Modern maps are always depicted with the northern hemisphere as the "top" of maps, and the southern hemisphere as the bottom, as seen here: http://www.mapsofworld.com/world-political-map-2000px.jpg

I have seen recently, however, some maps that show the opposite as true, more as a teaching point, possibly to show a "northern hemisphere bias," shown here: http://cdn.shopify.com/s/files/1/0071/5032/products/upside_down_2.png?v=1357282201

Or here:http://en.wikipedia.org/wiki/South-up_map_orientation

So at what point did this become the norm for map drawers and historians? Is this a result of western imperialism? A natural result of the geography of the earth?

Thanks for your expertise!