Vertically when you look at it on a map, where north is facing up.

But we both knew that.

My science teacher in middle school insisted rivers didn’t flow north, because on a globe north was up.

Yes, we tried showing her the Nile.

You are ruining the joke due to lack of sense of humor.

Also:

What if north is facing down?

What if map is not on a wall but on a table?

What if map is not oriented not in north-south direction?

You're right. I thought you were being a pedant, and I stand corrected. Vertical is a relative orientation, so the usage applies in all of the cases you indicated.

I guess a vertical navigable canal is a lock.

You can also have a lift.

https://en.wikipedia.org/wiki/Falkirk_Wheel

The Panama canal has some vertical portions, but the Suez has none.