What is the best way to resize an image based on a scale reference in that image?

[dbastien]

I received an image that contains a scale reference grid.  I need to resize the image based on that reference to make sure the rest of the elements on this image get used properly.

The reference grid value is 10 tiny squares = 1cm

These are sewing pattern pieces for a doll garment.

[c.pfaffenbichler]

I would recommend using the Ruler Tool to measure 100mm and set the Ruler Units to Pixels … that should get about 728px. 

Then use Image > Image Size to set the Resolution to the corresponding 72,8ppcm with Resample disabled which should, after changing the Ruler Units to mm, get you 100mm for the previously set measurement. 

[dbastien]

Never used the ruler tool before, will try it now.

[dbastien]

When using the ruler tool I do not understand how the read the results.

creating a line that should be 1cm I get x:6.69 y:56.82 h:-0.01 à:0.3 L1:2.50

what does that tell me?  Not sure.

[c.pfaffenbichler]

Are the Ruler Units set to Pixels? 

What don’t you understand? And please post screenshots instead of typing away … 

[c.pfaffenbichler]

And don’t focus on such a small length – the smaller it is the more likely rounding and other issues are and you should have a perfectly fine 10cm length available (if I understand the grid correctly). 

[dbastien]

Sorry to be so thick today, I am usually pretty smart in understanding new stuff.  Must be having a bad day.  I simply do not get it.  Maybe I will wait until tomorrow for this.

[c.pfaffenbichler]

In my screenshot the value for L1 (the length) is 728,48. As the Ruler Units are px that’s 728,48px. 

As the length corresponds to 10cm (unless I miscounted the grid) that means 1cm should have ~72,8px. 

So setting the file’s resolution to that without resampling will not chage the pixels, but the way their size are interpreted. 

Setting the Ruler Units to cm should then indicate that the length of the ruler is 10cm. 

[dbastien]

Thank you very much.  I get it now.  At first I had no idea where to look for the numbers and which number was the one I needed.  Your screenshot helped me understand.

[Stephen Marsh]

If you are happy with the response, could you please mark the appropriate answer/s as the "Correct Answer".