Final answer:
High-level controlled airspace is a defined altitude range that starts at a specified altitude above sea level to separate aircraft and ensure safe navigation, though this can vary based on regulations by country.
Step-by-step explanation:
The phrase 'the northern high level control airspace is _____ feet ASL and above' refers to a specific altitude range that is designated for managed air travel. High-level controlled airspace typically starts at a specified altitude above sea level (ASL) and extends upwards to a certain point. There are no universal altitudes for airspaces as they can vary by country and are subject to aviation authorities' regulations. However, based on the altitude measurements provided - 10,000 ft., 5,000 ft., 2,000 ft., 1,000 ft., and Below sea level - it can be inferred that the high level control airspace would start at an altitude higher than the surrounding terrain and is meant to separate aircraft from lower flying traffic, ensuring safer navigation.