Georeferencer Help - a step-by-step guide to georeferencing

This application offers an easy and enjoyable way to georeference historical maps, so that they can be displayed as an overlay on top of the real world inside your web browser, or in Google Earth, or the Google Earth browser plug-in. You will need to enable Javascript in your web browser, and we recommend having the Google Earth browser plug-in installed. We also recommend that you have Google Earth downloaded for viewing the map in 3D, but this is not essential.

  1. Select the map you would like to georeference by clicking on it to enlarge it. (The selected map can also be shared in Facebook and Twitter using the Button to link to map in Facebook and Button to link to map in Twitter buttons).
  2. Double-click on the selected map, or Button to open map in georeferencer to open the item in Georeferencer.
  3. If the map is not yet georeferenced, you will be prompted to register in Georeferencer with your e-mail address and a password - or use an existing Gmail, Facebook or Twitter account. Georeferencer registration can be done at: http://nls.georeferencer.com/user/register. For initial registration you will receive a confirmation email with a link you have to visit before you can login for the first time. Registration is quick and simple and allows us to manage and control Georeferencer better.
  4. Once logged in, in the Georeference window, the historic map to be georeferenced is shown on the left-hand side.
     
    The mapping on the right-hand side is for placing Control Points (see Section 6 below).
     
    Georeferencer - two screen view
    Historic map to georeference on the left                    Modern mapping on the right

    The default mapping on the right-hand side is modern OpenStreetMap . It is possible to change this to the NLS historical mapping from the 1930s, modern Ordnance Survey Opendata, Google or Bing mapping and satellite imagery by clicking on the Georeferencer - layer select icon on the far right side and selecting your preferred map layer.
  5. The right-hand map can also be navigated to a particular place by using a place name search. Just type a place name in the box, click the 'Find a Place' button, and if the name is in the gazetteer, the map should be repositioned with this place at the centre.
  6. Georeferencing involves placing at least three Control Points (more for a better fit) in both windows. Control Points are simply the same known location that you place on both the historical and the modern map. For best results, spread your points as evenly as possible over a wide area.
     
    Zoom in (by double-clicking with your mouse or using the vertical slider) on either window and click once on the map to place a Control Point on your chosen location. Zoom in the other window to place the Control Point on exactly the same location.
     
    If you make a mistake, Control Points can be moved by clicking on them to select them and then dragging them to their correct location. Control Points can be deleted by clicking on the Control Point with your mouse pointer to select it and then pressing the Delete button on the keyboard. When you have placed your Control Points, the georeferenced map can be saved by selecting the SAVE button. You will also be asked whether you would like to clip) the extents of the map around the map border or neat line, so that only the geographical or real-world part of the map is visible.
  7. The historical georeferenced map will then appear as an overlay in the Visualise window. The Visualise window allows a standard 2D overlay and side-by-side presentation of the historic map, as well as a 3D Google Earth browser plug-in view. (In the Google Earth browser plug-in, there are various options in the header at the top of the page). The transparency slider at the top left allows the transparency of the historical map to be adjusted, so you can fade it against a background of a modern map or satellite image. You can also choose to display the georeferenced map in Google Earth..
  8. There are three types of transformation based on the control points that can also be visualised:

    • Affine
      The affine transformation results in a basic shift, scaling, and rotation of the original map. It preserves the overall look of the map, but specific control points usually do not stay in the same location. Straight lines on the original map usually stay as straight lines, whilst squares and rectangles on the original map are usually changed into parallelograms of different sizes and angles.
    • Polynomial
      The polynomial transformation allows the map to be warped or rubber-sheeted, taking greater account of control points but allowing them to move to new coordinate locations depending upon the overall positions and deviations of control points from geographic reality across the map. It also allows scaling and rotation.
    • TPS or Thin Plate Spline
      The Thin Plate Spline is a more elastic transformation that maintains the assigned map coordinates of all control points in the same positions, but places between these control points may well all move. It maximises the importance of the control points rather than the overall map.

  9. In the Accuracy window, you can view the geometrical accuracy and distortion of the map calculated from the control points using a distortion grid overlay. This visualization can help to identify wrongly assigned control points and can also help to understand the compilation and geometric properties of the early map better. It uses functionality developed by MapAnalyst, where further information can also be found.
  10. In the This map window there is a link to the original ungeoreferenced image on the NLS website under the 'Original web presentation' link. There is also some more information about the map, and downloadable metadata as an ESRI World file or OGC KML relating to the georeferenced map.