public override double?GetAltitude(HealCoor basin)
{
var altitude = Relief.GetAltitude(basin);
var altitudeShape = ReliefBed.GetAltitude(basin) + Earth2014Manager.Radius2Add;
var thetaTan = Math.Tan(basin.Beta.Value); /* may be out of range */
var undulation = altitudeShape - altitude
- Ellipsoid.Radius(Ellipsoid.CalcVarPhi(thetaTan));
return(undulation);
}