What is Georeferencing?

Georeferencing of historical map images involves assigning spatial information so that they align with real world geography. The essential process typically consists of adding control points to an historic map that have a real-world location; once there are sufficient control points, the historic map can be transformed so that it correctly aligns with geographic reality (Fleet et al., 2012).

What is the Use of Georeferencing?

  • Georeferencing data render maps more discoverable, supporting map-based searching and presentation of the collection applicable in any geographic interface, the way researchers prefer to find and look at maps. Under the This map tab you can load the corresponding geodata.
  • Gazetteer searching links spatial and textual attributes.
  • Georeferencing allows visualization in a more useful, flexible way. Historical maps can be viewed alongside or overlaid on other mapping on line, enabling comparison of maps over time and research into landscape change and the development of places (Kowal and Přida, 2012:287).

Thank you very much for your help!

After more than 30 map enthusiasts located 912 maps with about 24,000 control points in the first project phase in 2016, more than 2,000 maps from GeoPortOst II have got now georeferenced as well.

In a short period of time, numerous volunteers have set many control points and have assigned even difficult maps with their geography.

Many thanks to all participants for their great commitment and the impressive result – in particular to the three top contributors Tucker Jones, Georg Zimmermann and Dominik Stoltz!

In addition to georeferencing, GeoPortOst offers numerous attractive features for working with the maps:

Under This map you will find the following offers:

  • Georeferencing (Georeference)
  • Geodata download
  • Transcription / annotation tool
  • Edit features
  • Coordinates for accuracy calculation and correction of your georeferencing
  • 3D view of the georeferenced maps

Under Overlay & compare you will find the following offers:

  • Drag different maps into the Overlay & compare window and compare them directly or connect them.
  • Use the Swipe mode to compare the map with today’s terrain.
  • With the Spy glass you can focus on individual map sections.


  1. By clicking the button in the Participate! section (Georeference a Random Map), you will be assigned a map at random. In the application, the historic map will appear on the left; a modern map will appear on the right.
  2. If you are interested in a region or a topic, you can search specifically in the Geoportal. After selecting a map, click the button Georeference this Map under the map image in the main window. This takes you to Georeferencer for your map.
  3. Sign in by using email/password (necessary to register upon first use) or social media account (Facebook/Google/Twitter).
  4. Zoom into the old map enough to read it, and then find the corresponding location on the modern map. Use the Find a place-symbol (magnifying glass) at the left if necessary. Click on the This map tab for more information. In some cases, it may be useful to consult historical gazetteers, which you can access in the Map Guide.
  5. Click on one map to assign a point, then assign a point at the same location on the other map. Look out! Check that the correct points are linked together. Assign as many control points as you can, distributed as much as possible over the entire map.
  6. Define the content area of the map by clipping the borders (see video instructions).
  7. Click the blue Save button down right when you are finished. Now, you cam see the overlay of the georeferenced map.
  8. Use the transcription tool (Transcribe), for example, to indicate current place names or to clarify legends that are difficult to read.
  9. Check the location of the map as overlay, adjust your control points if necessary by using the Coordinates control, add more control points or go on to the next map.

Thank you! The map is now georeferenced and you can use other functions for editing, analysis or comparison.

  • Edit map: Under This map you will find the button Open in GeoEditor on the right. Here you can draw on the map, mark borders, define polygons and save your editing as png.
  • Overlay & compare: Under This map you will find the button "Overlay & Compare" on the right. Here you can select different views (Swipe, Spy glass) on the map. Choosing Grid you can compare different maps. You can use the menu in the right frame to create a personal map selection and upload your own maps (My private maps), search the entire GeoPortOst collection (GeoPortOst) or access other maps from OldMapsOnline (All maps).

Enjoy and thank you!

Download the Detailed Instruction

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Maps Already Georeferenced

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GeoPortOst II

GeoPortOst collection