Monday, February 26, 2007

Helping Amazon's Mechanical Turk Search for Jim Gray's Yacht

Regrettably, Jim Gray is still missing, but I thought deserved more kudos than they got in the press for their extraordinary effort to help in the search for Gray's yacht. Google got a lot of press coverage for talking up the idea of using satellite image sources, but Amazon did it. Why is that? One reason is that Amazon has a lot of experience operating and maintaining very large distributed databases (VLDBs). Another reason is that it's not just Google that has been developing interesting Internet tools. Amazon (somewhat quitely, by comparison) has also developed their own Internet tools, like the Mechanical Turk. These two strengths combined at Amazon and enabled them to load a huge number of satellite images of the Pacific into the Turk database, thereby facilitating anyone (many eyes) to scan them via the Turk interface, and all that on very short order. Jim would be impressed.

I spent several hours on Sunday, Feb 4th, using Amazon's Mechanical Turk to help look for Gray's yacht. The images above (here about one quarter the size displayed by the Turk) show one example where I thought there might have been an interesting object; possibly a yacht. Image A is captured by the satellite at a short time before image B (t1 < t2). You can think of the satellite as sweeping down this page. Things like whitecaps on the ocean surface are going to tend dissipate and thus change pixels between successive frames, whereas a solid object like a ship will tend to remain invariant. The red circle (which I added) marks such a stable set of pixels which also have approximately the correct dimensions for a yacht i.e., about 10 pixels long (as explained by Amazon). Unfortunately, what appears to be an interesting object here has not led to the discovery of Gray's whereabouts.

Use of the Turk satellite images was hampered by a lack of any way to reference the images (about 5 per web page) by number, and there was no coordinate system within each image to express the location of any interesting objects. These limitations could have led to ambiguities in the follow up human expert search of flagged images. However, given that time was of the essence for any possible rescue effort, omitting these niceties was a completely understandable decision.

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