[Gimp-user] setting the cms in GIMP using a ruler in the photo

[rich404 <forums gimpusers com>]

> Hi All,
>
> Apologies, I am new to GIMP - I am trying to figure out if there is a
> way to scale the image in GIMP using a ruler that is in the image. I
> have taken photos of a number of different fungi in the field and now
> I need to measure their size. In each photo there is a ruler (that
> goes both horizontally & vertically) in cm - so basically what I want
> is a way to tell GIMP that this part of the image is 1cm in size. Is
> this possible to do?
>
> Thanks, Sam

You need to adjust the Image -> Print Size resolution values to match the
captured ruler scale.

First set the canvas units to whatever you are using - cms The example I used is
millimetres. That is the drop down units menu (7) shown here:
https://docs.gimp.org/en/gimp-image-window.html

Then use the Gimp measure tool to show the on canvas size. screenshot 1 
measures 10 mm as 74.2 mm Use some guides if they help, the measure tool can
snap to intersections.

Then it is a matter of simple proportions to get a scaling factor in my case
74.2 / 10 = 7.42

Open up Image -> Print Size and multiply whatever resolution is there (mine is
180 ppi - comes from the photo) by the scaling factor. Do by hand or Gimp can do
simple arithmetic * multiplies / divides.

With that changed the measure tool should now show dimensions corresponding to
the image. screenshot 2

rich: www.gimp-forum.net

Attachments:
* http://www.gimpusers.com/system/attachments/827/original/01-scale.jpg
* http://www.gimpusers.com/system/attachments/828/original/02-scale.jpg

-- 
rich404 (via www.gimpusers.com/forums)