flock bearing on compass

flockompass.kv

@@ -73,3 +73,14 @@
                         origin: self.center
                 canvas.after:
                     PopMatrix
+            Image:
+                id: to_flock        
+                source: 'needle_to_flock.png'
+                canvas.before:
+                    PushMatrix
+                    Rotate:
+                        angle: app.fbearing
+                        axis: 0, 0, 1
+                        origin: self.center
+                canvas.after:
+                    PopMatrix

main.py

@@ -10,7 +10,7 @@ from kivy.uix.screenmanager import ScreenManager, Screen, RiseInTransition
 
 from kivy.vector import Vector
 from kivy.animation import Animation
-from math import floor
+from math import atan2, sin, cos, degrees, floor
 
 
 class MapScreen(Screen):
@@ -26,6 +26,7 @@ class FlockompassApp(App):
     gps_data = DictProperty()
     session_data = DictProperty()
     needle_angle = NumericProperty(0)
+    fbearing = NumericProperty(0)
 
     def dump(self, dt):
         print(dt, self.gps_data, self.session_data)
@@ -68,6 +69,18 @@ class FlockompassApp(App):
 
             self.ms.ids.mapview.center_on(self.session_data['dest_lat'],
                                           self.session_data['dest_lon'])
+        else:
+            lat1 = self.gps_data['lat']
+            lon1 = self.gps_data['lon']
+
+            lat2 = self.session_data['dest_lat']
+            lon2 = self.session_data['dest_lon']
+
+            self.fbearing = atan2(sin(lon2 - lon1) * cos(lat2),
+                                  cos(lat1) * sin(lat2)
+                                  - sin(lat1) * cos(lat2) * cos(lon2-lon1))
+            self.fbearing = self.needle_angle + degrees(self.fbearing)
+            self.fbearing = (self.fbearing + 360) % 360
 
     def get_field(self, dt):
         needle_angle = 7
@@ -88,7 +101,9 @@ class FlockompassApp(App):
         # animate the needle
         if self.cs._anim:
             self.cs._anim.stop(self)
-        self.cs._anim = Animation(needle_angle=needle_angle, d=.2, t='out_quad')
+        self.cs._anim = Animation(needle_angle=needle_angle,
+                                  d=0.2,
+                                  t='out_quad')
         self.cs._anim.start(self)
 
     def compass_enable(self):