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