A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Oct 8, 11:44 -0700
You're absolutely correct that we can determine altitude from this image (even if it's photo-realistic artwork --always possible!). As you have described, the trick, which I had hinted at in my original posting of the image, depends on a perspective effect. When you're closer to a planet, the proportion of the apparent disk of the planet occupied by features at the center of the disk (like North America in this case) increases. This is entirely independent of the total angular size of the planet and therefore independent of magnification. It's only useful when the observer is nearer than a few planetary diameters. This diagram may help others to see what's going on...