Talk:Square kilometre

Value in acres

I was checking the values listed on this page. The page currently says that 1 sq km equals 247.105383 acres.

The GNU units program gives 247.10439 acres

The National Oceanographic Data Center say 247.105381 acres [1]

The Google calculator also says 247.105381 acres [2]

I think 247.105381 is probably right. It is a pretty minor thing but it would be good to get it right. I will try to do some more checking, but welcome any comments from others. -- Popsracer 11:21, 29 Sep 2003 (UTC)

1 sq km = 1,000,000 sq m

0.3048² sq m = 1 sq ft

16.5² sq ft = 1 sq rod

160 sq rods = 1 acre

1 sq km = 247.105381467 acres

er... I think... Evil saltine 11:44, 29 Sep 2003 (UTC)

GNU Units assumes the U.S. survey acre instead of the international acre. The figure 247.105383 has an error in its last digit. -- Indefatigable 16:58, 30 Sep 2003 (UTC)

That's why the metric system is the best. A metre is a metre is a metre, and a kilometre is a kilometre is a kilometre. None of this business of having to remember whether you're British or American, land-based or nautical, civilian or military, and so on. JIP | Talk 18:30, 22 September 2008 (UTC)

Oh yeah. metric system is the best.78.106.148.243 (talk) 16:15, 3 February 2009 (UTC)

You'll certainly get a 'squire deal' of of this measurment, sir! --Arma Martin 3nd 03:34, 26 January 2007 (UTC)

Trouble is that we have two slightly different values for both the acre and the square mile, one for the International measurements and the other for the US Survey measurements. The text will have to be changed to reflect this fact. Michael Glass (talk) 06:28, 12 June 2012 (UTC)

Proposal to change the text to reflect the difference between the International and US Survey measurements

At the moment, the text reads:

• 0.386302 square miles
• 247.105381 acres

and

• 1 square mile = 2.58998811km²
• 1 acre = 0.004047km², 0.4047ha or 4047 m²

2.47 acres/ha

No source is given for these figures, and the text does not reflect the fact that there are in fact two very slightly different miles in use in the United States. My proposal is to use the figures from unitconversion.org. This would make the text read as follows:

• 0.386102159 square miles (International)
• 0.386100614 square miles (US Survey)
• 247.105381467 acres (International)
• 247.104393047 acres (US Survey)

and

• 1 square mile (International) = 2.58998811km²
• 1 square mile (US Survey) = 2.58999847km²
• 1 acre (International) = 0.004046856km², 0.4046856ha or 4046.856m²
• 1 acre (US Survey) = 0.004046873km², 0.4046873ha or 4046.873m²

This, of course presumes that UnitConversion.org is right. What do others think? Michael Glass (talk) 12:39, 13 June 2012 (UTC)

Michael Glass (talk) 05:09, 17 June 2012 (UTC)

Obviously the US survey foot was preserved so it could continue to be used, as one option, to report the results of geodetic surveys. The National Geodetic Survey and its predecessor organizations, right back to the Survey of the Coast organized in 1807 used metric units, but some of the results, such as the State Plane Coordinate System were made available in survey feet. Today the difference is only significant for state plane coordinates, where the origin is many miles from the points being surveyed. Since areas are based on the relative location of the points defining the parcel, the differences in using survey feet or international feet to compute acres or square miles would be insignificant compared to the uncertainty in the area. So I see no need to use or mention US survey feet in this article. Jc3s5h (talk) 20:19, 19 June 2012 (UTC)

Speaking only of US practice, I don't think you're quite right here; equally important to the geodetic and coast surveys were the Land Survey which was the basis of much land ownership and was firmly rooted in traditional units and the Geological Survey, which employed both metric and traditional units. Prior to 1959 the US inch was defined as 1/39.37 meters and the survey foot was correspondingly 1200/3937 meters. In 1959 the relations between metric and traditional units were redefined, with the yard exactly exactly 0.9144 meters, the foot 0.3048 meters, and the inch 25.4 mm. Although US surveys used metric units, when those surveys were related to customary units, the old conversion was consistently used until 1959, so rigorously speaking, converting square kilometers to acres or square miles should use the conversion of feet to meters being used at that time.

Before making such a change, we should look into whether British practice made significant use of traditional units. I know their early maps were denominated in inches, but they made the change to metric mapping (and land survey) earlier than the US. Since the international foot was midway between US and British units, if their case is relevant to survey measurements it will lead to further complications. SteveMcCluskey (talk) 04:07, 21 June 2012 (UTC)

Thank you for the two comments above. Actually my first concern was about accuracy. The figures in the article at the moment are unsourced and are significantly different from the measurements given in UnitConversion.org.

• My first concern is whether UnitConversion.org is a reliable source for the information.

Before making any change I think it is necessary to determine this question first. Michael Glass (talk) 13:21, 21 June 2012 (UTC)

I don't know if UnitConversion.org is a good source or not. I do not see any citation to it in the article. In my view the best source for US measurements is the NIST Handbook 44. This is the document relied upon by state weights and measures departments. If your measuring device used in commerce does not meet the requirements in this handbook, state weights and measures officials will seize your device and fine you. Jc3s5h (talk) 13:32, 21 June 2012 (UTC)

It's a 497 page document but it doesn't seem to refer to square kilometres, perhaps because land measurements of that size are not sold very often in the United States. Is there any other source? Michael Glass (talk) 12:21, 23 June 2012 (UTC)

I found a site from the National Geodetic Survey that answered at least some of the questions at [3] The relevant section says:

First, remember this rule: There is only one meter, BUT, there are two types of feet.

The two types of feet are:

1. The U.S. Survey Foot

It is defined as: 1 meter = 39.37 inches.

If you divide 39.37 by 12 (12 inches per foot), you get the

conversion factor: 1 meter = 3.280833333... U.S. Survey Feet.

2. The International Foot

It is defined as: 1 inch = 2.54 centimeters.

If you convert this to meters and feet, you get the conversion

factor: 1 International Foot = 0.3048 meters.

These two conversion factors produce results that differ by 2 parts per million; hence for most practical work it does not make any difference to the average surveyor which one is used since they usually do not encounter distances this large. For example, converting a distance of 304,800 meters (about 1,000,000 feet) to feet using the two conversion factors, these are the results:

304,800 meters = 999,998.000 U.S. Survey Feet

304,800 meters = 1,000,000.000 International Feet

A difference of 2 feet in 1 million feet.

Michael Glass (talk) 13:32, 24 June 2012 (UTC)

The conversion factors in Handbook 44 are in Appendix C. It does not attempt to give every combination; a certain amount of multiplication may be required on the part of the reader.

As for 2 part per million in a linear measurement not being important for the average surveyor, this is true if the measurement is from one physical monument to another on a parcel or project. But it is important if it represents the distance from the origin of the state plane coordinate system to a monument on a parcel. For example, the y coordinate of the Vermont-Quebec border in the Vermont State Plane Coordinate system is about 279000 meters. Suppose a boundary of a lot near there runs from (470000 m, 278970 m) to (470000 m, 279000 m). Suppose the points are converted to feet; the southern point is converted to survey feet and the northern point is converted to international feet. The two y coordinates come out to 915254.08 ft and 915534.33 ft, or a difference of 100.25 ft. But if the procedure were carried out correctly and the same conversion factor was used for both points, the difference would be 98.42 ft. It would't take a land surveyor to detect a blunder of 1.83 ft; a homeowner could measure a property line to that tolerance. So when dealing with state plane coordinates, survey feet and international feet cannot be considered interchangeable. Jc3s5h (talk) 01:20, 25 June 2012 (UTC)

A two part per million difference works out as 2mm per kilometre, a small but appreciable difference. These differences can add up, especially in square measures. For example, when I multiplied the number of acres by 43560 to find out how many square feet there were in a square kilometre and used the unitconversion.org figures, the result came out at more than 10 million square feet for each square foot, but there were 43 more International square feet than the slightly larger US Survey square feet!

What I am striving for in the text is clarity, simplicity and accuracy. If we're after clarity and simplicity, ignoring small differences would be appropriate. However, that would be done at the expense of accuracy.

One way round this would be to put a reasonable approximation in the text and more precise figures in a footnote. In checking my maths I found that my calculations using unitconversion.org were about the same as other websites I consulted, so I think they are reasonably reliable. How do you feel about these two proposals:

• Using the figures from unitconversion.org?
• Putting reasonable approximations in the text and more precise figures in footnotes?

Michael Glass (talk) 00:16, 26 June 2012 (UTC)

Consider the context of this article. The square kilometre is an exact multiple of the square metre, the SI unit of area and the basis of all area measure in the developed year. Even the US defines its customary units in terms of SI units. This article is not specifically about surveys or maps, nor is it US-oriented. Therefore there is no real reason to even mention US survey feet. In an acre, if it is square, the difference in area between international or survey feet are used is only about 0.17 square foot, which is always within the measurement tolerances in any land survey work. So for this article, just forget about US survey feet. Jc3s5h (talk) 01:03, 26 June 2012 (UTC)

Even though only the US uses survey feet, acres and miles I believe that this is still notable. My proposal is not to overstate the difference between the US Survey measurements and the International measurements. That is why I am happy to mention this difference in a footnote. However, I think that not mentioning it at all is going too far. In an effort to take into account your concerns I have been happy to confine the mention of this difference in a note. Is it asking for a footnote really too much for you? Michael Glass (talk) 14:10, 27 June 2012 (UTC)

Brugge and Chester

Though the information about Brugge and Chester is very interesting, their areas are nowhere near one square kilometre. Brugge is more than 3.5 square kilometres and Chester is just over half a square kilometre. Neither city is a good illustration of a square kilometre so their presence in an article about the square kilometre is not justified. What do others think? Michael Glass (talk)

I included them to show the range of sizes that one might encounter when estimating the areas enclosed by city walls and to encourage readers to make their own estimations when they arrive at an unknown city. Both have a well-defined boundary, unlike London, Rome or Paris where the medieval boundaries have all but vanished from the maps. Martinvl (talk) 06:21, 9 October 2012 (UTC)

Personally, I found these two examples very useful, having spent a while "googling" around to find some examples of metric area to help me visualise how big a piece of land with an area of 4000 sq m was. --Tickerhacker (talk) 09:52, 27 June 2013 (UTC)

Merge discussion

Proposal: merging Square kilometre and 1 E+6 m²

Rationale. It was proposed one time earlier[4] but no discussion seems to have come of it—and no merger. These to articles have overwhelmingly overlapping topics. Therefore I suggest they be merged, though I have no preference for which way. __meco (talk) 12:46, 30 November 2012 (UTC)

Oppose: These articles have very different objectives - 1 E+6 m² is a list of areas that are between 0.1 km² and 1 km² while Square kilometre is a description of the unit of measure with a few examples that are approximately one square kilometre in area added as part of the description. Martinvl (talk) 21:45, 28 January 2013 (UTC)

kilo–square metre

Is there a better wording for k(m2) than "kilo–square metre"? --Backinstadiums (talk) 11:43, 5 March 2019 (UTC)

This is a correct wording, and no better one was ever considered, because this unit is only fictional and not in practical use. Purgy (talk) 14:35, 10 March 2019 (UTC)