Determining the area in m2 of a polygon

[Hannes]

Hi GeoDjango, 

I am struggling with a simple problem, but I can't find a solution (via google or stackoverflow). I am hoping someone of the GeoDjango group knows the answer.

In my model I have a polygon field defined via

polygon = models.PolygonField(srid=4326, geography=True, null=True, blank=True)

When I want to determine the area of the polygon, I call

area_square_degrees = object.polygon.area

But how can I convert the result in square degrees into m2 with GeoDjango? This answer does not work, sincearea does not have a method sq_m. Is there any built-in conversion available in GeoDjango?

Thank your for your reply!

Best regards, 
Hannes

[jabrane]

why do you choose the wgs84?
instead you could try with a metric srid then the aerea would be in m2 

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jabrane%

[jabrane]

you can try out these links :

http://gis.stackexchange.com/questions/9621/openlayers-geometry-how-to-convert-area-from-degrees-to-square-kilometres

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

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jabrane%

[Hannes]

Do you know an appropriate srid for the metric system?

Thank you for your reply!

[Max Demars]

3857 is the most common. Web Mercator.

[Hannes]

Hi Max, 

Thank you for your reply.

I have implemented the conversion to 3857, save the area size and convert back to 4326, since the coordinates are stored in degrees.

        object.polygon.transform(3857)

        object.polygon_size = object.polygon.area

        object.polygon.transform(4326)

It seems non-pythonic to convert twice but it works for now.

Do you know how resource intense the conversion is?

[Max Demars]

Hi Hannes,

It's true that transforming data twice is not ideal. What is your database backend? If you use PostGIS 1.5 or newer, with geography=True it should return distance in meters. Perhaps you can try spheroid=True instead of geography=True. see: https://docs.djangoproject.com/en/dev/ref/contrib/gis/model-api/#geography

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[Max Demars]

Other solution would be to translate degree to meters, with a custom python method instead of transforming the data.

[jabrane]

I can suggest google epsg 90013
as I know geodjango works with openlayers so you can calculate aerea
with srid 90013 ( use on-the-fly reproection or coordinate conversion)

good luck

>>>>>>> degrees<http://en.wikipedia.org/wiki/Square_degree> into
>>>>>>> m2 with GeoDjango? This
>>>>>>> answer<http://gis.stackexchange.com/questions/9673/area-units-for-geos-area-property>