I read about this method for flattening large slabs in a publication (Woodwork I think) a year or two ago. Unfortunately, I couldn’t find the article in order to reference it. As I work with irregular slabs, too large for my machines, I expect it to be a reasonable and effective method for flattening large slabs, in this case, eastern red cedar.
The router is mounted to a 4′ sled which rides across two parallel fences (jointed 2×4’s smoothed and waxed). The router has an 1-1/4″ straight cut bit, and mounted on the sled can cover the entire face of the slab, removing about 1/32″ of material per pass.
a couple of turned walnut handles help to steer
the whole endeavor sits on my table saw
The School of the Transfer of Energy 
Seen similar setup before, been meaning to build one. Thanks for reminder. It’s perfect for leveling slab bench seats I often rough mill with a chainsaw.
Also useful in relief carving.
I did the same..my Router didn’t have a good way to mount on the sled so I have an angle iron sled whereas my Router goes back and forth on that. The sled has outside rails to keep in it place. I set it up on a level picnic table so I could sit and work. Worked very well…my Cedar came out exactly like yours in the picture!
I will forever be making enhancements to the jig, but this one is nice…can be taken down and stored in a minimal of space. I would love to submit a pict if I could…I don’t see where I can share it…
Great job on yours!