[Tagging] Reviving the conditions debate
ewoerner at kde.org
Wed Jun 13 13:35:07 BST 2012
I want to revive the discussion on how to tag restrictions that depend on certain conditions. My idea is to finalize the proposal described in http://wiki.openstreetmap.org/wiki/Proposed_features/Extended_conditions_for_access_tags and finally accept it.
The reasons for picking this proposal:
* The proposal mostly describes syntax that is already used for tagging, e.g. a maxspeed in a certain direction is almost always tagged as maxspeed:forward, maxspeed:backward
* The proposal is intuitive and simple to use: the key of a tag is the base tag + a set of conditions that all have to be true for the key to apply (i.e. logical AND).
* The proposal is complete: every logical formula of conditions can be expressed in it (technically, it's pretty similar to disjunctive normal form).
* The proposal is consise: it follows the pattern most dominant with road restrictions, namely overriding general restrictions with special restrictions. It normally uses no more than one tag per sign.
* The proposal is extensible: the set of conditions is not fixed and can be extended to new conditions. It is possible to set a sane default for new conditions that are experimental.
* The proposal is easily implementable: I implemented it in a prototype already.
The only real negative aspect that has been mentioned until now:
* the proposal puts a lot of information into keys and theoretically allows for an unlimited set of keys. (The reality however shows that those keys tend to be the same, e.g. taginfo shows no less than 335 elements with key "maxspeed:(22:00-06:00)").
* http://wiki.openstreetmap.org/wiki/Proposed_features/access_restrictions_1.5 - in my opinion a horrible and incomprehensible syntax with arbitrary distinctions, taginfo revealed almost no uses (for example, "maxspeed:hgv:wet" in the Extended Conditions proposal above is "access:lgv?wet.speed" in the Access Restrictions 1.5 proposal)
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