CONVERT function - acres, statute miles

["David A. Wheeler" <dwheeler@dwheeler.com>]

I'm walking through all the units of CONVERT to make sure we define them, with _specificity_.

I've found a nasty oddity: acres and statute miles.  It turns out that there are TWO definitions of acre, and TWO definitions of mile, that are in active use.  Here's the explanation, and a recommended solution.  In short, let's NOT define 'acre', but have 'acreif' and 'acresf' for the two acres, and also add an 'smi' for statute miles.  We'll need to do something, because OpenOffice.org handles acres.

Why do I suggest this?  Here are the details, which get painful quickly :-(.

In US customary and in Imperial units (which are NOT always the same),
5280 feet = 1 mile.  43,560 square feet = 1 acre.  That's always true, for both systems.

To convert to SI, an international foot = 30.48cm.

That's enough to handle these non-metric lengths, right? Well, no :-(. It turns out that in the U.S., U.S. maps and land are normally NOT measured using the international foot.  Instead, for historical reasons they are measured using the "survey foot", which is 1200⁄3937 meters instead.  NIST (who officially handles U.S. definitions) officially defines "acre" in the U.S. as being based on the "survey foot", NOT the "international foot", so "acre" in the U.S. officially has a different meaning than everywhere else :-(  :-( .  Normally miles in the U.S. are measured using the international foot, but in map surveys the "survey foot" is often used instead (depending on which state you're in, believe it or not). This odd measure is the "statute mile", which is close but not the same as a normal international mile.

So there are two different acres: the U.S. statute (or survey) acre (used for nearly all land area measurements in the U.S.) and the international acre:
1 international acre = 4046.8564224 m^2 (exactly)
1 survey/statute acre = 43560*12^2/39.37^2 m^2 (exactly)
 = 4046+(13525426/15499969) m^2 (exactly)
They're close but not the same; with 100 acres, you get a difference of ~1.62 m^2.

OpenOffice.org's "acre" uses the international acre. Okay, that's understandable. Except that almost no one uses acres anymore, except the U.S., which DOES NOT use international acres.  Having a term this easily misused is dangerous.  Excel doesn't have 'acre', and if they added it, they'd probably add the U.S. survey acre (because they'd probably just pick up the NIST definition and not even be AWARE of an alternative).  In short, we have one term with two meanings.

I propose that we NOT define 'acre', and instead define two other units:
'acreif' - acre (international acre, based on the international foot)
'acresf' - acre (U.S. survey/statute acre, based on the U.S. survey/statute foot)
I put the adjectives AFTER the word 'acre', because they'll be next to each other in alphabetic listings and where people would look for them; I used "sf" and "if" because just an "s" looks like the plural and would be ignored. Later on, if there's a single 'acre' definition that's used everwhere, we can make 'acre' be a synonym for it.... but by NOT having an 'acre' to start with, the resulting error will force users to be specific.

If we do that, we may as well define "statute mile" ("smi"), the other common unit (in U.S. maps).  The error here is very small (it's not squared), but we may as well do it for completeness.  Yes, this means that when people look at a map, they'll sometimes get statute miles, and when they look at their car odometer, they get the not-the-same international miles :-(. They're close to each other; most Americans probably think that a "statute mile" is the same thing as a mile.

I don't see a need for CONVERT to support statute feet (its use is very specialized).  Please don't tell me about rods, chains, and furlongs.  Indeed, I'd like to kick this whole measurement system into a very deep trench.

For more info about the U.S. measurement system, see:
http://physics.nist.gov/cuu/Units/bibliography.html

Those of you who universally use the metric system: Count your blessings.

--- David A. Wheeler