Hi Paul Norman
Not so long time ago you have triggered a discussion with your article “Moving to stricter multipolygon parsing”. The discussion lasted two/three days, was intense and rather divergent (demonstrating that the multi-polygon issue is still not “only for beginners”). After that I have not seen or heard anything related to the issue on this forum. So, my question is what happened?
As I understand, your suggestion is to allow tags only on the MPR level and these tags should be none-conflicting (well, the WIKI documentation shows that this was the original intention with MPR). The suggestion could have large positive impact on the (area class) data quality. Therefore I fully support the suggestion but yet I am not sure about the implementation.
To shorten the discussion I would mention just a few arguments causing the dilemma to me.
1. Your implementation is based on the assumption that the mappers will check (lookup) the edits in a map that uses osm2pgsql as a parser. What if the MPR conversion to geometries is not using osm2pgsql? Here the mappers will still probably see the edits no matter where they put the tags. So, the restrictions should come much earlier, probably in the editor systems.
2. At the same time, inserting the suggested restrictions in editors will cause contradictions with the fundamental OSM documents. The WIKI sections defining and illustrating the Relation and MPR notions not only allow but even suggest putting tags on the members (even on border segments, on holes…). So, in my opinion, the restrictions should be first implemented in the OSM wiki documentation by refining/correcting the related sections.
3. Finally, the assumed “do-ocracy” (someone, once in the future, will detect and correct the error) does not work very well. There are many reasons to that. Let me mention two. There is a huge number of errors (significant and “systematic” not counting POI related and of semantic nature). So, it is maybe illusory to assume that the do-ocracy can cope with so many of them. Further, many of these errors are never visible in raster maps, mostly used by mappers. Consequently, do-ocracy will probably even not detect them. But the errors are there and in layered vector mapping these will be probably immediately visible. Just take the large number of river sections tagged as lakes (or the contrary), replicated or almost replicated areas/MPRs with different structures or just take the thousands of closed riverlines (waterway=river).
Now, if these dilemmas are not only mine, then I would suggest an alternative implementation model:
1. An OSM voluntary expert team should go through the WIKI documentation and refine the MPR related notions and implement the restrictions.
2. The editor systems, used by mappers, should accordingly implement the mentioned restrictions.
3 The expert team should use programs to detect the MPR related “systematic” (versus random) errors and programmatically correct them in the source data.
Thanks for the attention, Sandor.