Final answer:
The southern high level control airspace generally starts at 10,000 feet above sea level (ASL). This altitude provides separation between levels of airspace for safe and organized air travel.
Step-by-step explanation:
The southern high level control airspace is defined by a specific altitude above sea level (ASL), which is usually standardized for purposes of air traffic control and aviation regulations. Although the exact value can vary by region and is subject to change by aviation authorities, a common altitude for high level control airspace starts at 10,000 feet ASL and above. This altitude is used as a reference to ensure the safety and organization of aircraft travel by providing a clear separation between different levels of airspace, where different rules and levels of control may apply.