This narrative map explains how a map is georeferenced. The Van Berckenrode map of Amsterdam from the Allard Pierson collection is used as an example. If you want to try to georeference a map yourself, you can do so in Allmaps Editor .
In this slide, you can explore the original scan; IIIF lets you zoom into the image without downloading the whole map at once.
The pink outline marks the part of the image we want to place on a map. This mask will hide the paper outside that area. For now, the full image stays visible so you can see the boundary.
The pink dots mark recognizable places on the image. Each ground control point pairs a position on the old map with geographic coordinates, such as the location of a church or a street corner.
You can zoom into the map to explore which ground control points are used.
Allmaps uses the control points to position the image in geographic coordinates. This first fit uses a polynomial transformation. This means that the image is translated, scaled, rotated and skewed to fit on top of a modern map, all in the browser! The mask now hides the surrounding paper.
The blue dots show the geographic control points; the pink dots show where the transformed image places them. White lines connect each pair. Longer lines show larger differences between this fit and the chosen coordinates.
A thin-plate-spline transformation removes the vectors between the control points on the image and the underlying map, and distorts the image locally in between those points to create a better fit.
Compare this view with the previous slide: a closer fit at these points also changes the shapes between them.
The map is now fully opaque. Pan and zoom to explore Amsterdam, or go back a slide to compare the two transformations. A close match at the control points does not guarantee that every detail between them is accurate.
Modern tram and metro routes are drawn over the historic map. Compare the routes with the streets and waterways on the map.
This GeoJSON overlay is hidden by default in the project configuration and enabled only on this slide.
These points mark historic buildings recorded by the City of Amsterdam. Each
label reads the building’s name from the GeoJSON’s Naam property. Zoom in to
see more labels where buildings are close together.
The previous slide’s transport routes disappear here: each slide starts from the global layer defaults.