How are TAA boundaries primarily defined?

The correct answer highlights that Terminal Area Arrival (TAA) boundaries are primarily defined by magnetic course. Specifically, TAAs are structured to facilitate safe arrival in controlled airspace, especially around busy airports. The boundaries are designed based on various magnetic courses that guide pilots on the correct paths during the approach phase.

Using magnetic courses helps to ensure that aircraft are properly aligned with the arrival paths and can maintain safe separation from other traffic. The defined courses make it easier for pilots to maintain situational awareness and navigate effectively, especially in complex terminal environments where there may be multiple runways and approaches.

While elevation restrictions, landmarks, and distances from navigation aids play roles in aviation navigation, they are not the defining factors for TAA boundaries. Instead, the focus is on ensuring that paths align with the established magnetic courses to streamline approach procedures and enhance safety as aircraft enter the terminal area.