Tagging for the renderer
The phrase "Tagging for the renderer" and especially "Don't tag for the renderer" has a long history within OSM. More broadly it is sometimes phrased as "Don't map for the renderer" since it's not just about tagging. In any case the phrase is often misunderstood. The meaning of it is probably closer to the following:
- Don't deliberately enter data incorrectly for the renderer
but we need to live with the phrase that gets used most often. The basic good practice principle is that you avoid using incorrect tags, or otherwise skewing the data you enter, to make things show up in a specific way on the map rendering.
For example, if landuse=industrial shows up as a pink area on one of maps, and you have a flowerbed full of pink roses, then tagging your flowerbed as landuse=industrial would be incorrect and must be avoided. Instead, you should accurately tag the flowerbed with the type of plants, and improve the renderers instead so that they understand how to show it.
Another example is when someone abbreviates names, or puts spaces into them like T o w n H a l l S t r e e t or puts Kashida like علـــــــم to ensure that the names show up "nice" on the one particular map. This should be avoided as it is tagging for the renderer. It breaks the search, other map styles, and, in addition, map prettiness is subjective. It should be the renderers' job to abbreviate street names, interspace characters or stretch characters.
CERN particle accelerator rings were at one point tagged as highway=trunk and highway=primary (with tunnel=yes) (see image) even though they aren't major roads of any kind. It has since been retagged to correct highway=path/tunnel=yes/access=restricted and later changed to highway=corridor + access=private + tunnel=yes.
To collect information about how buildings look use 3D tagging rather than map fake buildings, barriers and landuses that in some renderers will look like 3D image of building.
The Great Lakes were tagged as natural=coastline, even though that is for the ocean, because the coastline ways are rendered at low zoom levels. They were changed to water=lake and as a result disappeared from the default map layer. Renderers should show big lakes at low zoom levels, without tagging them as seas.
Also, although the most common form of data skewing would be misuse of tags, this rule can apply to the manner in which elements are geometrically arranged.
A misunderstanding comes when people say that you shouldn't tag something "for the renderer" even though the tags being used are accurate and not misleading. For example, if a specialist map renders a particular specialist tag (e.g. the details of power plants) then using the tags the renderer understands is a perfectly reasonable thing to do, even if they haven't been formally "approved".
It is also perfectly fine to use various renderings as one of tools to find mistakes in what is mapped. Just remember that it is a tool, not a final and ultimate arbiter of a correct mapping.