Tuesday, August 24, 2010

Browsing a digital library using a map

Every so often I revisit the idea of browsing a collection of documents (or specimens, or phylogenies) geographically. It's one thing to display a map of localities for single document (as I did most recently for Zootaxa), it's quite another to browse a large collection.

Today I finally bit the bullet and put something together, which you can see at http://biostor.org/maps/. The website comprises a Google Map showing localities extracted from papers in BioStor, and a list of the papers that have one or more points visible on the map.


In building this I hit a few obstacles. The first is the number of localities involved. I've extracted several thousand point localities from articles in BioStor. Displaying all these on a Google Map is going to be tedious. Fortunately, there's a wonderful library called MarkerCluster, part of the google-maps-utility-library-v3 that handles this problem. MarkerCluster cluster together markers based on zoom level. If you zoom out the markers cluster together, as you zoom in these clusters will start to resolve into their component points. Very, very cool.

The second challenge was to have the list of references update automatically as we move around or zoom in and out on the map. To do this I need to know the bounding box currently being displayed in the map, I can then query the MySQL database underlying BioStor for the localities within the bounding box, using MySQL's spatial extensions. The query is easy enough to implement using ajax, but the trick was knowing when to call it. Initially, listening for the bounds_changed event seemed a good idea. However, this event is fired as the map is being moved (i.e., if the user is panning or dragging the map a whole series of bounds_changed events are fired), whereas what I want is something that signals that the user has stopped moving the map, at which point I can query the database for articles that correspond to the region that map is currently displaying. Turns out that the event I need to listen for is idle (see Issue 1371: map.bounds_changed event fires repeatedly when the map is moving), so I have a function that captures that event and loads the corresponding set of articles.

Another "gotcha" occurs when the region being viewed crosses longitude 180° (or -180°) (see diagram below from http://georss.org/Encodings).


In this case the polygon used to query MySQL would be incorrectly interpreted, so I create two polygons, each with 180° or -180° as one of the boundaries, and merge the articles with points in either of those two polygons.

I've made a short video showing the map in action. Although I've implemented this for BioStor, the code is actually pretty generic, and could easily be adapted to other cases where we want to navigate through a set of objects geographically.