Measurement tool incorrect against ruler in image by VARYING amounts I'm writing asking for a little help.
I used an image as a template plane underneath my 3D design. Things were going well but then I realized there was a strange phenomenon with measurements. The picture here is taken from the top. The view in Blender 2.8  is taken from the top. The measurement tool is overlaid on the image. The image happens to have a metric ruler in it.
Notice carefully that for shorter distances, the measurement tool is in error by a larger amount. The measurement tool is in error by a smaller amount for longer distances:
For 40mm on the ruler, the measurement tool reads 34.96mm
For 50mm on the ruler, the measurement tool reads 44.52mm
For 100mm on the ruler, the measurement tool reads 92.3mm
For 170mm on the ruler, the measurement tool reads 163.2mm
For 280mm on the ruler, the measurement tool reads 290.2mm
The difference at 40mm is ~5mm (12% difference). The difference at 100mm is ~ 7.7 (7.7% difference). The difference at 170mm is ~7mm (4.1% difference). Amazingly, at 280mm, the difference is 0.8!
I used proportional scaling on the image. If I make it bigger or smaller, this strange result remains. If I measure it from the bottom of the ruler, the results are the same. This doesn't seem to be related to the Apply Scale when in Object mode problem. There is no modifier applied.
Bottomline: My 3d project is 280mm. That distance is correct but the smaller internal details are all wrong by varying amounts. I've tried to rescale one part but then another part is wrong. If someone could please help me, I'd really appreciate it.
Thanks,
Dennis
The photo has barrel distortion, so the ruler scale isn't linear. Hence the inconsistencies.
Still, you can align it to the grid.

Adjust transparency for the image, to see the grid underneath
Move the 3D cursor to 0mm markings and set it as Pivot Point (using the overlay menu)
Scale the image until the 170mm markings match the grid

(The gif starts with the ruler perfectly aligned)