What is georeferencing
A scanned map is initially just an image without geographic information.
Georeferencing makes it possible to associate real coordinates with map pixels, transforming the image into cartography usable in GPS and GIS environments.
Control points and calibration
Calibration is performed by identifying known points on the map, such as grid intersections, geodetic vertices or coordinates shown on the margins.
These control points allow the software to calculate the mathematical relationship between the image and real geographic coordinates.
Coordinate systems and datums
To obtain correct georeferencing, it is essential to know the coordinate system and datum used by the original map.
Errors in datum or projection selection can cause significant shifts of the cartography relative to the real position.
Cartographic transformations
Cartographic transformation is the mathematical process that converts image coordinates into geographic or projected coordinates.
Depending on the quality of the map and the number of control points, linear, affine or more complex polynomial transformations may be used.
Georeferencing accuracy
Final accuracy depends on scan quality, the correctness of calibration points and the original deformation of the paper map.
Very old or distorted maps may require a larger number of control points to improve geographic accuracy.
Georeferencing maps with OkMap
OkMap allows raster maps to be calibrated using control points and different coordinate systems.
Once georeferenced, the map can be used to display GPX tracks, waypoints and outdoor routes with geographic accuracy.