When OpenStreetMap put a slippy map on its front page on its front page in 2005 it used a Mercator map. This made excellent sense at the time: true north was at the top everywhere, angles were good (spherical Mercator less so than geoidal Mercator) and the raster tiles at then in use could be scrolled without spending computational power on changing their shape. Other maps at the time made the same decision for the same reasons.

Twenty years on desktop and even mobile processors can render vector tiles with consummate ease, keeping the labels in the right shape while they do so. The need to keep tiles the same shape has evaporated and many maps can be spun to put whichever direction the user wants at the top. But the disadvantages of Mercator remain: at small scales Canada and Greenland are exaggerated in size compared to Africa. At large scales this is more subtle, each zoom level maps Helsinki at a linear scale twice as large as Bogota with four times the area and shifting a zoom level doesn’t mix well with map styles that use the level to show differing amounts of detail. There is more than a further zoom level of difference between Helsinki and Longyearbyen. The widest used scheme for tile URLs leaves a hole 1000km across around each pole; this was tolerated because the is no land in the Arctic hole and, as any Australian will tell you, the southern hemisphere doesn’t count.

Many vector tiles, however, are still based on Mercator leaving the different scale at each zoom level depending on latitude in place along with the holes at the poles. This made the transition from raster to vector tiles easier but is less relevant now apart from holdouts like the OSM-Carto layer. An alternative tiling scheme is how OSM stores its coordinates: latitude and longitude. A large scale map of a small area could simply use the coordinates directly scaled by the cosine of the latitude and the scale will stay the same as the map pans north and south. A small scale map could use the Equal Earth projection favoured in a United Nations vote, another equal-area projection or one that isn’t such as an orthographic globe view — or even Mercator.

These opinions are my own.