Warn when boundary relation is no longer closed

[ZeLonewolf]

I operate a site that is a data consumer of administrative boundaries. I am seeing a handful of administrative boundaries get broken each day by changesets from iD users, and I am constantly having to go in and fix these boundaries. Often when contacted, the changeset authors noted that there was no warning given and everything "looked fine".

The most common errors are gaps in the boundary and extra segments added to the boundary.

Below are some recent changesets where this issue occurred.

https://www.openstreetmap.org/changeset/96835302
https://www.openstreetmap.org/changeset/97199863
https://www.openstreetmap.org/changeset/97060209

It would be great if there was a better way to detect that a change was made to a boundary relation and do some additional validation to prevent breakages.

[ZeLonewolf]

Here is an additional case (iD 2.19.5): https://www.openstreetmap.org/changeset/98004483

[1ec5]

The most common errors are gaps in the boundary and extra segments added to the boundary.

Boundaries normally have similar geometry requirements as multipolygons. Multipolygons do enjoy geometry validation, plus inner-outer role validation, though there are situations where iD has only partially downloaded the relation so the validation wouldn’t take place. Boundaries have some additional complications: if part of the boundary is disputed, there will be some redundant outer or inner ways. Also, a boundary relation can contain admin_centre and label nodes.

[ZeLonewolf]

Issue is still present as of iD 2.20.4:
https://www.openstreetmap.org/changeset/118554695

[natfoot]

I use ID mostly for my edits and if I edit and an existing multipolygon (...Node Line Node Line Node Line Node Line Node...) and remove the center node in the multi polygon there is now a hole in the multiploygon. Save. When I save I get no errors that the multipolygon is now (broken). No warning no errors are being displayed.

[ZeLonewolf]

An additional case of boundary breakage is when one of the members which should be inner is outer (or vice versa), for example, in this changeset.

[1ec5]

Multipolygons do enjoy geometry validation, plus inner-outer role validation, though there are situations where iD has only partially downloaded the relation so the validation wouldn’t take place.

For boundaries, it’s usually the case that the boundary hasn’t been fully downloaded yet, which would prevent any validator rule from being able to detect a gap, fork, or incorrect role.

#4903 tracks automatically downloading the members of a relation as soon as you select it, which would unblock this feature. #6656 tracks automatically downloading the members of a relation while you use iD, so that hopefully all the members would be available by the time you open the panel. #5420 tracks a button for explicitly downloading all the members at once; perhaps iD could refuse to perform certain risky operations until the user clicks this button.