Add missing files for sphere

projection_sphere.go

+package radolan
+
+import (
+	"math"
+)
+
+// values described in [1]
+const (
+	earthRadius = 6370.04 // km
+
+	junctionNorth = 60.0 // N
+	junctionEast  = 10.0 // E
+)
+
+func (c *Composite) projectSphere(north, east float64) (x, y float64) {
+	rad := func(deg float64) float64 {
+		return deg * (math.Pi / 180.0)
+	}
+
+	lamda0, phi0 := rad(junctionEast), rad(junctionNorth)
+	lamda, phi := rad(east), rad(north)
+
+	m := (1.0 + math.Sin(phi0)) / (1.0 + math.Sin(phi))
+	x = (earthRadius * m * math.Cos(phi) * math.Sin(lamda-lamda0))
+	y = (earthRadius * m * math.Cos(phi) * math.Cos(lamda-lamda0))
+
+	// offset correction
+	x -= c.offx
+	y -= c.offy
+
+	// scaling
+	x /= c.Rx
+	y /= c.Ry
+
+	return
+}