Since we've decided to build a square to place in our pictures, I tried correcting red-DSCN0032.jpg using only a square from the floor with Hugin.  I tried with two different squares and got decent results.  I don't think it's as good as the longer lines, but it seemed to mostly work.

Some next steps:
1) Once we get the lens calibration done, it might be a lot better.  I think there is a tutorial for this.
2) We need to work out how we quantify the error from the pictures, maybe the same method we'll use at the stream, or maybe something more suited to all the information we have about our gridded stream.
 
We had an exciting power outage today, which prompted an impromptu trip to Menard's.  Here's a picture.

Menard's trip