Georeferencing

The georeferencing toolbar is used to align raster, CAD, and image data with real-world map coordinates.

Preparation

Tool	Description	Geometric meaning	Recommended use
Move	Translates the entire raster on the map canvas.	Translation matrix: x' = x + tx, y' = y + ty	Move imagery roughly into the correct area before adding control points. This does not change scale or orientation.
Scale	Changes the raster size.	Scaling matrix: x' = sx x, y' = sy y	Correct scale differences from scanned data. Uniform scaling is recommended.
Rotate	Rotates the raster around its center by any angle.	Rotation matrix: x' = x cosθ - y sinθ, y' = x sinθ + y cosθ	Roughly rotate the raster close to the target orientation before fine-tuning with control points.
Flip - Horizontal flip	Mirrors the raster from left to right.	Transformation: x' = -x, y' = y	Correct scanned imagery that is reversed horizontally. Use with care because an incorrect flip can prevent control points from matching.
Flip - Vertical flip	Mirrors the raster from top to bottom.	Transformation: x' = x, y' = -y	Correct scanned imagery that is reversed vertically. Recheck orientation and symbols after flipping.
Fixed rotation - Rotate left	Rotates the raster 90° counterclockwise.	90° rotation matrix: x' = -y, y' = x	Quickly correct a scanned image with portrait/landscape orientation reversed.
Fixed rotation - Rotate right	Rotates the raster 90° clockwise.	270° rotation matrix: x' = y, y' = -x	Similar to Rotate left. Use it to quickly correct orientation.

Tool	Description
Add control point	Selects a control point from the layer and adds it to the map.
Select	Selects and highlights control points within a specified area.
Zoom to	Centers the view on the selected control point and zooms in.
Delete	Deletes the selected control point link.
Delete all	Deletes all added control point links.

Transformation types

Affine transformation: A linear transformation that maps source image coordinates to the target coordinate system by using translation, rotation, scaling, and skewing. It usually requires at least 3 control points.
Translation transformation: The simplest geometric transformation. It only shifts data in the X and Y directions and does not apply scaling, rotation, or skewing. It usually requires only 1 control point.
Similarity transformation: A transformation that preserves geometric similarity. It allows translation, rotation, and uniform scaling, but does not include shear or non-uniform deformation. It requires at least 2 control points.

Workflow example

The following workflow uses a scanned or image-based raster as an example and avoids relying on screenshots.

For JPG, PNG, or similar image formats, set the project coordinate system to EPSG:3857 if that is the coordinate system used by your reference map and it helps with positioning.
Add the image or raster layer to the project so it appears in the Layers list.
Select the image or raster layer, then open the Georeferencing tab on the toolbar.
Use Add control point. First add the source point on the image, then locate the matching target point on the reference map. You can enter longitude and latitude values or click directly on the map. Add 3 to 5 control points in sequence.
Review the residuals in the control point list. If a residual is too large, delete that control point and select it again until all residuals are within an acceptable range.
Review the final georeferencing result and confirm that the raster aligns correctly with the reference data.