http://ts.nist.gov/ts/htdocs/230/235/appxc/appxc.htm#1

Ok look at this:

2. TABLES OF U.S. UNITS OF MEASUREMENT
In these tables where foot or mile is underlined, it
is survey foot or U.S. statute mile rather than
international foot or mile that is meant.

3 feet (UNDERLINED) = 1 yard (yd)

Ok so here it defines a yard as 3 SURVEY feet.

But then scroll down to:

5. ...
1 yard (yd)---------------------------- 0.914 4 meter
(exactly)

Here it defines a yard as 0.9144 meters EXACTLY, which
is exactly 3 INTERNATIONAL feet.

So what's the deal here???

--- Ron McConnell <rcmcc@...> wrote:

>
> Two definitions of a foot in the USA
>
> 1. standard ("international") foot = 0.3048
> meters
>
> 2. U.S. survey foot = 1200/3937 meters =
> 0.3048006096012 m
>
> <http://www.vterrain.org/Projections/sp_feet.html>
>
> - Full note with more links below this message body
>
> This topic may have been covered already in BP,
> but for a new folk like me, the extract below
> was very interesting, including the statement
> "... there have been instances where the
> conversions
> were done using the wrong conversion factor
> or the computation was otherwise done incorrectly
> [by the professionals]..."
>
> So, we need to look closely at the data
> we get from anywhere in the USA to see
> _if_ we can tell which foot was used or converted.
> That last "millimeter" of calculated distance
> may be correct to only the last several meters.
> This type of multiple definition of a unit/name
> is not exclusive to the USA.
>
> There are _many_ other interesting, well written
> notes
> links and resources on geography at
>
> Virtual Terrain Project (VTP)
>
> <http://www.vterrain.org/index.html>
>
> "The goal of VTP is to foster the creation
> of tools for easily constructing any part
> of the real world in interactive, 3D digital form.
> The VTP software consists of a set of entirely
> free and open source applications and libraries."
>
> I've added a link to VTP to BP.
>
> Cheers, 73,
>
> Ron McC.
> w2iol@...
>
> Ronald C. McConnell, PhD
>
> WGS-84: N 40� 46' 57.6" +/-0.1"
> W 74� 41' 22.1" +/-0.1"
> FN20ps.77GU31 +/-
> V +5058.3438 H +1504.2531
>
> http://home.earthlink.net/~rcmcc
>
> Don't argue about facts.
> - G. Robert Leopold
>
> ===============================
>
> State Plane Coordinate Systems and the U.S. Survey
> Foot
>
>
> <http://www.vterrain.org/Projections/sp_feet.html>
>
> There is a unit of measure called the "U.S. Survey
> Foot."
> It is almost exactly equal to a standard foot,
> but it's definition is slightly different:
>
> * standard (or "international") foot: .3048
> meters
>
> * U.S. survey foot: 1200/3937 meters
>
> In other words, one is defined in relation to the
> meter
> by a decimal expression, the other by a fraction.
> Fortunately, the difference is very small,
> amounting to less than a few meters over an area
> as large as a U.S. state.
> However, this small difference can cause problems
> in some cases, so it's advisable to always know
> which units are being used.
>
> More information about how this relates to the
> U.S. State Plane Coordinate System is contained
> in this fascinating and informative document:
>
> POLICY OF THE NATIONAL GEODETIC SURVEY CONCERNING
> UNITS OF MEASURE FOR THE STATE PLANE COORDINATE
> SYSTEM OF 1983
> January 1991
>
> Key excerpts:
>
> * The State Plane Coordinate System of 1927
> was published in a unit of measure identified
> as the "U.S. Survey Foot."
> By 1959, except for surveying and mapping
> applications,
> the United States had switched to the unit of
> measure
> identified as the "International Foot."
>
> * (In 1983) the U.S. Survey Foot was specified
> in 11 states and the International Foot was
> specified
> in 6 states. In all other states the meter
> is the only referenced unit of measure in the SPCS
> 83 legislation.
> The remaining 19 states do not yet have any
> legislation
> concerning SPCS 83.
>
> * Because SPCS 83 is presently published by NGS
> only in meters, many surveyors, mappers, engineers,
> and LIS users are converting these metric values to
> feet.
> [...]
> there have been instances where the conversions
> were done using the wrong conversion factor
> or the computation was otherwise done incorrectly.
>
> More background for those interested:
>
> The "Foot-to-Meter" Conversion
>
> *
> <http://www.wsdot.wa.gov/Metrics/ftmtr2.htm>
>
> [USA] State Plane Coordinate System of 1983
>
> *
> <http://www.ngs.noaa.gov/PUBS_LIB/ManualNOSNGS5.pdf>
>
>

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