How to isolate a single colour and remove everything else.

[Achtung_boy]

I am trying to take a scanned document of a house plan, and scale it back to just the think black structural diagram. Surely there must be a way to either remove everything that isn't black? Any help would be appreciated.

Thanks

 

[Paul]

Trouble with scanned is you usually end up with way more bad artifacts than good image.
You could bump curves up slightly maybe.

 

[Tom Mann]

Why don't you post the image (or at least a section of it), and we'll be happy to take a look at it. As Paul said, often, such scans are full of artifacts, but OTOH, sometimes one can work around the problems. We won't know until we see it.

Tom

 

[Steve]

This might work.
Scan the plans at it's native resolution, look at the documentation for that.
You need a high quality scan.
Assuming you have red, blue, and black, open a curves adjustment layer under Presets click on "Sample image to set for white point".
Make sure you're zoomed in and sample size is set to point sample.

When you sample red it should disappear because you set red to white.
Open another curves layer and do that for every other color.

It works on a test that I did with a few lines on a white background not sure how well on your scan but it's worth a try.
Good luck.

Posting part of the scan like Tom said would help.