I have a photo of a barn. It is taken at an angle to the front wall instead of looking straight on at the wall. In other words, you can not measure (with the measure tool) the wall length. How can I adjust image to allow true measurement?
I included the image.
True measurement... don't count on it if you don't know some details like the lens used, especially if that photo is a scan or a picture of a photo paper
1) if you know the focal length of the lens (=field of view), distance to object, and maybe size of the film (negative film) for an accurate calculation
This method will allow you to calculate angles from the photographer position, then some trigonometry... But the photographer took it at an angle, so no
True.
2) If you don't have the above infos, but you do know the size of 1 object in the picture (like the fence height or any other object) you can try to report it on the barn, with some degradation as you don't know the field of view/focal length and distance to the camera, but it might be okey-ish enough. But no
True as it's an angled view.
3) try this calculator (scroll down)
https://www.scantips.com/lights/subjectdistance.html , there is also a lot to read on that very page which can be a big help for you.
But, for your
True measurement I'll say no, just because of the angle the photographer took it. You will have a great percentage of error (2-5% in the best case, a lot more if you don't have any info I've mentioned above)