Georeferencing in QGIS is an essential technique that allows you to align an image with real-world geographic coordinates using control points.
Whether you’re working with a scanned map, a PDF plan, or an image without spatial reference, QGIS makes it easy to georeference your data accurately.
In this step-by-step guide, you’ll learn how to georeference an image in QGIS and avoid the most common mistakes.
What is Georeferencing in QGIS?
Georeferencing is the process of assigning real-world coordinates to an image so it can be correctly positioned in a GIS environment.
Typical use cases include:
- scanned historical maps
- architectural or urban planning PDFs
- satellite or aerial images without coordinates
Without georeferencing, an image cannot be properly overlaid with other spatial datasets in QGIS.
Example:
One image (A) is already georeferenced, while another (B) is not. The goal is to align the second image so it perfectly matches the reference layer.
Step 1 – Open the Georeferencer in QGIS
In QGIS, open the Georeferencer tool:
Layer → Georeferencer
This tool is specifically designed to georeference raster images in QGIS.
Step 2 – Load the Image to Georeference
In the Georeferencer window:
File → Open Raster
Select the image you want to georeference (JPEG, PNG, TIFF, etc.).
Step 3 – Add Control Points
Adding control points is the most important step in georeferencing.
Click:
Add Point
Then:
- click on a clearly identifiable location in the image
- assign real-world coordinates to that point
You can do this in two ways:
✔ Enter coordinates manually
✔ Click directly on the map (recommended)
For better accuracy, use a reference layer such as OpenStreetMap or any already georeferenced dataset.
Step 4 – Choose Georeferencing Settings
Open:
Transformation Settings
Recommended settings for most cases:
- Transformation type: Polynomial 1 (Affine)
- Resampling method: Nearest neighbor
- Coordinate Reference System (CRS): EPSG:4326 or EPSG:3857
These settings provide a good balance between accuracy and simplicity.
Step 5 – Start Georeferencing in QGIS
Click:
Start Georeferencing
QGIS will generate a new georeferenced raster file aligned with your chosen coordinate system.
Step 6 – Verify the Georeferenced Image
Load the output raster into QGIS and check that:
- the image aligns correctly with your reference layers
- geographic features match accurately
This validation step is crucial to ensure your georeferencing is correct.
Best Practices for Georeferencing in QGIS
To improve accuracy:
✔ Use at least 4 control points (more is better)
✔ Distribute points evenly across the image
✔ Select precise features (road intersections, building corners, landmarks)
✔ Avoid blurry or ambiguous areas
Common Georeferencing Errors (and How to Fix Them)
Image is shifted
Possible causes:
- inaccurate control points
- incorrect coordinate system
Image is distorted
Possible causes:
- unsuitable transformation method
- poorly distributed control points
Incorrect overall positioning
Check:
- CRS (Coordinate Reference System)
- precision of control points
FAQ – Georeferencing in QGIS
How many control points do I need?
Minimum 3, but 4 to 10 points are recommended for better accuracy.
Which transformation method should I use?
- Polynomial 1 (Affine): best for most use cases
- Polynomial 2 or 3: for more complex distortions
Can I georeference a PDF in QGIS?
Yes, by opening the PDF as a raster layer in QGIS.
Conclusion
Georeferencing an image in QGIS is a fundamental skill for any GIS workflow.
By carefully placing control points and selecting appropriate transformation settings, you can accurately align any image with real-world geographic data.
Mastering georeferencing in QGIS will significantly improve the quality and reliability of your spatial analysis.
