Solution: A is AE⊥BC, and the vertical foot is E. According to the meaning of the question, the quadrilateral ADCE is a rectangle.

∴EC=AD= 15，

In Rt△AEC, tan∠EAC=,

∴ AE = = 5 (m),

At Rt△AEB, tan∠BAE=,

∴BE=AE？ tan∠EAB=5？ Tan 30 = 5 (meters),

∴BC=CE+BE=20 (meters).

A: The height of the flagpole is 20 meters.

The depression angle is the angle formed by overlooking something, of course, when this thing is in your sight, take what you see as the horizontal line and the line between glasses and that thing as the edge.

Elevation is the angle formed by looking up at something, holding it above your line of sight, taking what you see as a horizontal line, and taking the line between glasses and that thing as an edge.