Template:IFD tag ResolutionUnit

ResolutionUnit (The unit of measurement for XResolution and YResolution)

Tag = 296 (128.H)

Type = SHORT

N= 1

To be used with XResolution and YResolution. 1 = No absolute unit of measurement. Used for images that may have a non-square aspect ratio, but no meaningful absolute dimensions. The drawback of ResolutionUnit=1 is that different applications will import the image at different sizes. Even if the decision is arbitrary, it might be better to use dots per inch or dots per centimeter, and to pick XResolution and YResolution so that the aspect ratio is correct and the maximum dimension of the image is about four inches (the “four” is arbitrary.) 2 = Inch. 3 = Centimeter. Default is 2.