Not sure how NIST defines the international survey foot nor do I particularly care but here is the result of dividing 1 39 37 and represents the actual relation between U S Survey and International feet 2 54000508001016002032004064008128016256032512065024130048260096520193040386080772161544323088646177292354584709169418338836677673355346710693421386842773685547371094742189484378968757937515875031750063500127000254000 Notice the value is doubled until it repeats back to the same series after 210 digits While 2 ppm 4 ppt 8 ppqu 1 000002000004000008000016 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 1 000002 is close enough but that doesn t make it the definition of the survey unit compared with the international equivalent 1 39 37 meter defines the survey inch 1 000002 is an approximation