How to reverse engineer a schematic from a circuit board

Step 6 Mark the free holes

A multi-layer board may have some pads which don't look to be connected to anything.  If so, they probably terminate some through plating which connects the other side of the board to an inner plane or track.  You need to mark these "free" holes another way so you can relate them to the other side of the board later on. I have read that these inner layers are often used for ground and power planes, so shouldn't be too hard to trace when you've done everything else.

Create another new brush in a cross shape - use the guides to help you.  You may want to create a "x" shape and a "+" shape, which can interleave slightly.

Repeat the procedure you used to circle the ground-plane holes, only this time only put an x or a + over the free holes.  This way, the centre of the mark still shows where the holes go.  I made these brushes by making triangular and rectangular selections on a 64 pixel square canvas, stroking them, then erasing the outer edges, however I have since realised you could probably use a text "x" and "+".  Put a circle selection around the "x" the full width and height of the canvas, invert the selection, and delete the ends of the arms, so that it is not disproportionate to the "+".

Copy the alignment marks to the layer, and save a copy to a new .png file.