The clacking shutters of amateur photographers and cellphone-wielding livestreamers has become a familiar sight to those attending Occupy Wall Street's public demonstrations against political corruption and corporate greed. Ever since reports of police brutality spiked in response to the intense surge of nonviolent protests last year, demonstrators have been keen on this loose-knit system of citizen surveillance to keep tabs on the abuses being visited upon them by law enforcement officers. But at yesterday's May Day demonstrations in New York City, one group of tech-minded organizers were doing things a little differently: rather than darting through crowds clumsily pointing phones and laptops, they took to the skies with balloon-mounted cameras that construct live maps of the demonstrations below.

Slowly moving its airborne rig over the massive crowds gathered in Union Square with giant spools of string, the Grassroots Mapping project hopes to turn the usual authoritarian paradigm of digital cartography on its head. From the park's northwest corner, volunteers assembled a plastic housing for two cameras, one which was timed to capture an image every three seconds for mapping, and another (a cameraphone) which provided video for the group's livestream. The images are then run through custom software which stitches them together and uploaded to an online archive hosted by the Public Laboratory for Open Technology and Science (PLOTS).

The group, which began as an effort to chart the Deepwater Horizon oil spills that devastated the Gulf of Mexico in 2010, is collaborating with DIYers around the world to build an open library of maps created in this way. Blaine, a volunteer present at the Union Square rally, pointed out that Google has also begun using this technique to chart selected areas, and is currently integrating some of the crowdsourced mapping data with Google Maps. The groups also provide instructions on how to assemble your own mapping rig, which requires nothing more than a cheap point-and-shoot and as little as $100 in materials.

Check the gallery below for some shots of Occupy's eye-in-the-sky in action at yesterday's May Day rallies.