This is a bread and butter method for any layout man or woman.

I'm not sure the gal in the video did all that much site layout. Her theory of the 3 4 5 triangle method  is correct though.

Here is what I would suggest. After establishing your 3′ (or 4′)  base line along the known layout line, Hold the 4′ of your tape on one end of the base line and draw an arc 4 ‘ radius. Then hold the 5′ of your tape on the other end of the base line and draw the 5' radius arc.

The intersecting arc point is the layout point you are looking for.  Now connect the first end of your base line to the intersection of the arcs and you have your square angle.

Here are a few more tricks for using this method out in the dirt. — Click Here —