World files and QGIS
A world file is a tiny text file that tells GIS software where an image sits on the ground.
What is in it
Six numbers, one per line: the pixel size in x, two rotation terms, the pixel size in y (usually negative), and the coordinates of the centre of the top-left pixel. Together they map every pixel to a ground position.
Which extension
| Image | World file |
|---|---|
.jpg | .jgw |
.png | .pgw |
.tif | .tfw |
| Anything else | .wld |
The world file must have the same name as the image and sit in the same folder.
Coordinate system
The numbers are in the coordinate system of the reference that was used, such as a UTM zone for Sentinel-2 or Web Mercator for a map rendering. A world file does not record the system, so tell your GIS which one it is when you open the image.
Open it in QGIS
- Put the image and its world file in one folder.
- Add the image as a raster layer.
- If asked, set the layer's coordinate system to the one the tool reported.
- Add a basemap beneath it and check that features line up.
The points file
The tool can also write a .points file in QGIS Georeferencer's format. Load it in the Georeferencer to see the matched points, delete bad ones, and refit.