AP_Math: use DEG_TO_RAD in longitude_scale

Also increased accuracy of RadiansToCentiDegrees although it is like the
compiler will throw away the extra digits anyway.

libraries/AP_Math/AP_Math.h

@@ -25,7 +25,7 @@
 #define DEG_TO_RAD 0.017453292519943295769236907684886f
 #define RAD_TO_DEG 57.295779513082320876798154814105f
 
-#define RadiansToCentiDegrees(x) ((x) * 5729.578f)
+#define RadiansToCentiDegrees(x) ((x) * 5729.5779513082320876798154814105f)
 
 // acceleration due to gravity in m/s/s
 #define GRAVITY_MSS 9.80665f

libraries/AP_Math/location.cpp

@@ -36,7 +36,7 @@ float longitude_scale(const struct Location *loc)
         // the same scale factor.
         return scale;
     }
-    scale = cosf((fabsf((float)loc->lat)/1.0e7f) * 0.0174532925f);
+    scale = cosf((fabsf((float)loc->lat)/1.0e7f) * DEG_TO_RAD);
     last_lat = loc->lat;
     return scale;
 }