Calculate Coord from start point, endpoint, and distance

Question

I have two points on a map and I know the distance between them. Now, I need to get a new point between them X meters away from start point. However, i cannot figure out, how to find the new coords.

var nextTrazadoPoint = new Coord {Lat = ....,  Lng=...., Alt=...};
var previousTrazadoPoint = new Coord {Lat = ....,  Lng=...., Alt...};
var fromCoords = new GeoCoordinate(nextTrazadoPoint.Lat, nextTrazadoPoint.Lng, nextTrazadoPoint.Alt);
var toCoords = new GeoCoordinate(previousTrazadoPoint .Lat, previousTrazadoPoint .Lng, previousTrazadoPoint .Alt);
var distance = fromCoords.GetDistanceTo(toCoords); //Let's say 1000 ¿meters?

Now I want to walk 200 meters from previousTrazadoPoint to nextTrazadoPoint

//Vector from previousTrazadoPoint to nextTrazadoPoint
var vectorDireccion = new Vector(
    (double)(nextTrazadoPoint.Latitud - previousTrazadoPoint.Latitud), 
    (double)(nextTrazadoPoint.Longitud - previousTrazadoPoint.Longitud)
    );

//Normalize
vectorDireccion.Normalize();

//meters from previousTrazadoPoint 
var distanciaARecorrer = 200;

//New coords
var vectorDestino = distanciaARecorrer * vectorDireccion;
point.Latitud = (decimal)vectorDestino.X + previousTrazadoPoint.Latitud;
point.Longitud = (decimal)vectorDestino.Y + previousTrazadoPoint.Longitud;

However, when i draw the new point on Gmaps, it is not placed between both. Any ideas?

Answer 1

Thanks to @HansKilian and @cletus (Calculate distance between 2 GPS coordinates), I could find the solution

private const double EARTH_RADIUS = 6378.1;
private static double ConvertToRadians(double angle)
{
    return (Math.PI / 180) * angle;
}
private static double ConvertToDegree(double angle)
{
    return angle * (180.0 / Math.PI);
}

private static double CalculateBearing(CoordsDto from, CoordsDto to)
{
    var from_lat_rad = ConvertToRadians(from.Latitud);
    var to_lat_rad = ConvertToRadians(to.Latitud);

    var dif_lng_rad = ConvertToRadians(to.Longitud - from.Longitud);

    double x = Math.Cos(from_lat_rad) * Math.Sin(to_lat_rad) - Math.Sin(from_lat_rad) * Math.Cos(to_lat_rad) * Math.Cos(dif_lng_rad);
    double y = Math.Sin(dif_lng_rad) * Math.Cos(to_lat_rad);

    // Math.Atan2 can return negative value, 0 <= output value < 2*PI expected 
    return (Math.Atan2(y, x) + Math.PI * 2) % (Math.PI * 2);
}

public static CoordsDto GetPointFarAway(CoordsDto from, CoordsDto to, double meterAwayFromStart)
{
    var resp = new CoordsDto();
    var bearing_rad = CalculateBearing(from, to);
    var d = meterAwayFromStart * 0.001; //KM

    var input_lat1_rad = ConvertToRadians(from.Latitud);
    var input_lon1_rad = ConvertToRadians(from.Longitud);

    var newPoint_lat_rad = Math.Asin(
        Math.Sin(input_lat1_rad) * Math.Cos(d / EARTH_RADIUS) + Math.Cos(input_lat1_rad) * Math.Sin(d / EARTH_RADIUS) * Math.Cos(bearing_rad)
    );
    var newPoint_lon_rad = input_lon1_rad + Math.Atan2(
        Math.Sin(bearing_rad) * Math.Sin(d / EARTH_RADIUS) * Math.Cos(input_lat1_rad),
        Math.Cos(d / EARTH_RADIUS) - Math.Sin(input_lat1_rad) * Math.Sin(newPoint_lat_rad)
    );

    resp.Latitud = ConvertToDegree(newPoint_lat_rad);
    resp.Longitud = ConvertToDegree(newPoint_lon_rad);

    return resp;
}