On Thu, Jul 8, 2010 at 2:19 PM, Roark D Hennessy
<[email protected]> wrote:
> First I hope this is the right place to post.
> I want to get the square miles (or metres) from a given polygon (I used a
> well known area feature as test data).
> I took the example:
>
> import org.geotools.geometry.jts.JTS;
> import org.geotools.referencing.CRS;
>
> CoordinateReferenceSystem sourceCRS = CRS.decode("EPSG:4326");
> CoordinateReferenceSystem targetCRS = CRS.decode("EPSG:23032");
>
> MathTransform transform = CRS.findMathTransform(sourceCRS, targetCRS);
> Geometry targetGeometry = JTS.transform( sourceGeometry, transform);
>
> and
> The sourceGeometry is a polygon representing a state boundary.
> I did a targetGeometry.getArea();
> The result is way off ( I tested against CT) 5.2836281456E9 for CT. it
> should be around 5400 sq miles give or take a couple hundred. One site has
> it at 5544 sq miles.
> I also did Nebraska and got 6.70387746...E10, should have been approximately
> 77,300 sq miles.
> questions:
> What is the return value units?
The returned value is the units of your projection squared. So
according to units
You have: m^2
You want: miles^2
* 3.8610216e-07
/ 2589988.1
You need to calculate
5.2836281456E9 * 3.8610216e-07 = 2040.02024
which looks off (but much closer) to your answer of 5400 (though the
census bureau says 4844
http://quickfacts.census.gov/qfd/states/09000.html)
> Should I be using a different CRS for north america as target?
EPSG:2163 is probably better.
Ian
--
Ian Turton
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