I bumped into Shaper Trace today, and don’t remember it being mentioned on the forum before. It produces SVG files from a phone camera that are both scaled correctly and are corrected for perspective. At $99 + shipping it is a bit pricy for me as an impulse purchase, but I could see me purchasing it for the right job. For example, this would really help making custom tool inserts.
That looks really interesting! Not gonna buy it though. 
There’s a been an hint about it here
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It does seem pretty slick, there must be some more options like this out there. An app then picture and a couple rulers or something?
Nothing really precise emerged from the discussion tbh
The most common solution seems to rely on your phone’s spirit level and hope for the best
I’d love to find an app like trace but without suscription, or even an app where i could slap a few arrow stickers on things/walls, shoot it and get a measurement…
I learned about https://www.toolkaiser.com/ elsewhere on this forum. It’s free and easy to use but I have never gotten results that I found to be usable. You might have better luck than I did.
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I might have posted about it. And if the results were bad, then something went wrong, it has lots of cool features like nesting etc. It’s only for tool outlines though.
Shaper Trace claims it doesn’t require a subscription.
For the most part, reprojection is a solved problem and someone could easily make an inkscape plugin if there weren’t already one. On the other hand, instances I’ve seen of centerline trace are usually pretty poor and if they have a novel solution that has cracked that then it might be worth it.
I bought a Trace (two actually, one for my mum who’s big into papercraft). It works great for what it is, but scaling the imported files is annoying. Fusion 360 doesn’t respect the svg metadata and imports as pixel scale, which depends on the dpi of the device… (or something, point being it has no connection to real units). I traced a known square and calibrated based on that.
Haven’t come up with a ton of real world uses yet, but my first project was 3d printing a sheath for a paring knife that’s been rattling around the drawer.
When I scan or photograph and object, I try to do it on graph paper to have good rectangles for the deskew. You’ll still have distortion, and 3D objects have perspective. But for flat stuff in the middle of the image, it’s not bad.
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I have one of the Shaper Trace Frames right here. I use it at work to save a lot of time integrating customer parts into premade furniture I supply to them for congresses and events and at home for laser cutting, engraving, at the moment mostly for xmas crafting with the kids (my daughter is especially in love with it in back-to-back combo with the laser cutter. Ask me anything if you want to know something so I can provide info into this community for a change instead of asking for advice 
There is no subscription. (good)
Also not another app to be installed. (good)
But also no software you can download or easily integrate into an offline-“backup” (which is not good), just a web application at trace.shapertools.com - it’s gonna be alive as long as Shaper is. Since it is owned by the same company that owns Festool it is probably here to stay for now, but… well… still easy to vanish the entire service and product. 
I am astonished in how fast and good it works, I bought (and now use) it for the company mainly to save time. A lot of time. And in the end by now one of my customers paid for it Not sure whether it’s worth just for a home shop. It’s a fun gadget and it’s QUICK (did I mention how fast it is). I would buy it again for my purpose. Not sure where they are priced at for now.
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A little trick I’ll add to Jeff’s simple approach of taking a picture: if you move the camera farther away from the subject and zoom in, the resolution gets worse but the distortion improves (decreases). Depending what you are doing, this might be better overall.
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