Pine Needle Tracks

Guest Post by Dan Drake of the University of Puget Sound:

The attached picture is of the entrance to the library at the University
of Puget Sound. It's autumn here in Tacoma and there are lots of fallen
pine needles outside which the students track in. The door opens on the
right (on the left, to entering students) and you can see the
distribution is shifted that way near the door. The pine needles are
concentrated in the middle but show a few outliers, and the light color
in the middle shows dust and dirt brought in -- so there's a discrete
and continuous distribution.

The two dimensions of the carpet show a sort of time series: near the
door, people (and hence the dirt and pine needles) are shifted towards
the side of the door that opens, and as they walk into the library, they
tend to move to the center of the entryway.

Thanks for maintaining the blog -- I (and hopefully my students...)
enjoy it!

Thank you Dan for your interest and submission.
I'll have more on pine needles next week.