Do I really need to measure something? Why cannot I just give you two pictures?
Theoretically speaking, you cannot obtain true 3D dimensions without knowing at least one true length.
Think this way: If one views a video clip of, say, the Grand Canyon, it is not possible to ascertain if the video was shot from a helicopter flying over the true landscape in Arizona or if the video was synthesized in a Hollywood movie studio using computer graphics techniques based on a scale-down model of the Grand Canyon - if the model is sufficiently detailed and photo-realistic. Hence, it is not possible to obtain true 3D dimensions without a reference length of some kind.
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