Dart code to calculate distance between two lat/lng decimal coordinates

distance.dart

import 'dart:math';

/// Describes a distance unit for conversion
enum Unit {
  miles,
  kilometers,
  nauticalMiles,
}

/// Describes a decimal coordinate
class Coordinate {
  num latitude;
  num longitude;
  String name;
  
  Coordinate({this.name, this.latitude, this.longitude});
}

///  This routine calculates the distance between two points (given the
///  latitude/longitude of those points). It is based on free code used to
///  calculate the distance between two locations using GeoDataSource(TM)
///  products.
///
///  Definitions:
///    South latitudes are negative, east longitudes are positive
///
///  Passed to function:
///    Coordinate a = Latitude and Longitude of point 1 (in decimal degrees)
///    Coordinate b = Latitude and Longitude of point 2 (in decimal degrees)
///    optional: Unit = the unit you desire for results
///           where: 'Unit.miles' is statute miles (default, or omitted)
///                  'Unit.kilometers' is kilometers
///                  'Unit.nauticalMiles' is nautical miles
///
///  Worldwide cities and other features databases with latitude longitude
///  are available at https://www.geodatasource.com
///
///  For enquiries, please contact sales@geodatasource.com
///
///  Official Web site: https://www.geodatasource.com
///
///          Dart code James Robert Perih (c) All Rights Reserved 2021
///
///           GeoDataSource.com (C) All Rights Reserved 2017
///
///
num distanceBetweenTwoPoints(
  final Coordinate a,
  final Coordinate b,
  final Unit units,
) {
  final num radlat1 = pi * a.latitude / 180.0;
  final num radlat2 = pi * b.latitude / 180.0;
  
  final num theta = a.longitude - b.longitude;
  final num radtheta = pi * theta / 180.0;
  
  num distance = sin(radlat1) * sin(radlat2) + cos(radlat1) * cos(radlat2) * cos(radtheta);
  if (distance > 1) {
    distance = 1;
  }
  
  distance = acos(distance);
  distance = distance * 180.0 / pi;
  distance = distance * 60 * 1.1515;
  
  switch (units) {
    case Unit.kilometers:
      distance = distance * 1.609344;
      break;
    case Unit.nauticalMiles:
      distance = distance * 0.8684;
      break;
    case Unit.miles:
    default:
      // intentionally left blank
  }
  
  return distance;
}

/// test implementation
void main() {
  Coordinate winnipeg = Coordinate(name: 'Winnipeg', latitude: 49.895077, longitude: -97.138451);
  Coordinate regina = Coordinate(name: 'Regina', latitude: 50.445210, longitude: -104.618896);
  Coordinate calgary = Coordinate(name: 'Calgary', latitude: 51.04369632775411, longitude: -114.04254321432143);
  
  print('Distance between ${winnipeg.name} & ${regina.name}: ${distanceBetweenTwoPoints(winnipeg, regina, Unit.kilometers)}');
  print('Distance between ${regina.name} & ${calgary.name}: ${distanceBetweenTwoPoints(regina, calgary, Unit.kilometers)}');
  print('Distance between ${winnipeg.name} & ${calgary.name}: ${distanceBetweenTwoPoints(winnipeg, calgary, Unit.kilometers)}');
}