The pilot knows where they are at all times. They know this because they know where they aren't. By subtracting where they are from where they aren't, or from where they are - whichever is greater - they obtain a difference or deviation. The pilot's sense of direction uses deviation to generate corrective commands to drive the pilot from a position where it is to a position where they aren't, and arriving at a position that they weren't. Consequently, the pilot is where they are now.

