Monday, March 11, 2013

Capital Bikeshare: Space & Time

Capital Bikeshare: Space & Time

This week I've been studying the spatial and temporal components of the Capital Bikeshare data-set. First off what do I mean by the spatial component? Well the bike stations have fixed locations (actually they don't, I'm lying). The stations were actually designed to be movable, they are not fixed to the ground at all (apart from the obvious effect of gravity). They use solar panels to power their rental system software and to log data to a remote server which provides the live availability feed.

According to Capital Bikeshare "Typically a station is only moved due to road construction or some other temporary issue and then it is moved back". Unfortunately this movement data is not contained in the quarterly data dumps so unless you've been logging it for the past few years (which I haven't) then you won't have access to it. If anyone does have access to this data and they would like to share it with me then please leave a comment below.

Anyways back to our discussion about space and time.

This is not a simulator

The first thing I did in order to study the spatial and temporal components was to build a tool which simulates the bikes moving on the network. In reality this is not really a simulator at all because its using the real historic rental data.

The blue dots are the bike stations and the moving red dots are bikes travelling from station to station. The data-set does not contain GPS traces, it contains tuples of (origin, destination, duration) so the red dots are simply moving along an interpolated path from origin to destination where the entire journey takes the correct duration of time.

This is a really interesting exploratory tool, you can move the simulator to any date and time you want, you can also choose from a variety of speeds [1x, 10x, 100x and 1000x]. At 100x you can clearly see different spatial and temporal patterns. You can watch people moving towards the city center during the morning compute and home again during the evening commute. You can watch certain areas of the city light up with activity at different times. You can also use the tool for general exploration, you can watch for example, the very first bike rental which was on Sept 15th 2010 or you can see how federal holidays disturb typical travel patterns for example on Thanks Giving.

If your not satisfied by simply watching the video then you can explore the full data-set yourself by following this link.

You might have noticed that different stations appear as the simulator approaches December 2012. This is because Capital Bikeshare is an ever growing network. If you go all the way back to the beginning of the data-set you will see all of the stations fade away accept for the very first station at 27th & Crystal Dr.


Okay so the simulator is cool, you could use it to watch the entire data-set play out from the very first rental on Sept 15th 2010 until the very last rental (released as of this date) December 31st 2012. Its unlikely that anyone will every do this tho (I certainly won't). I think the simulator is better suited for exploring rental activity on specific dates and or times.

About the visualization

The visualization combines quite a few different elements. The map is being provided by Leaflet an awesome Javascript  library for mobile-friendly interactive maps. You can change the map provider by hovering over the icon in the top right corner of the map. My favorite alternative is the visually appealing Stamen-Watercolor.

The data has been stored in a PostgreSQL database with an index on the time column. This allows the simulator to quickly change to any date/time. Why PostgreSQL? I have found PostGIS and pgrouting very useful for some of my other projects.

The transition animations are being powered by D3. I love D3, it makes this sort of thing so simple and elegant to code. The bike stations and bikes themselves are not actually Leaflet layers. D3 is being used to manipulate an SVG element that is sitting on top of an equally sized Leaflet canvas.

The datepicker is provided by Pickaday. I haven't used this in a project before but I quite like it.
Last but not least, the timepicker is provided by jQuery.

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