NIST Handbook 44, Appendix C is a great compendium of information inter-relating Customary, Imperial, and metric measure. However, the 5 subsections can be a little confusing as conversions are stated to different precisions, in different sections. A recent comment by Pat on confusion of exact vs approximate values caused me to look closer. Section 4 is probably the best source of exact conversions. However, some are not correctly marked as exact or not, and in several cases the exact value can be deduced, but is not explicitly stated.
As a result of my review, I have submitted the comments below to the NIST Weights & Measures office: Comments on NIST Handbook 44, Appendix C, 2010 Edition Handbook 44, Appendix C is an extremely useful resource for defining U.S. Customary measure and its relation to the metric system. The data provided has more than sufficient accuracy for any practical purposes. However, I believe a few errors exist in the use of underlining to denote exactness. Further, I offer a few suggestions for possible improvement. All comments relate to section 4 of Appendix C; however, I have relied on some of the other sections to define exactness in my reasoning. I did not do a methodical review of prior versions, but these comments generally apply to prior editions as well. Those figures which are underlined are intended to be exact, and may therefore serve as definitional. I believe the following issues exist: 1) In the first table of length - International, the figures 12 in/ft, 36 in/yd, 1 ft/ft, 3 ft/yd and 1 yd/yd are not underlined, but are exact. Underlining should be added. 2) In units of capacity - liquid volume measures, it is not possible to state exactly the volume of the minim decimally in terms of drams, ounces, or gills (there is a division by three), yet they are underlined. Underlining should be omitted. Note: The exact value of the minim can be stated as 0.003 759 765 625 in³ if desired. 3) In the cubic inch column of this table, the figures 1728 in³/ft³ and 1 in³/in³ are not underlined, and should be. The value of 0.225 585 94 in³/fluid dram is marked as exact, but is not. The underline should be deleted, or the exact value stated and underlined, 0.225 585 937 5 in³/fluid dram (this value can be calculated from other entries in the table.) I recommend the exact value for completeness. 4) In the first table of Units of Volume, exact definitions of the cubic inch and cubic foot in liters are given and underlined. However, these figures are rounded (and not underlined) in the dry and liquid measure tables. The use of two different values is somewhat distracting and confusing. I recommend the exact values be used in the dry and liquid measure tables (also in the milliliter column of these tables). I have some additional suggestions for exact values. They will involve more significant figures than could ever matter in practical measurement. However, they aid in the rigorous definition of US Customary measure in metric terms and I urge their consideration: 5) Since the release of a new datum (NAD83) and related geodetic data, USGS has allowed States to pass legislation and receive their geodetic data in meters, International feet, or Survey feet, according to their preference. This results in a mess; however, some States have adopted the International foot. As a result, I believe the table for units of area - International measure requires the addition of an International acre, 4046.856 4224 m², exactly, based on 43560 International square feet. The link, chain, rod, furlong, and related areas are perhaps sufficiently obsolete as to not require "international" definition. Note: This issue may also indicate a need for some rewrite of NIST SP811, Appendix B.6. 6) The gallon and bushel are defined as 231 in³ and 2150.42 in³ respectively. By multiplying these values by (0.254 dm/in)³, it is possible to give exact values in liters for these quantities, as 3.785 411 784 L, and 35.239 070 166 88 L. The first requires no more decimal places than the values for cubic inch and cubic foot, already given, the value for bushel requires two more. Starting from these values, each submultiple may also be stated exactly and decimally, if desired.
