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I'm not sure what I've screwed up here - there's a body of water near where I am known collectively as the "five lakes". Today, I went in and tried to correctly mark the five sections by splitting the outer polygon in a few places and drawing separating ways, then giving the appropriate names to the individual sections (and tagging them "natural=water" and "name=[fill in name here]").

In potlatch2 it looked like I was doing things correctly and everything looked fine in the browser at zoom level 12, but at 13 or closer sections of the lake (including some that I would swear I hadn't touched) "disappear" (with the islands showing up in blue as though those sections had gotten "inverted" somehow).

The section I was editing may be seen at this section of the map, and you should be able to see the effect I'm describing by zooming in one more level from that link. South Twin Lake (the left side of the dividing line that I added there) appears "filled in" while North Twin Lake (the right side, obviously) looks okay. A similar effect appears to be happening at the northernmost section of the lake, and to a couple of lakes next to this one that I swear I didn't knowingly edit at all (the "Jo Mary" lake et al).

(At least, I'm ASSUMING everyone else can see it - I see the effect in both Firefox and Chromium.)

Can anyone tell what it is that I screwed up, what I should have done instead, and how to fix it short of finding some way to just revert changesets 17322213 and 17320149 (also an option assuming I can figure out how, since I'm pretty sure I'm the only one making edits in this area right now)?

asked 12 Aug '13, 23:40

Epicanis's gravatar image

accept rate: 0%


by the way: both lakes (North and South) overlap - but that is likely not really causing the problem you see.

(13 Aug '13, 11:33) aseerel4c26 ♦

I see that now too - now I'm trying to figure out how to fix THAT - but at least now I think it should all be showing up correctly as water again now that I know what's going on and have hopefully gotten it enclosed.

(13 Aug '13, 15:41) Epicanis

Hello There, The area of South Twin Lake is not a closed area. The problem is at the bottom of the join to North Twin Lake, this corner :-


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answered 13 Aug '13, 07:06

BCNorwich's gravatar image

accept rate: 20%


screenshot - illustration of the break

(13 Aug '13, 08:56) andy mackey

Thank you! I had missed that somehow. I found the similar gap up at the "Ambajejus Lake" end as well and (hopefully) fixed it.

(Is there a way to remove a single connection between nodes? It appears I've managed to select two separate nodes on this section of the lake for the dividing line instead of the same one, giving me a tiny triangular space where the two "lakes" overlap. There's also a (pre-existing, that one isn't mine) small section of "Pemadumcook Lake" that for some reason has a dividing line parallel to the county line (whereas "Pemadumcook lake" is that plus the whole section to the north of it) so it'd be handy to be able to just remove the dividing line to combine the two polygons into one.)

Next time I'll zoom in closer so I can see what I'm doing better...

(13 Aug '13, 15:39) Epicanis

Hi, in P2, read the help page at the button in the left upper corner. Watch out and be careful before you save it.

(13 Aug '13, 16:06) Hendrikklaas

One last followup - it looks like I've managed to fix my mistakes thanks to all of your help!

For any other new editors who might benefit from knowing, my main problem seems to be that I wasn't zoomed in closely enough to properly distinguish which nodes I was clicking on in some places, so when I tried to split and then draw the new dividing line, I was ending up making links to more than one node on the other end. In one case, that left me with an unclosed polygon (and the divide being two segments next to each other instead of one) and in another case with a piece of line-segment sticking out the corner of the polygon.

Solution: Just like bathroom etiquette: "If your aim's that bad, move closer". Now that I know what I did, I'll be zooming in closer and picking the link nodes more carefully.


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answered 13 Aug '13, 21:58

Epicanis's gravatar image

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edited 13 Aug '13, 21:58

Selecting "none" background helps as well.

(14 Aug '13, 07:54) andy mackey

I see the answer above is accepted (and correctly so), but wanted to point out this additional issue: the multipolygon relation (relation 2851500) that used to cover the whole lake area is now messed up. Both Ambajejus Lake and Pemadumcook Lake are still in that same multipolygon relation, with multiple islands within each. Typically in OSM, each lake and its islands will be in its own multipolygon relation. When "inner" ways are outside of "outer" ways, often rendering gets weird. Also, often the tags (natural=water, name, etc) will be moved to the relation and off the ways. If there is a reason to have those lakes joined together (say, if they're known collectively as the Five Lakes), it's possible to use a "superrelation" which gathers the smaller relations.

In Potlatch 2, relations are listed in the Advanced tab at the bottom. In iD, any relations are listed at the bottom of the Edit Feature window.

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answered 13 Aug '13, 17:26

neuhausr's gravatar image

accept rate: 21%

I think I understand how I screwed this up now and have partially fixed it. Now I'm trying to figure out if I've done this whole thing completely wrong in the first place or what...

Is there something specific I need to do to the inner (island) polygons to resolve this or should my corrections to the dividing lines I've been making to the outer polygons resolve the issue?

(Also, when you say "often the tags (natural=water, name, etc) will be moved to the relation and off the ways.", so you mean I SHOULD do that? I imagine that would correct the location of the label when it's rendered...)

(13 Aug '13, 18:21) Epicanis

I have screwed up many multipolygons in my time, don't sweat it! ...although it's great if you can clean up any messes you make :) Basically, I'd suggest moving all of the Ambajejus Lake ways (the outer and all inner ways, aka islands) to a new relation and removing them from the other relation. Along the boundary between the lakes, the same way will be included in both relations.

Regarding tags, I should've been stronger: on the wiki it says "Tags describing the multipolygon (e.g., landuse=forest) should go on the relation"

(15 Aug '13, 17:00) neuhausr

I'll have to look up how to reassign relations, but that makes sense.
I'm not quite as worried about it now that I've got it displaying correctly again (i.e. I'm not screwing up everybody's maps for the lakes any more) but I'll make a note to go back and see about doing this.


(16 Aug '13, 17:28) Epicanis

Hi There, In Potlatch2, click and hold on the node you want to move, drag the node onto the node you want to merge it with, release the node, then press "J" join nodes. The two nodes should be joined/merged. I don't use P2 but I think that's the correct sequence, JOSM is so much easier, (after some practice). Regards

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answered 13 Aug '13, 16:30

BCNorwich's gravatar image

accept rate: 20%

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question asked: 12 Aug '13, 23:40

question was seen: 4,685 times

last updated: 16 Aug '13, 17:28

NOTICE: is no longer in use from 1st March 2024. Please use the OpenStreetMap Community Forum