Final answer:
Safe Altitude within 100NM is measured from the highest obstacle in the area; it ensures sufficient clearance for aircraft and is published in aeronautical charts like Enroute and Area Charts. An effective air defense perimeter may alter altitude considerations near SAM sites, where current information is provided through NOTAMs and advisories.
Step-by-step explanation:
The Safe Altitude within 100NM (nautical miles) is typically measured from the highest obstacle within the area, which could include mountain peaks, structures, or other obstacles that impact the safety of flight operations. This altitude ensures a sufficient clearance around these obstacles for aircraft navigation safety. Safe altitudes and information on other airspace-related elements are published in aeronautical charts and publications such as the Enroute Chart and the Area Chart, provided by aviation authorities like the Federal Aviation Administration (FAA) in the United States.
When operating within an effective air defense perimeter, such as near a confirmed or possible Surface-to-Air Missile (SAM) site, different rules and safe altitudes may apply to ensure the safety of civilian aircraft from potential hazards associated with these sites. Pilots and aviation personnel rely on NOTAMs (Notices to Airmen) and defense-related advisories for current information in these scenarios.