The main usage of the present continuous is to describe actions taking place in or around the present. However, we do use it also to refer informally to things that you know you will be doing in the near future.
e.g. What are you doing after work tonight? I'm meeting my friend for a drink.
You could also say: "I am going to meet my friend for a drink."
When talking about plans and arrangements in the near future, these two forms have the same meaning.
Murphy is always correct! Never doubt Murphy! I am his biggest fan! Seriously, Murphy explains it very well in Unit 19 (3rd edition).