The coastline paradox demonstrates that the measured length of a natural geographic feature like a river depends on the scale or ruler size used, making this a matter of common knowledge in geometry and geography.
The claim is a well-known geometric and geographic principle (often related to the coastline paradox and fractal dimensions where measuring scale alters the calculated length). It is a foundational observation in fractal geometry and geographical measurement that needs no citation, fitting the definition of common knowledge in this context.