Dennis writes:

I can't tell you where it would be written in regs or fars but Troutdale
coordinates with PDX to direct traffic to turn prior to entering PDX
airspace or to stay below their airspace and flight paths. Heading west
from Troutdale you are instructed to turn before Blue Lake.

The part that confuses me is that the Class D for Troutdale supposedly
tops out at 2500, but the overlapping sectors of the Class C for PDX
start at 1700 and 2000. The dotted lines for the Class D are inside
the Class C boundaries. So at what altitude would an aircraft pass
from the Class D into the Class C?

I had not previously seen this on a chart. I've seen Class C
impinging on Class B but then the Class C is usually marked T for the
upper limit, so that you know it ends just where the Class B begins.

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