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I get a multypolygon from my web service and want to calculate the area of that polygon. I was trying to use this piece of code:

var poly = new OpenLayers.Geometry.MultiPolygon(poligon);
var area = geom.getArea();

But that returns the area in square degrees. The projection is EPSG:3243.
I then tried to use 

OpenLayers. Layer. SphericalMercator

But that didn't help me a lot. Is there any other solution or some other hint?

Cheers!

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2 Answers

up vote 2 down vote

Try this:

var poly = new OpenLayers.Geometry.MultiPolygon(poligon);
var area = geom.getGeodesicArea();

var kmArea = area * 1000;

Reference: http://dev.openlayers.org/docs/files/OpenLayers/Geometry/Polygon-js.html#OpenLayers.Geometry.Polygon.getGeodesicArea

EDIT: Sorry, don't multiply by 1000 DIVIDE by 1000000.

var kmArea = area / 1000000;
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I think you need to divide by 1e6 to get square km not multiply by 1000. – iant♦ May 13 '11 at 14:42
Not sure, but wouldn't it be divide by 1000 since the function returns square meters? I definitely goofed by multiplying by 1000 but what i meant was divide by 1000. – CaptDragon May 13 '11 at 14:49
1  
there are 1000m in a Km so 1000*1000 sq m in a sq km – iant♦ May 13 '11 at 14:54
+1 Okay i follow you, excuse my math disorder, but then it would be area / 1000000? – CaptDragon May 13 '11 at 15:02
2  
that's what I said :-) – iant♦ May 13 '11 at 15:09
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up vote -1 down vote

as far as I know it is not possible. Some references to it:

http://support.esri.com/en/knowledgebase/techarticles/detail/27851

http://forums.esri.com/Thread.asp?c=9&f=85&t=132922

You will need to transform to meters first.

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4  
actually there is another function in geometry getGeodesicArea() wich returns aproximated area in square meters! – vaske May 13 '11 at 9:10
1  
@vaske: Is your comment the answer to your question? – underdark♦ May 13 '11 at 9:25
2  
This is OpenLayers not ArcMap. – CaptDragon May 13 '11 at 13:59
I know that, but the question was how to convert square degrees to square meters and I suggested to do it before getting area. – Mykolas Simutis May 13 '11 at 14:24
@underdark yes that's the answer :) – vaske May 16 '11 at 6:18
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