Explanation of magic constants

prog/data_utils.py

@@ -28,9 +28,14 @@ def dist(a, b):
     assert 40 < a[0] and a[0] < 60, a[0]
     assert  5 < a[1] and a[1] < 25, a[1]
     assert 40 < b[0] and b[0] < 60, b[0]
-    assert  5 < b[1] and b[1] < 25, b[1]
+    assert  5 < b[1] and b[1] < 25, b[1] # Check that we are somewhere near Prague
     #           lat                           lon
     return math.sqrt(((a[0]-b[0])*111.2)**2 + ((a[1]-b[1])*71.50)**2)
+    # ... and then use distance approximation based on length of one degree there.
+
+lon_muntiplicator = 71.50/111.2
+# This is magic constant for approximation globe to plane working near Prague.
+# If lon is multiplied with this constant, one unit of lat and multiplied lon will have approximately the same distance
 
 def shape_indexer(shape, i: float):
     ii = int(i)
@@ -40,7 +45,6 @@ def shape_indexer(shape, i: float):
     return shape[ii]*(1-rest) + shape[ii+1]*rest
 
 
-lon_muntiplicator = 71.50/111.2
 async def get_data_of_trip(trip_id, date_from, date_to):
     c = await get_communication()
     dts = await c.list_realtime_data(date_from, date_to)