- From: John Halleck <John.Halleck@utah.edu>
- Date: Mon, 26 Apr 2004 09:02:03 -0600 (MDT)
- To: www-math@w3.org

At http://www.w3.org/TR/mathml-units/ section: "5.3.3" "In this case, the context is necessary as the standard U.S. mile is equal to the international mile." This is a typo... this should read "... standard U.S. mile is NOT equal to the international mile." "... the only which ..." should read "... the only units which ..." Note that the unit discussion is in your document is terms of the two "miles", while the legal definitions CURRENTLY IN USE are (as mentioned below) actually in terms of inches, although the correct legal terms are actually "international foot" and "US Survey foot". (As commented on later in this letter.) I would be curious as to where the discussion in terms of miles comes from. In any case, since the US legal distinction states the units as US feet vs International feet, I would argue for the units document having URI's in those terms instead of in terms of miles. Aside: Also note that not every country theoretically using SI actually use the same value for the meter. For example, Plane coordinates for Namibia use what they claim as the "German Legal Meter" (although Germany and the rest of the world call it the "Namibian Meter".) (See for example the Namibian Geological survey at http://www.gsn.gov.na/map_system.htm ) So this might make a good additional example. (Since they use the "standard" SI units for everything other than surveying.) section: "6.4 Foot-Pound-Second System of Units You say: "Due to the ubiquitous nature of the foot-pound-second system of units in some countries some mention of this system of units is necessary." Note that wile section 5.3.3 acknowledges the two different systems, this section doesn't. And worse, the conversion examples the ignore the issue, and imply that the proper conversion values on feet are aproximations. Different states in the US * CURRENTLY * mandate different State Plane Coordinate systems, some with meters, some Survey feet, some with International feet. (These are documented in "NOAA Manual NOS NGS 5 - "State Plane Coordinate System of 1983", and in detail at the National Geodetic Survey web site.) Because of the bias offset values built into the state systems the difference between using a system with the correct foot and using it with the incorrect one can easily be 30 feet or more for state coordinates. So this distinction is a direct and current legal issue for Surveyors in the US, and not just an oddity of history. People familiar with the history can tell you that the difference between the values are mandated as follows: US survey foot: 1 m = 39.37 in EXACTLY. International foot: 1 in = 2.54 cm EXACTLY. Note that both conversion values are exact [legally mandated] and are only approximate when truncated in the reciprical forms. Note that for conversions to and from SI, for scientific use (not surveying) are officially in terms of the international foot, which is exactly [not aproximately] 0.3048 . But since the unit in the table is "foot" without labeling which one, there could be some confusion in the minds of the reader. Your table says exact conversion values are given in bold, and the exact 0.3048 conversion for international feet is not bolded. You then display what is apparently more significant digits than the "not assumed exact" conversion value your table gives would allow in a conversion. But, at least your table does give the correct (international) foot for the value. Note also, as an aside, that distinguishing things only by bolding is difficult for the handicapped when trying to read the page using screen reading software that doesn't always preserve such a typographical convention. -- John Halleck. John.Halleck@utah.edu 801.585.9527 P.S. My heart felt thanks to the MathML team for their work towards making "portable" mathematical notation practical.

Received on Monday, 26 April 2004 11:10:07 UTC