the small squares are able to be measured with a cheap digital caliper (similar
to https://www.harborfreight.com/6-in-composite-digital-caliper-63586.html )
longer distances are with a tape measure.
take a look at the ‘in search of accuracy’ thread
In search of accurate measurements where I
am working on a 3d printed part to pair up with a tape measure to get sub-mm
accuracy. I used a set a couple days ago and it worked well and was
substantially faster than just using a tape measure (or so I’m told by someone
who has calibrated more machines manually than I have)
David Lang