Pierre, I have a different "which inch" question. Perhaps you know from your surveying. It is really "which foot, mile, and acre?"
NIST asserts in SP 811 (Appendix B.6) that the fathom, rod, chain, furlong, mile and acre are all based on the Survey foot. This was certainly true for geodetic data on the NAD27 datum. As the rod, chain, and furlong are probably obsolete, this is probably true now for those units. For NAD83, the intent was to release geodetic data in meters only, but the State outcry resulted in USGS releasing conversions to either Survey or International foot, provided the State pass a law clarifying what it intended to use. Some States HAVE adopted the International foot (Michigan being one). So, in those States, are the fathom, mile and acre based on the International foot that the State has specified, or does the State insist on International values, and the Feds insist on Survey values within those States? It would seem to me that the fathom, mile and acre (and modern rod, chain, and furlong, if such exist) depend on whether a particular State has specified International or Survey foot as its standard for surveying. (Land stays in the State, so hopefully whichever the State has chosen) Taken to absurd, does an acre foot of water change size slightly as a river flows through Survey and International foot States, or does the Federal government use preemption and insist on the Survey foot for Interstate commerce (and water management). (What happens when a river borders both an International and Survey foot State?) I recognize the difference is too small to measure, but I wonder if the ramifications of the absurd "what foot would you like" position of USGS has ever been fully thought through. ________________________________ From: Pierre Abbat <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Sun, September 5, 2010 1:00:51 PM Subject: [USMA:48472] Re: Which inch? On Saturday 04 September 2010 20:49:33 Pat Naughtin wrote: > Dear John, > > In the definitions you report below, which inch applies? > > Is it now the 1960 (metric) inch set at 25.4 millimetres exactly? > > Is it the survey (metric) inch set at 1/36 of 36/39.37 of the > international prototype metre set in 1893? The gallon is defined in terms of the 25.4 mm inch, even when using the survey foot to measure distance. However, the 6 ppm difference in volume between gallons defined by the two inches is dwarfed by other uncertainties. When a surveyor computes a volume, usually it's either dirt being moved or storm water flowing. Neither the packing ratio of the dirt nor the friction of water on ground is known with anywhere near that precision. Pierre -- When a barnacle settles down, its brain disintegrates. Já não percebe nada, já não percebe nada.
