How useful is a tag that exists only once?
The OSM database is simple: A key and a value for every node you want to store into that database. And because it is a liberal database with no checking at all, you can put anything you like, in it.
Of course we have some rules and guidelines:
From that we can learn that amenity=hospital is the preferred way to mark (you guessed it) a hospital on the map. Which is used 122164 times on the map (taginfo)
I think most map users have a clear idea on what exactly is meant by that specific tag combination.
But what about this one:
According to the wiki, man_made is:
A tag for identifying man-made (artificial) structures added to the landscape.
Below you see what this man_made=1417-32 looks like: Can you see what it is?? I don’t!
Looking it up in the wiki for that specific value gives (you guessed it) an empty page. Why does one create a specific value for a key, without explaining to others what it means??
On taginfo you can find 1132 entries (75 screen pages) for man_made with a uniqe (used only once) value. This is the last page of taginfo on the man_made key:
And the last entry is “junk”.
I looked that one up again: Of course (you guessed it) there is no wiki page on man_made=junk.
In terms of data storage, those 1132 entries don’t take much room, but why use them in the first place? Could the mapper - at least - not have taken the steps (setting up a wiki on that value) to help other mappers with this obscure value?
So please, mappers. If you create a value for a key that is not yet used anywhere else in the database (and think thrice before doing so), be so kind to the other mappers and explain what you are doing and why you are doing it! And even better: discuss this first on the appropriate forum or talk-list.