[Tagging] Super-keys are evil

[Frederik Ramm]

Hi,

On 02/23/2015 02:43 AM, Kurt Blunt wrote:
> Right now, tags serve two distinct purposes. There are attribute tags
> like name=Wall Street, and there are category tags like
> amenity=parking or aeroway=helipad. 

This works for many things but not all; the border can be blurred. You
will not be able to fit every key into your "category or attribute" schema.

> Many tags don't even make sense. What does highway=track mean? Is it
> a highway that acts as a track? A track is clearly not a type of
> highway. A track is just a track.

And if you travel along a dark track in the night then you might be
robbed by a highwayman.

> A contributor is left to feel like
> an idiot for not understanding the logic behind this system.

Woe betide all who mistake highway=unclasssified for a street that lacks
classification.

> For these reasons, I believe there is a case to be made for an
> overhaul of category tags. My personal opinion is that we should get
> rid of super-keys altogether and instead promote all categories to
> keys with empty values: "amenity"="reception_desk" becomes
> "reception_desk"="", "highway"="track" becomes "track"="", 
> "aerialway"="gondola" becomes "gondola"="", "barrier"="city_wall"
> becomes "city_wall"="", "historic"="city_gate" becomes
> "city_gate"="", "sport"="volleyball" becomes "volleyball"="", etc.

And power=line becomes line="" and barrier=line becomes, uh, wait a minute.

It is not so simple, even leaving aside the fact that many programs
would simply dismiss your empty values.

> Now, I don't actually think such an overhaul is currently feasible
> given the massive burden it would put on the database system.
> However, it might be something to think about for the future.

I think that in theory what you call "super keys" is a good thing to
have because it gives you a layered level of understanding. For example,
if someone tags

natural=water
water=lake
lake=turlough

then you have a chance to understand "this is a natural feature" (and
not man-made) even if you don't know what a lake is; you can understand
"this is a lake" (and not a reservoir) even if you don't know what a
turlough is; or you're so much into water bodies that you can actually
understand the full message.

If the tag was instead the space-saving

turlough=""

then you'd be stumped without recourse to the giant tag dictionary that
explains to your renderer that something tagged turlough="" should
perhaps be drawn in a blue-ish colour.

Matter in a nutshell: Certainly the way we use these "super tags" has a
lot of historical baggage but I don't think it is a stupid idea per se,
*especially* if your goal is (like you're claiming yours to be) making
tags easy for mappers.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"