not sure how nist defines the international survey foot nor do i particularly care but here is the result of dividing and represents the actual relation between u s survey and international feet notice the value is doubled until it repeats back to the same series after digits while ppm ppt ppqu etc may diminish in importance rapidly when measuring a particularly large distance with a high degree of resolution required that differential could conceivably come into play practically is close enough but that doesn't make it the definition of the survey unit compared with the international equivalent meter defines the survey inch is an approximation