Lass uns nach oben gehen / Lass uns nach unten gehen

I have previously seen a reply, where it was explained that “nach” is used in the following situations:
 - City names
 - Countries without articles
 - Continents
 - Geographic directions

However, I have just seen these sentences, where “nach” is used in the different way. Is there something, which I have missed?
 - Lass uns nach oben gehen
 - Lass uns nach unten gehen

If you look up nach in the Leo dictionary, you will see many examples of using nach with directions such as up and down east and west etc.