It's been a while since I unfolded one but I have a fair few Landrangers etc on the bookshelf. On OS (UK - Ordnance Survey) maps, there is always a set of thre

It's been a while since I unfolded one but I have a fair few Landrangers etc on the bookshelf. On OS (UK - Ordnance Survey) maps, there is always a set of three arrows that show grid/true/magnetic north. I suspect that most countries will have a similar system. When you consider how hard it is to represent geography and direction on paper, a localised grid is an obvious starting point. I am pretty sure that all countries will have their own version of grid north, just as they have a mean sea level(s). In the UK we have mean sea level in Cornwall, specifically in Newlyn harbour under a small hut. Bit odd but no odder than most countries that also maintain a sea level datum, which is pretty much all of them. Some have more than one and in some really strange places. The world is a really odd shape! Mercator is a projection, ie a method of how to make a 3d thing into a fairly useful 2d model of the 3d thing. No idea what a web Mercator is but I doubt the Flemish bloke minds too much. As you imply Mercator does preserve "north" but it is curved in that projection so not "up", unless you allow for "up" to be a curve.