Function to calculate distance between two lat and lon points.

distance_between.dart

import 'dart:math' as math;

void main() {
  /// Online Tool https://www.meridianoutpost.com/resources/etools/calculators/calculator-latitude-longitude-distance.php?
  final distance = DistanceUtils.distanceBetween(40.689202777778, -74.044219444444, 38.889069444444, -77.034502777778);
  
  print(distance);
}

abstract class DistanceUtils {
  static String formatDistance(int meters) {
    final km = (meters * 0.001).toStringAsFixed(1);

    return km;
  }

  /// Implementation of [this algorithm](https://stackoverflow.com/questions/27928/calculate-distance-between-two-latitude-longitude-points-haversine-formula)
  static double distanceBetween(double lat1, double lon1, double lat2, double lon2) {
    final p = math.pi / 180;

    final cos = math.cos;

    final value = 0.5 -
        cos((lat2 - lat1) * p) / 2 +
        cos(lat1 * p) * cos(lat2 * p) * (1 - cos((lon2 - lon1) * p)) / 2;

    return 12742 * math.asin(math.sqrt(value));
  }
}