Calculate angle between two Latitude/Longitude points

Question

Is there a way to calculate angle between two Latitude/Longitude points?

What I am trying to achieve is to know where the user is heading. For example, user is head

Answer 1

For those who use C/C++, below is the tested code:

static const auto PI = 3.14159265358979323846, diameterOfEarthMeters = 6371.0 * 2 * 1000;

double degreeToRadian (double degree) { return (degree * PI / 180); };
double radianToDegree (double radian) { return (radian * 180 / PI); };

double CoordinatesToAngle (const double latitude1,
                           const double longitude1,
                           const double latitude2,
                           const double longitude2)
{
  const auto longitudeDifferenceRadians = degreeToRadian(longitude2 - longitude1);
  auto latitude1Radian = degreeToRadian(latitude1),
       latitude2Radian = degreeToRadian(latitude2);

  const auto x = std::cos(latitude1Radian) * std::sin(latitude2Radian) -
                 std::sin(latitude1Radian) * std::cos(latitude2Radian) *
                 std::cos(longitudeDifferenceRadians);
  const auto y = std::sin(longitudeDifferenceRadians) * std::cos(latitude2Radian);

  return radianToDegree(std::atan2(y, x));
}

double CoordinatesToMeters (const double latitude1,
                            const double longitude1,
                            const double latitude2,
                            const double longitude2)
{
  auto latitude1Radian = degreeToRadian(latitude1),
       longitude1Radian = degreeToRadian(longitude1),
       latitude2Radian = degreeToRadian(latitude2),
       longitude2Radian = degreeToRadian(longitude2);
  auto x = std::sin((latitude2Radian - latitude1Radian) / 2),
       y = std::sin((longitude2Radian - longitude1Radian) / 2);

  return diameterOfEarthMeters *
         std::asin(std::sqrt((x * x) +
                             (std::cos(latitude1Radian) * std::cos(latitude2Radian) * y * y)));
}