[OSM-talk] Proposal superseded_by tag for ID stabilisation

[Gregor Horvath]

Hi,

critique welcome:

Proposal superseded_by tag
--------------------------

OSM ID's do not describe physical objects but geometrical objects on a
map. If an a geometrical object supersedes another one with a different
ID it should be possible to tag this relationship for some better
stabilisation and traceability of ID's.

* new TAG superseded_by=type:ID
  
  example:
  superseded_by=way:7867678
  superseded_by=node:67668
  superseded_by=rel:7897789

  I do not think that a superseded tag on the new object is necessary
  because it is redundant data and an unnecessary maintenance burden.

* Use Cases ID deletion

Below are the cases for deleting a ID mentioned on the mailing list.
I've added the proposed solution.

** expansion of POI nodes into buildings
   node is deleted and gets superseded_by tag pointing to the building

** splitting of ways (old ID would then point to only half)
   A split to A,B:
     A.superseded_by = B
     A does not get deleted

** superseded by several objects (a split operation).
   A gets replaced by B,C
     A.superseded_by = B
     A.superseded_by = C

** merging (old ID would become invalid in 50% of cases)
   - example 1: Relations for long-distance routes created in several
                places until they "meet", and are merged, with one of
                them being deleted.
   - example 2: a POI node might later be joined to be part of an area
                representing a shop;  this shop area might later be
                joined to others to represent an entire building that
                contains several shops 

   A,B gets merged to A: 
     A.superseded_by = B
     B deleted (visible=false)
     B.superseded_by = A

** supersede multiple objects (a merge operation) 
   A,B,C  gets replaced by C
     A.superseded_by = C
     B.superseded_by = C
     C.superseded_by = A
     C.superseded_by = B
     A,B deleted (invisible)

** re-mapping of stuff in the course of the license change
   - re-structuring of relations
   - 0.7 area data type (lots of existing areas get a new ID)
   
   superseded_by tag

** mapping error (mistake)
   delete ID, no superseded_by
   
** restaurant moves, should it keep the same ID? If it is renamed? If
   it goes bankrupt, is sold, renovated, and reopened by a different
   owner 

   the node describes a point on the map, not a restaurant,
   therefore no new ID in this cases.