When we (I didn’t really participate being a noob at the time) imported the municipality borders back in 2012 from 2011 data from swisstopo what we didn’t really know at the time that that data was still very very much a work in progress. As we typically did our following annual updates much much earlier than swisstopo we relied on information from the BfS which in general was and is OK but certain aspects were not covered, for example as I noted in the relevant threads we didn’t catch certain cantons changing the way they handled lakes and municipality borders.
As @habi is running QA checks on the borders now it would be advantageous to adjust the borders to follow the lake line as the swisstopo data set does to avoid false positives, but it could just as well be argued that nobody noticed to now so who cares. Then there’s the separate question if we should then add the fake municipality in just as swisstopo has.
Just so that we are all on the same page, I’m pausing after Kilchberg Relation: Kilchberg (ZH) (1682152) | OpenStreetMap so there is time to stop the process if anybody is very unhappy, but I’ll probably do all of the remaining (around Lake Zürich) in one go after this.
What I haven’t done is added the BfS fake municipality for the lake. Do we need a poll on that?
Just a quick update: I’ve finished work on all the municipalities of the Canton Zürich along the lake, leaving the borders in St. Gallen and Schwyz to work on (and the other lakes in the canton and the issues in Berne :-)). There was one slightly unexpected hick-up: there are a couple of post code boundaries, for example Männedorf, that extend in to the lake that I have left as is, this may or may not be correct and I’ll check those at a later date.
Actually St. Gallen and Schwyz have “the lake area belongs to the municipality” doctrines, so Lake Zürich is finished, leaving the Greifen- and Pfäffikersee in the canton Zürich and the lakes in Berne.
Thunersee done now (was more work than the whole of Zürich), leaves the Brienzersee that I’ll do tomorrow. Note that in Bern I’ve kept the existing fake municipality for the Thunersee for now in case anything depends on it.
Bieler- and Brienzersee done. That should remove the largest remaining offenders in swissBOUNDARIES3D <-> OpenStreetMap (the top 5 and many others in the top 20). I haven’t looked at the issues in Lucerne yet, but they are unlikely to be doctrine related.
PS: I’ve replaced the lake side boundaries for Vilnelz, Hagneck and Lüscherz too, Hagneck and Vilnelz inclued parts of the lake, the border in Lüscherz was roughly in the right place but otherwise had little to do with the actual border. The Lüscherz border was also glued to everything and the kitchen sink, making it a PITA to fix (if you are wondering why I had to do the whole thing twice and revert the first attempt), please please do not do that.
Meggen was clearly simply due to a geometry difference in the lake that I’ve fixed.
More interesting is the question of the high number of municipalities in Ticino with large differences. I’ve manually improved the border of Aranno and the differences there were clearly not due to the simplifications made when the data was originally imported. My guess would be that the borders have probably been resurveyed in the process of digitalisation of the cadastre in TI. If we do anything systematic for those I would open a new topic.