Georeferencing — Recipe (QGIS)

Tool-bound implementation of the Georeferencing (Task) Schema. Also referenced as a prerequisite step from Backdrop digitalisation.

Tool: Raster → Georeferencer

1. Open the Georeferencer and load the un-referenced image (scanned map, un-referenced aerial photo).
2. Place control points on the image and, for each, either type in its known real-world coordinates directly, or (with a reference layer such as an existing orthophoto loaded in the main QGIS canvas alongside the Georeferencer window) click the matching location on that reference layer to pick the coordinate pair visually.
3. Set the target CRS.
4. Choose the transformation type:
   • Linear/Helmert/first-order (affine): scales, rotates, and translates only. Assumes a flat surface with no lens distortion — appropriate for a flat scan or paper map with no optical distortion to correct.
   • Polynomial 2/3 (higher-order): warps the image using polynomial equations to correct non-linear distortion — appropriate for aerial or satellite photographs, where lens/perspective distortion is present, especially toward the image edges. Higher polynomial order corrects more precisely but needs more control points and more computation.
5. Run the transformation and check the reported residual/RMS error before accepting — a control point with an unexpectedly high residual is usually a mis-clicked point, not a transformation-order problem, and should be re-picked rather than compensated for by raising the polynomial order.
6. Save the georeferenced output.

(Open item: minimum recommended control-point count per transformation order in QGIS’s specific implementation, and the acceptable residual/RMS threshold — to fill in once verified against QGIS’s own documentation or a worked example.)

Use cases (tool-agnostic context, kept here for now)

• Aerial photographs and satellite images — typically need higher-order rectification because of lens/perspective optical distortion.
• Scanned maps — typically only need first-order/affine, since there is no lens distortion to correct.
• Urban planning / GIS work generally — georeferencing aerial photographs is usually the prerequisite step before overlaying vector data such as roads, buildings, and zoning boundaries.

ArcGIS

(To fill in: the equivalent Georeferencing toolbar/pane workflow in ArcGIS Pro — same Task Schema, different interface.)