers.
In this case:
- check the layer CRS
- check the project CRS.
The CRS is not defined
Some datasets do not contain information about their projection.
In this case, use:
Set Layer CRS
Important: this does not reproject the data, it only tells QGIS which CRS to use.
6. Defining a CRS vs Reprojecting a Layer
This distinction is important.
Defining a CRS
- tells QGIS which coordinate system is used
- does not modify the coordinates.
Reprojecting a layer
- actually transforms the coordinates into another CRS.
Conclusion
Projections are a fundamental element in geographic information systems. In QGIS, understanding the difference between defining a CRS, reprojecting a layer, and changing the project CRS helps avoid most display issues.
Once these concepts are understood, it becomes much easier to work with geographic data coming from different sources.
