You are casting from double to integer (not enough capacity).
Integer is 32 bit double is 64 bit.
You could perhaps use Int64 but probably better to convert to a unit system where you can retain similar accuracy required but only use the integer portion of the double. e.g. Int32 can represent over 7 million feet to the nearest mm I believe (and that's just the positive portion). You are unlikely to need anything near that.
You may also check you C# overflow checking options, strange that you encounter it by default, but I've never attempted casting a double to an integer in C#.
The good and the bad thing about dictionaries is that they require unique keys. Keys should also have an equality function to distinguish them.
