How do I find the lat/long that is x km north of a given lat/long?

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I have a very similar piece of code. It got me very close results when compared to another implementation.

I think the problem with yours is that you are using “distance” as linear distance in meters instead of angular distance in radians.

/// <summary>
/// Calculates the end-point from a given source at a given range (meters) and bearing (degrees).
/// This methods uses simple geometry equations to calculate the end-point.
/// </summary>
/// <param name="source">Point of origin</param>
/// <param name="range">Range in meters</param>
/// <param name="bearing">Bearing in degrees</param>
/// <returns>End-point from the source given the desired range and bearing.</returns>
public static LatLonAlt CalculateDerivedPosition(LatLonAlt source, double range, double bearing)
{
    double latA = source.Latitude * UnitConstants.DegreesToRadians;
    double lonA = source.Longitude * UnitConstants.DegreesToRadians;
    double angularDistance = range / GeospatialConstants.EarthRadius;
    double trueCourse = bearing * UnitConstants.DegreesToRadians;

    double lat = Math.Asin(
        Math.Sin(latA) * Math.Cos(angularDistance) + 
        Math.Cos(latA) * Math.Sin(angularDistance) * Math.Cos(trueCourse));

    double dlon = Math.Atan2(
        Math.Sin(trueCourse) * Math.Sin(angularDistance) * Math.Cos(latA), 
        Math.Cos(angularDistance) - Math.Sin(latA) * Math.Sin(lat));

    double lon = ((lonA + dlon + Math.PI) % UnitConstants.TwoPi) - Math.PI;

    return new LatLonAlt(
        lat * UnitConstants.RadiansToDegrees, 
        lon * UnitConstants.RadiansToDegrees, 
        source.Altitude);
}

Where

public const double EarthRadius = 6378137.0;   //  WGS-84 ellipsoid parameters

and LatLonAlt is in degrees/meters (conversion takes place internally).
Adjust as needed.

I assume you can figure out what the value for UnitConstants.DegreesToRadians is 🙂

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