I do agree that if a pipe is open to atmosphere, the exit pressure is atmospheric. However, I am not sure about the pressure profile along the pipe from the upstream to downstream (atmospheric tank). For example in my case, the pressure gauge about 30ft before entering to the atmospheric tank is about 50psig. Does "atmospheric pressure on the exit" mean that pressure is smoothly decreasing along the pipe from 50 to 0 psig or pressure sharply decreases right before the exit? 

It gets more interesting if the fluid is moving downward through a vertical pipe to an open to atmospheric tank. I believe the pressure along the downward movement increases (elevation head to static head), and then opens to atmosphere.

Here I attached another example to elaborate more. Draining water from a tank through a horizontal pipe. How is the pressure profile along the pipe? If we calculate the pressure on the pipe&tank connector as 50 psig, and the pipe is open to atmosphere, what is the pressure in the middle for example?

It'd be great if you could help me out.