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Hi Brynn, hi all,
Thanks, Maren. I will try to find some time to read those. But I was really hoping for a shorter answer.
Only the third one (SVG) is really useful for this matter :)
How to change the display units, so that the file is using those units, and so that the drawing is as close to the real life units as the user's screen resolution permits (like for printing or cutting). In this case, I mean for a new drawing in the current version of Inkscape (not opening a file which was started in an earlier version).
There are a few settings pertaining to units in the doc properties : Display units, custom size (which should probably be just "size") and "scale".
(1) The most important setting is the "(Custom) Size", which will determine what final size your document will be. If you aim for real-world use (like printing/cutting), it will have to be in a realworld unit (mm) to avoid any problem.
(2) The "Display Unit" is just a convenience. It will allow you to always see the rulers and size of stuff labeled in that unit, in the *outside* coordinate system (real world size). Whether you plan to use units in the file xml code or not will not affect the values shown and I would recommend setting it to the unit you make your measurements in.
(3)The "Scale" is to tell Inkscape what should a user unit be worth in the outside coordinate system. If "the file is using those units" means that you do want to use mm or in or cm IN THE XML CODE[1] (which Inkscape can do, but does not do by default I think), you HAVE TO set it to 1user unit per PIXEL (temporarily set display unit to px, set scale to 1, put display unit back to whatever you need), otherwise a centimeter in the xml code will NOT be worth a centimeter (because… reasons, the details about that are in my article). If you only measure your stuff with the display units (and do not use units in the code, it should not matter (tools "should" take the scale into account, and if some do not it's an bug))
(Also, the "zoom correction factor" in the prefs→ Interface only control the behavior of View→Zoom→1:1 1:2 2:1 so that going to "Zoom 1:1" will show you on screen something worth 1cm at 1cm if you have set it correctly, but does not impact the file in any way.)
I hope it's not toooo confusing,