Georeferencing with Allmaps

bit.ly/allmapsgeoref

What is georeferencing?

Georeferencing is the process of adding spatial coordinates to a digital image of a map to enable it's use in geospatial tools.

Common locations are found between the map image and a GIS framework (a desktop GIS or a web GIS application).

Link pairs are created between those locations. The link pair assigns a real world coordinate from the GIS to specific pixel locations in the image.

When enough links have been added to the image it can be transformed from Image Space to Coordinate Space allowing the scanned map to be placed in the GIS in the correct location.

IIIF Images

International Image Interoperability Framework.

Shared protocol.
Scalable images.
Hosted online at various institutions.
IIIF Manifest endpoints.

Allmaps.org

Allmaps is a set of open-source tools built for georeferencing, warping, and exploring IIIF maps.

Allmaps Editor is a simple to use online geo referencer that uses the IIIF Manifest URL to allow quick and easy georeferencing of already hosted map scans.

The Allmaps Tile Server turns Georeference Annotations from the Allmaps Editor into XYZ map tile layers, which can be used to view warped maps in existing mapping websites or applications like QGIS.

editor.allmaps.org viewer.allmaps.org here.allmaps.org latest.allmaps.org

Georeferencing workflow

Demo map: Map of Crater Lake National Park (1933)

Copy the IIIF Manifest URL for a scanned map.
Open the Allmaps Editor and paste in your IIIF Manifest URL.
Some manifests can have more than one image, choose the image you want to georeference.
Use the Draw mask tool to mask out any areas of the map you don't want in the final version (e.g. anything outside the neatline).
Use the Georeference tool to add links between the map scan and the Editor background map.

Best practices:

Start by panning and zooming the background map to match the extent of the map scan.
Look for permanent features that are clearly identifiable in both the map scan and background map.
Focus on cultural features first, like road intersections and specific buildings or landmarks.
Avoid ephemeral features like coastlines and rivers.
Place a minimum of 4 links for a basic map transformation.
For more complex map transformations use 8 or more links.
Try to distribute links throughout the map area to balance the amount of warping in the final image.
Change the background map type under the settings menu (gear icon in lower right corner of the Editor).
Use the rotate (Shift + option click and drag) keyboard shortcut to align non N up facing maps with the Editor Background map.
Avoid oblique maps.

Allmaps Editor shortcuts

Shift + mouse drag to zoom to extent of drag.
Shift + option click and drag to rotate map in Editor.

Allmaps Viewer shortcuts

Space – Toggle transparency on/off
B – Toggle background removal
M – Display the mask
T – Change the transformation algorithm
G – Display a grid over the image
D – Cycle display of distortions: surface deformation, angle distortion, or none

Find a map to georeference

Use the IIF Manifest links below to select a map to georeference or pick any map from the Allmaps Editor Collections.

Shared IIF Manifest Table - Google Doc

Add your XYZ tiles to ArcGIS Online

Copy the XYZ map tiles url from the Allmaps Editor.
Go to ArcGIS.com and log-in using the Your ArcGIS organization's URL method.
Open a new Map Viewer.
Use the Add menu dropdown and select Add layer from URL.
Paste in your XYZ map tile url from Allmaps.
Click Next, add a layer Title and Attribution and click Add to map.

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