src/Polygon.php
public function getArea(): float
    {
        $area = 0;

        if ($this->getNumberOfPoints() <= 2) {
            return $area;
        }

        $referencePoint = $this->points[0];
        $radius         = $referencePoint->getEllipsoid()->getArithmeticMeanRadius();
        $segments       = $this->getSegments();

        foreach ($segments as $segment) {
            /** @var Coordinate $point1 */
            $point1 = $segment->getPoint1();
            /** @var Coordinate $point2 */
            $point2 = $segment->getPoint2();

            $x1 = deg2rad($point1->getLng() - $referencePoint->getLng()) * cos(deg2rad($point1->getLat()));
            $y1 = deg2rad($point1->getLat() - $referencePoint->getLat());

            $x2 = deg2rad($point2->getLng() - $referencePoint->getLng()) * cos(deg2rad($point2->getLat()));
            $y2 = deg2rad($point2->getLat() - $referencePoint->getLat());

            $area += ($x2 * $y1 - $x1 * $y2);
        }

        $area *= 0.5 * $radius ** 2;

        return (float)abs($area);
    }
public function getArea(): float
    {
        $area = 0;

        if ($this->getNumberOfPoints() <= 2) {
            return $area;
        }

        $referencePoint = $this->points[0];
        $radius         = $referencePoint->getEllipsoid()->getArithmeticMeanRadius();
        $segments       = $this->getSegments();

        foreach ($segments as $segment) {
            /** @var Coordinate $point1 */
            $point1 = $segment->getPoint1();
            /** @var Coordinate $point2 */
            $point2 = $segment->getPoint2();

            $x1 = deg2rad($point1->getLng() - $referencePoint->getLng()) * cos(deg2rad($point1->getLat()));
            $y1 = deg2rad($point1->getLat() - $referencePoint->getLat());

            $x2 = deg2rad($point2->getLng() - $referencePoint->getLng()) * cos(deg2rad($point2->getLat()));
            $y2 = deg2rad($point2->getLat() - $referencePoint->getLat());

            $area += ($x2 * $y1 - $x1 * $y2);
        }

        $area *= 0.5 * $radius ** 2;

        return (float)abs($area);
    }
Calculates the polygon area.

This algorithm gives inaccurate results as it ignores
ellipsoid parameters other than to arithmetic mean radius.
The error should be < 1 % for small areas.

@return float