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