ment since the time of image capture. A drift-modeling team was formed, ultimately consisting of two sub-teams of oceanographers with access to two alternative drift models. As image processing proceeded, these sub-teams worked in the background to parameterize their models with weather and ocean-surface data during the course of the search. Thus, once targets were identified, the sub-teams could quickly estimate likely drift patterns.

The drift models utilized a particle-filtering approach of virtual buoys that could be released at an arbitrary time and location, and for which the model would then produce a projected track and likely endpoint at a specified end time. In practice, one must release a string of adjacent virtual buoys to account for the uncertainty in the initial location and the models’ sensitivity to local effects that can have fairly large influence on buoy dispersion. The availability of two independent models, with multiple virtual buoys per model, greatly increased our confidence in the prediction of regions to search.

Worth noting is that, although these drift models were developed by leading scientists in the field, the results often involved significant uncertainty. This was particularly true in the early part of the search, when drift modeling was used to provide a “search box” for Gray’s boat and had to account for many scenarios, including whether the boat was under sail or with engines running. These scenarios reflected very large uncertainty and led to large search boxes. By the time the image processing and weather allowed for target qualification, the plausible scenario was reduced to a boat adrift from a relatively recent starting point. Our colleagues in oceanography and the Coast Guard said the problem of ocean-drift modeling merits more research and funding; it would also seem to be a good area for collaboration with computer science.

The drift-modeling team developed its own wiki-based workflow interface. The analysis coordinator was given a Web site where he could enter a request to release virtual “drifters” near a particular geolocation at a particular time. Requests were processed by the two trajectory-modeling teams,

A more apt term
for the software and
workflow described
here might be
a polytechture,
the kind of system
that emerges
from the design
efforts of many
independent actors.

and the resulting analysis, including maps of likely drift patterns, were posted back to the coordinator via the drift team’s Web site. Geolocations in latitude/longitude are difficult to transcribe accurately over the phone, so using the site helped ensure correct inputs to the modeling process.

Analysis results. The goal of the analysis team was to identify qualified search coordinates. During the search, it identified numerous targets, but only two were qualified: One was in ER- 2 flyover imagery near Monterey, originally flagged by Mechanical Turk volunteers; the other was in Digital Globe imagery near the Farallon Islands, identified by a member of the more experienced image-processing team. 3 Though the low number might suggest our filtering of targets was overly aggressive, we have no reason to believe potential targets were missed. Our conclusion is simply that the ocean surface is not only very large but also very empty.

Once qualified, these two targets were then drift-modeled to identify coordinates for search boxes. For the first target, the drift models indicated it should have washed ashore in Monterey Bay. Because this was a region close to shore, it was relatively easy to send a private plane to the region, and we did. The second target was initially not far from the Farallon Islands, with both models predicting it would have drifted into a reasonably bounded search box within a modest distance from the initial location. Given our knowledge of Gray’s intended course for the day, this was a very promising target, so we arranged a private offshore SAR flight. Though we did not find Tenacious, we did observe a few fishing vessels of Tenacious’s approximate size in the area. It is possible that the target we identified was one of these vessels. Though the goal of the search was not met, this particular identification provided some validation of the targeting process.

Discussion. The image-processing effort was the most structured and technical aspect of the volunteer search. In trying to cull its lessons, we highlight three rough topics: polytechtural design style, networked approaches to search, and civilian computer-vision research targeted at