calculate polygon area in square meters

polygon-area.js

function calculatePolygonArea(coordinates) {
    // Check if the polygon is closed (first and last points should match)
    const first = coordinates[0];
    const last = coordinates[coordinates.length - 1];
    if (first[0] !== last[0] || first[1] !== last[1]) {
        coordinates.push([...first]); // Close the polygon if not already closed
    }

    // Calculate average latitude for longitude scaling
    const latitudes = coordinates.map(coord => coord[1]);
    const avgLat = latitudes.reduce((sum, lat) => sum + lat, 0) / latitudes.length;
    const latToKm = 111.32; // 1 degree latitude ≈ 111.32 km
    const lonToKm = latToKm * Math.cos(avgLat * Math.PI / 180); // Adjust longitude scale by latitude

    // Convert coordinates to kilometers relative to the first point
    const refLon = coordinates[0][0];
    const refLat = coordinates[0][1];
    const pointsInKm = coordinates.map(([lon, lat]) => [
        (lon - refLon) * lonToKm, // x in km
        (lat - refLat) * latToKm  // y in km
    ]);

    // Shoelace formula
    let area = 0;
    for (let i = 0; i < pointsInKm.length - 1; i++) {
        const [x1, y1] = pointsInKm[i];
        const [x2, y2] = pointsInKm[i + 1];
        area += (x1 * y2) - (y1 * x2);
    }

    // Final area in square kilometers
    area = Math.abs(area) / 2;
    return area;
}