[OpenLayers-Users] How to validate OpenLayers area calculation using PostGIS?

[João Rodrigues]

Hi

I am working on a GIS API that works on top of OpenLayers.

I have a feature that calculates the area of a drawn polygon. I have tested
it on the browser and the areas I got, appear to be correct. For example, I
have measured some country areas and they matched the real areas
approximately.

Surely, I have to run some unit tests to prove that the area calculations
are correct. I use PostGIS to generate the area value, but I get completely
different results than the ones from OpenLayers.

In PostGIS, I do this:

SELECT
st_astext(st_transform(st_geomfromtext('POINT(50 0)', 4326), 900913)),
st_astext(st_transform(st_geomfromtext('POINT(-60 -5)', 4326), 900913)),
st_astext(st_transform(st_geomfromtext('POINT(-30 -30)', 4326), 900913)),
ST_Area(
ST_GeomFromText(
'POLYGON(
(
50 0,
-60 -5,
-30 -30,
50 0
)
)'
), false
) as area

I get these results:

    "POINT(5565974.53966368 -7.08115455161362e-10)";
    "POINT(-6679169.44759641 -557305.257274577)";
    "POINT(-3339584.72379821 -3503549.84350437)";
    19582793771831 // area

I introduce these three points in my test, like this:

    var points = [
        new OpenLayers.Geometry.Point(5565974.53966368,
-7.08115455161362e-10),
        new OpenLayers.Geometry.Point(-6679169.44759641, -557305.257274577),
        new OpenLayers.Geometry.Point(-3339584.72379821, -3503549.84350437)
    ];
    var ring = new OpenLayers.Geometry.LinearRing(points);
    var polygon = new OpenLayers.Geometry.Polygon([ring]);

Then, I used the getArea() method from OpenLayers.Measure, but I got an
area of: 17050040174394.607 sq. meters

According to PostGIS, the area of this polygon should be: 19582793771831
sq. meters.

It is a big difference.

I understand hat the getGeodesicArea method gives only an approximation of
the real geodesic area, but I get results that are too different. Is there
a correction we could do in the getArea method? How could I calculate the
area with more accuracy?

Thanks

PS: I can't publish the whole code here, because it is work related.

João Rodrigues
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