Which coordinate system is used to calculate area of polygons?

[sairahul1526]

as the title says

And can we change the default coordinate system?

[peterstace]

Simplefeatures doesn't know about coordinate systems, everything is just a plain number.

So, for an area calculation, the input units will flow through to become output units. For example, take this polygon of a park:

package main

import (
	"fmt"
	"math"

	"github.com/peterstace/simplefeatures/geom"
)

func main() {
	g, err := geom.UnmarshalWKT("POLYGON((151.19536 -33.86561,151.19529 -33.86599,151.19621 -33.86624,151.19646 -33.86604,151.1964 -33.86575,151.19536 -33.86561))")
	if err != nil {
		panic(err)
	}
	fmt.Println("area (square degrees):", g.Area())
}

This will print:

area (square degrees): 4.939000000025871e-07

This will give you the area of the polygon in square degrees, which isn't very useful.

What you need to do is reproject the coordinates to a coordinate system that is equal area and has more appropriate units. Any equal area will do, so I normally use a simple one such as Lambert cylindrical equal area.

This will give you the area that you're looking for:

package main

import (
	"fmt"
	"math"

	"github.com/peterstace/simplefeatures/geom"
)

func main() {
	g, err := geom.UnmarshalWKT("POLYGON((151.19536 -33.86561,151.19529 -33.86599,151.19621 -33.86624,151.19646 -33.86604,151.1964 -33.86575,151.19536 -33.86561))")
	if err != nil {
		panic(err)
	}
	fmt.Println("area (square meters):", g.Area(geom.WithTransform(wgs84ToLambertEqualArea)))
}

func wgs84ToLambertEqualArea(xy geom.XY) geom.XY {
	xy = xy.Scale(math.Pi / 180) // Convert from degrees to radians.
	xy.Y = math.Sin(xy.Y)        // Project onto the cylinder.
	const earthRadius = 6371000  // Scale to meters.
	return xy.Scale(earthRadius)
}

This should give you the true area in square meters:

area (square meters): 5070.689245845211