User talk:Marc Mongenet/town and city

place=city tagging

The Place wiki page gives 5 levels of human settlements:

Settlement	subdivision level 1	subdivision level 2
place=city	place=suburb	place=neighbourhood
place=town	place=suburb for larger town or place=neighbourhood for smaller one	place=neighbourhood for a large town with suburbs
|place=village	place=neighbourhood for larger villages
|place=hamlet		place=isolated_dwelling

The criteria for tagging place=city given by Key:place is:

The largest urban settlements in the territory, normally including the national, state and provincial capitals. These are defined by charter or other governmental designation in some territories and are a matter of judgement in others. Should normally have a population of at least 100,000 people and be larger than nearby towns. See place=suburb and place=neighbourhood on how to tag divisions within a city. The outskirts of urban settlements may or may not match the administratively declared boundary of the city.

Orthogonality problem

The place tag is the generic tag to map human settlements. For capitals (national, state, provincial, any admin level), there is the admin_centre role.

Using place for both purposes (settlements and capitals) is an error, because many provincial, state, and even some national capitals, are not a large settlement, nor the largest settlement in the territory (examples: Privas, Appenzell, Vaduz). Settlement importance and capital status are orthogonal properties.

Besides, tagging capitals with place=city solves no problem, the admin_centre role being available to rendering software. But it creates problems, because tagging as city mere towns, or even villages, destroys all expectations (see below) the map user can have when seing a city on the map.

Locating is NOT mapping

This chapter could also be named: Map what's on the ground & Don't map your local legislation, if not bound to objects in reality

In many countries, there is one (or several) list(s) of cities, cornubations (or villes, or bourgs, or cités or agglomérations or whatever it is called in the local language). Quite often, these lists are based on statistics data, and give precious informations. But blindly tagging as city the list items on the map is NOT mapping. It is only using the map to show the location of items of some list. None of these lists was produced for OSM. So, all lists must be interpreted in the light of OSM criteria to decide if a place is a city (note that I write city in monospaced font, because it is only an OSM tag value).

Of course, some lists are of great help for the place tag, while other are of very limited use, and no sane mapper would consider that copying the list as is in the map makes sense (except, of course, with a custom tag).

International consistency

Places tagged as city are displayed at medium to low zoom levels. An low zoom levels, towns are generally not displayed.

At low zoom levels, several countries can generally displayed in the browser window without scrolling. On some renderers, Europe can almost be displayed west to east on a HD screen with city names.

This fact makes the place=city tagging consistency specially important to achieve a good, consistent, map, across the continent, and the world.

What we can expect to find a city

• It has more inhabitants than surrounding towns.
• People of surrounding towns comes to the place for work, shopping, administrative procedures, specialized medical care...
• It has an airport with regular flights to others cities assured by big passenger airplanes.
• It is connected to other cities by the fastest road network in the country (motorways and 2x2 lanes road in developped countries).
• It is connected to other cities by the fastest train lines in the area.
• It is an intersection for the fastest roads or train lines.
• It has an hospital able to handle the most severe conditions (intensive care unit...)
• It has a university.
• It has museum(s) of (inter)nationnal renown.
• It has a criminal court.
• It has a prison.
• It has a stadium big enough for national and international sporting events.
• It is the siege of national or regional institutions, like:
  • head of state
  • parlament
  • government
  • supreme court

Hints that a place is not a city

• It has less inhabitants than surrounding towns.
• People have to go to a nearby city to study, work, do shopping, do administrave procedures, specialized medical care...
• It is not connected to other cities by a fast train line.
• It is connected to other cities by fast train lines, but the faster trains do not stop in the town.
• It is not connected to other cities by a fast road network.

The population tag is not enough

It could be tempting to think that the rendering software should use the population tag to override the place=town and place=city tags. It could even be clever in in using a threshold depending on the territory population density. But it is not a good idea, because:

• A large population is only one of the feature we expect in a city.
• If several small towns form a major conurbation, the renderer can not deduce the name of the conurbation (if a name exists).