Georeferencing instruction page

Create an account and Sign in

Start by clicking ‘Fix a location of a map’ , which will open the VU Geoplaza Georeferencer site. At the top right of the page you’ll see ‘Sign-in’. It is advisable to create a Georeferencer account. An account with Georeferencer is free and the ‘Sign in with email’ option only requires entering your e-mail address, adding a username and password and then confirming it through your email. Alternatively, you may login using an existing account with Facebook, Google or Twitter.

Navigating in Georeferencer

The Georeferencer web application has four tabs in its header. From left to right (see also instruction video 1 below):

  • This map With this tab you find background information on the map. Here, a link to the map in our image viewer is also provided. Once the map is georeferenced, files to be used in GIS-software (Geo-TIFF, WMS, etc.) become available.
  • Georeference You are automatically guided to the georeference tab. A split screen displays the current topography on the left and the map to georeference on the right. Below we’ll discuss the process of georeferencing in detail.
  • View In this tab the map, once georeferenced, is shown in overlay modus. Changing the transparency level (the slider in the top-right-hand corner) allows for a comparison between map and current topography. Additional viewing options (grid, slider and spyglass) are provided by clicking the ‘Overlay & compare’ button (in the bottom right-hand corner), which opens a new window. Here it is also possible to compare your map with other georeferenced maps (from both the VU and other institutions).
  • Transcribe This less-used feature allows you to add toponyms to map locations.

Instruction video 1, ‘navigeren in Georeferencer‘ (Dutch spoken, subtitles in English to be added soon):

Georeference a map: a basic instruction

Georeferencing is all about placing control points. These control points are attached to corresponding points on the current topography and the old map. Georeferencing a map basically involves the following steps, which are also illustrated in instruction video 2 (below):

  1. Start: click on ‘Fix the location of a map’ You are automatically assigned a map to georeference. If the map is not to your liking, click the button ‘skip to next map’ (bottom right-hand corner) will assign you a new one.
  2. Preparation: match left and right screen Make sure that approximately the same area is displayed on both the current topography (left) and the old map (right). Sometimes it can be helpful to search for a certain location (magnifier button at the left-hand side) or change the base map (globe button at the left-hand side). Note that the old map (right) can be rotated, which may prove helpfull in case the orientation is not north.
  3. Place the first three control points Control points are placed by left clicking on the old map (left screen) and the current map (right screen). When a control point is added succesfully, a small dot with a number appears. Clicking on an existing control point shows a magnifier: the cross at the bottom allows you to abandon the point and by dragging the ‘move symbol’ (on the right of the magnifier), the point can be relocated.
    • Control points can be placed anywhere, as long as identical places on both the new and old map can be identified. It can be a city or town or a characteristic river bend (on a large scale map, e.g. a region, a country or a continent) or a church or a crossing (on a small scale e.a. a city map).
    • Spread the points as evenly as you can over the map.
    • It can be a challenge! You might find yourself relocating and zooming in and out time after time. This is especially the case when working with very old maps and/or territory unknown to you. Your patience might be tested, but at the same time you will experience the satisfaction of a close examination of a beautiful map!
  4. Improve by adding more control points The moment you have placed three control points the map is turned, scaled and warped accordingly, showing you the first result of your georeferencing. Now it’s time to improve the result.
    • Adding control points is necessary to improve the result. A minimum is five control points is required, but it is preferable to try to reach the stage in which new control points barely change the projection.
    • The coordinates view shows the control points’ coordinates and their error rates (in red). These errors might be helpful in identifying outplaced points, which can then be relocated or removed. However, keep in mind that many old maps are inaccurate and errors are inevitable!
    • Notice that once the first three points have been placed (step 3) it becomes easier to find new control points. The overlay mode, availabe after the first three points, may prove especially helpful.
  5. Clip and finish the map Finish your map by removing any unnecessary borders and margins. Using ‘clip the map’ displays the map in a blue frame with dots at the corners. Notice that the blue dots act in the same way as the controlpoints: a new point is added by left clicking and existing points can be removed or relocated by using the options once clicked. In case the map has an inset (of a smaller map), clip off the inset as well.

It might seem hard at first, but you’ll soon get the hang of it. Don’t worry about saving: this is done automatically. When you finished your map simply push ‘Next’ for your next map! Do not hesitate to contact us if you have any questions or remarks.

Instruction video 2 ‘basisinstructie georefereren’ (Dutch spoken, English subtitles to be added soon):