Georeference
Overview
Georeference is the process of assigning geographic coordinates to spatial data, allowing for accurate placement and analysis within a GIS. Georeferencing is commonly applied to scanned maps, aerial photographs, and satellite imagery to align them with real-world coordinates.
Key Concepts
Control points are known locations used to establish transformation. Ground control point (GCP) has both image coordinates and real-world coordinates. Transformation is the mathematical function mapping image to ground coordinates. Residual error measures the difference between estimated and actual GCP locations. RMS error is the root mean square of all residual errors.
Transformation Types
| Type | Minimum GCPs | Flexibility |
|---|---|---|
| Affine | 3 | Rotation, scale, skew |
| Polynomial (1st order) | 3 | Same as affine |
| Polynomial (2nd order) | 6 | More local adjustment |
| Spline | 10+ | Rubber-sheeting |
| Projective | 4 | Perspective correction |
Workflow
- Identify control points in source image
- Match control points to known coordinates
- Calculate transformation
- Apply transformation to image
- Evaluate accuracy (RMS error)
Appendix
Created: 2025-12-13 | Modified: 2025-12-13