Torsion boxes, episode 3: Building a flat reference surface

In this video I demonstrate how I built a flat reference surface so that I can have a place to build a flat torsion box. The method I'm using is mostly based on the technique shown in this @woodwhisperer video: https://youtu.be/1-Hbsou6cWo?si=ut_ud5-7HUFJGNOV which in turn is based on a demonstration of the concept by @DavidJMarks in episode 409 of WoodWorks, his TV show on the DIY network. I saw "mostly based on" because I'm doing a few things a bit differently, which are explained in the video. One of those differences is my use of the "crossed strings" technique for measuring flatness. I learned about this method from the following link, containing the notes from a talk given to the Bay Area Woodworkers Association by Jamie Buxton in 2002: https://bayareawoodworkers.org/Teck_Talks/TeckTalkTorsion%20Box%20Construction.html I also take a few minutes in this video to explain why the basic method works, including just a little bit of math. 00:00 Intro 01:56 Fundamentals 04:10 The WoodWhisperer and David Marks method 05:02 My setup 05:08 Making beams out of plywood 08:26 Checking for accuracy 10:58 A way to make the adjustments 12:42 How to measure flatness while making adjustments 12:56 Levels 14:20 Winding sticks 14:51 Crossed strings 20:10 Closing