Letters - December 2007
December 1, 2007
From the Ground Up
The article “The International versus U.S. Survey Foot” by Mark Meade discussed an item that, as the author implies, is part of an issue misunderstood by too many surveyors. Knowing conversion factors is one thing. Knowing which lineal unit to use and why is another. While the article contains more than sufficient discussion about foot-meter conversion, including some history, it does not delve into reasons for selecting either feet or U.S. Survey Feet for survey descriptions, reports and other documents.
A table showing each state’s legislation for defining metric-to-foot conversion factors is presented. A distinction is not made as to when and where the conversion factors are to be applied. It would be interesting to find out about the people behind each state’s legislation. What age were the people? How much experience did they have? How much education did they have? Were their decisions based on “it’s always been done this way,” or upon future surveying needs? How many people opposed the enacted legislation? What were their reasons?
Notice was presented in the Federal Register on July 1, 1959, that the “foot,” as a submultiple of the yard, would be the international unit. In the notice, attention was drawn to the fact that the American Standards Assocation, for industrial use, in 1933 defined the metric-to-English lineal unit conversion as 1 inch equal to 2.54 centimeters, exact (results are equivalent to the conversion “1 foot equals 0.3048 meter”). As a consequence, the accuracy of the article statement “Between 1893 and 1959 the U.S. further defined the yard as 3,600/3,937 meters” is debatable.
Particular attention should be directed to the following from the Federal Register notice:
Announcement. Effective July 1, 1959, all calibrations in the U.S. customary system of weights and measures carried out by the National Bureau of Standards will continue to be based upon metric measurement standards and, except those for the U.S. Coast and Geodetic Survey as noted below, will be made in terms of the following exact equivalents and appropriate multiples and submultiples:
1 yard = 0.914.4 meter
1 pound (avoirdupois) = 0.453 592 37 kilogram
The announcement explicitly states “… except those for the U.S. Coast and Geodetic Survey as noted below, will be made in terms of the following exact equivalents and appropriate multiples and submultiples.” The notice adds “… until such a time as it becomes desirable and expedient to readjust the basic geodetic networks…” Consequently, why would anyone, except USC&GS, employ the U.S. Survey Foot except to convert USC&GS values? Why certain sectors of the survey industry have chosen to use otherwise can only be attributed to tradition and lack of foresight. What surveyor wants to explain to his client that he is using a unit not recognized by American industry since 1933? How does the surveyor explain to a client a “U.S. Survey Acre”? The use of the U.S. Survey Foot by the survey trade is an unforgivable travesty which will burden, haunt and trouble many future surveyors. Continuing to pass this on to future generations should cease. The U.S. Survey Foot should be put to rest.
Terminology, in practice and in the article, relating to the “foot” is confusing if not misleading. When “foot” is used in discussion or prose, it is understood to be the “International Foot.” The other unit, not a “foot,” although it employs the word and is very nearly equivalent, is the “U.S. Survey Foot.” In technical discussion, reports and other documents, the distinction should be explicit. It should not be left up to the listener or reader to interpret or determine the unit employed.
Abbreviations cause another problem when it is necessary to distinguish between feet and U.S. Survey Feet. The National Institute of Standards and Technology’s (NIST) Guide for the Use of the International System of Units (SI), [written by Barry N. Taylor], Appendix section B.6 suggests using italic typeface for U.S. Survey Feet and derived units. Unfortunately, this is not done in practice. For all practical purposes within this context italic typeface is not noticeable and does not draw a reader’s attention. To reduce ambiguity, it is suggested that when U.S. Survey Feet values are presented, the abbreviation “usft” be used. The use of “ft” or foot mark (') should not be exhibited or associated in any case involving U.S. Survey Feet dimensions. Abbreviations “iFT” and “sFT” presented in the article were not found in NIST’s Guide.
The article does not address problems that are often encountered and associated with significant digits. To convert from feet to meters, a value should be multiplied by 0.3048, and from meters to feet divided by 0.3048. Using the reciprocal, incorrect results will be obtained when using, as an example, 3.28083 because insufficient significant digits are employed. In the latter case, it would be difficult, unless other information was provided, to conclusively ascertain from converted values whether lineal units were feet or U.S. Survey Feet. It is suggested that exact conversion factors be used instead of derived reciprocals. If the conversion is between meters and feet, 0.3048 should be used; between meters and U.S. Survey Feet, 1200/3937.
Exceptions aside, surveyors do not measure in or use chains although they convert to and from feet and U.S. Survey Feet. Just as chains have given way to feet, when will surveyors change to using the meter as the lineal unit? Which surveyors will be the leaders and which will be the laggards?
Readers interested in additional information, including abbreviations, numeric formatting and other measurement-related issues should refer to the Federal Register notice cited and others, and to NIST’s Guide for the Use of the International System of Units (SI).
Above all, regardless whether the foot or U.S. Survey Foot is used, surveyors should state in their descriptions and reports and on maps the unit they have used. In order to reduce probable confusion or errors, it is important when values are presented in U.S. Survey Feet units to call readers’ attention.
Maurice L. Schumann
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