There is another plugin: https://lukemiller.org/index.php/2011/09/digitizing-data-from-old-figures-with-imagej/Thanks for the tip - I installed Fiji. The thresholding tool is very powerful. However, I think it may be expecting a cleaner image than is often the case. This drawing is not AC or hydrofoil related, but it is typical of the kinds of things I'd like to convert to CAD. I'll play around with it some more, and actually import something into CAD to see how that works.
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With this, you have to click on data points you want to save, so you're back to manual drawing, but the nice thing is you can calibrate your image to export coordinates in the 0 - 1 range right away.
Cool project BTW!