Pilots must learn the meaning of dozens of symbols. What does this one mean?

Answer Reporting point

Aeronautical charts are full of symbols. This black triangle indicates a compulsory reporting point. In such cases, the point is at a geographical location, defined by fixes or navigation aids, in which pilots should report to Air Traffic Control. When ATC informs pilots that their aircraft is in "radar contact," pilots no longer have to report the points indicated.

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