Hello!
I am often mapping borders between forests and farmyards. Should I use regular areas that share nodes? Or it’s better to use multipolygon relations that share ways members?
Same question, but for administrative borders. Which approach is better?
I think you mean something like https://api06.dev.openstreetmap.org/changeset/237802#map=19/53.55935/-0.69296&layers=D which I’ve just created on the dev server? https://api06.dev.openstreetmap.org/relation/4304988379 is a forest, https://api06.dev.openstreetmap.org/relation/4304988378 is a farmyard, and https://api06.dev.openstreetmap.org/way/4305899095 is a way that’s part of both?
Personally, I wouldn’t map like that - it makes it really complicated to edit or even see the data in an editor. I’d suggest in this example just using two ways that share two nodes.
Of course, there will be times when a forest really does have holes in it or really is in several pieces, and then using a multipolygon for it is the only way to describe it - but I’d still have adjacent geographical features share nodes rather than ways.
it may depend on the editor, but generally this method is quite suitable to see the data, if you select the shared way you will see all areas it confines, even on the OpenStreetMap api:
if these were 2 overlapping areas you would typically select just one and might be overlooking the other one. Someone will also have a hard time in the future if she had to unglue tens or hundreds of nodes to put something in between and all the borders were duplicated and triplicated (area1, 2 and fence for example). If we speak about 1 or 2 segments it may be better to use overlapping polygons, but with complicated outlines I prefer if previous mappers have been sharing the ways in relations, for simplicity of further editing
There is no objectively correct answer to this, so if there is a predominant style in your area, it’s less likely to cause conflicts if you follow it. But my personal impression of the community sentiment suggests that if an area is simple enough to be mapped as a closed way, most mappers would prefer if you didn’t turn it into a multipolygon relation just so you can share ways with adjacent areas.
I use JOSM for editing.
The first approach (shared nodes) makes it hard to select the right area among the ones using the same nodes. There can be 5-10 different areas with partially overlapping ways, some to the left and some to the right, all sharing the same nodes.
The second approach (multipolygon, shared ways) makes it hard to make edits to all relations at once, or to copy/paste tags from one relation to another.
Among the two, I found the second approach is a bit easier to manipulate. But I’m still learning JOSM… so this could change in the future!
It really depends on the subject matter.
Boundaries will rarely need to be “unjoined” so using common ways tends to be totally OK.
If you are connecting landuse to landuse again using common ways would seem to be unproblematic as geometry improvements tend to effect both parts the same.
If you are gluing landuse to non-landuse objects, or any dissimilar objects to each other, something you arguably shouldn’t be doing, reusing the nodes makes it much easier to unjoin the objects (for example if the areas are modeled with multi-polygons there is no need to change the MP relations). Any editor worth its salt should be able to do this automatically as it simply requires replacing the existing nodes with new ones.