In 2015 Leiden University Library did a pilot project in the field of crowdsourcing. With a special Georeferencer-tool, map enthusiasts could add coordinates to a digital image of an old map.
They had to determine at least five geographical points on the old map, and link them to the corresponding location on a modern map. By georeferencing the maps in this way, the collection can be made searchable in new ways.
The project was a big success! In two weeks’ time all 393 maps were georeferenced by a crowd of enthusiasts. In total 41 individuals contributed to the project, but about 75% of the work was done by 3 top contributors. As a competition element, the ranking of the contributors was visible on the project website. The top contributor, who placed the most control points, could win a visit to the map collection and a print of one of the georeferenced maps of his choice. In this way we thanked Casper Brienen, the top contributor.
Manuscript sea charts
For this project we selected the Collection Van Keulen. This is a unique collection of 393 manuscript sea charts from the early eighteenth century by the Amsterdam publishing house Van Keulen. The georeferencing of these maps brought new insights to this collection. The georeferencing process itself forces you to study the maps in detail, so you discover details on the map that otherwise might be overlooked quickly. A lot of charts turned out to be less geometrically accurate than was previously assumed. This calls the contemporaneous use and purpose of these maps into question. With some large-scale, but not very detailed, maps of ports and bays it was very difficult to find the minimum of five control points. Such maps may almost be considered as ‘ungeoreferencable’. With more accurate maps, georeferencing gives the opportunity for comparison, where the transparent old map can be overlayed on a modern map. All georeferenced maps are now geographically findable by its geolocation.
Because of the success of this project, we are now preparing a follow-up. Over 7,500 maps of the KITLV (Royal Netherlands Institute of Southeast Asian and Caribbean Studies) collection are now prepared to be georeferenced by the crowd. The larger part of this collection consists of maps of the Dutch East Indies and dates from the nineteenth and twentieth centuries. In general, these maps will have a higher level of accuracy and are therefore easier to georeference. In this way the georeferencing of old maps contributes to a better catalogued and increased visible map collection of Leiden University Libraries.