Feature Request: split polygon on blade

[EvanCarroll]

Description

It would be nice if we had a way to split a polygon with a blade. This is a pretty common practice for mapping. I can think of two use value propsitions here,

1. It makes it easier to incrementally upgrade things: for example, let's say you map a parking lot. At first you may map the whole parking lot with a single polygon which is explicitly mentioned as a good start. However, the ideal is to have every parking lot aisle not overlap anything including parking aisles. This requires a lot of polygons for the parking lot. As a transition you may want to to cut the parking lot in half, and convert just half of it to smaller polygons for each aisle.

2. It makes it easier to reform and fix things, take for example this screen shot
   
   In this case an aisle overlaps a "planter". I would like to be able to

   1. split the the aisle on a blade creating two aisles of the same tags
   2. shrink one aisle.
   3. create the planter.
      While easier is an operative term, it's an understatement. For example, let's assume you have a long parking aisle and inside the parking aisle on opposite ends there are self-contained "Accessible Parking Space" polygons. This makes this task substantially harder with the blade-splitting the polygon. Because one of these must be deleted making the transition a bit more dangerous. For example, from the above, if I zoom out a bit you can see this,
      
      If I wanted to add a "Planter" as described above, I would have to delete the "Accessible Parking Space" first. A solution here would save me that work, and it would make it harder to do the wrong thing.

I know on the backend we typically use ST_Split for this

[RudyTheDev]

I would like to strongly endorse this proposal (which I know has been proposed before over the years in various forms). But, as a user commonly doing this, I would like to explain the steps I have to take to perform this relatively simple-to-explain operation.

This is where we start:

This is where we are going:

Now, these are the steps I currently take in iD editor to do this:

1. - remove (and copy) the tags from the way (otherwise, it will get converted into a multipolygon)
2. - split the way at the first corner
3. - disconnect the two new nodes
4. - split on second
5. - disconnect on second
6. - add a third corner/node
7. - split on third
8. - disconnect on third
9. - add the fourth corner
10. - split on fourth
11. - disconnect on fourth
12. - select the two "vestigial" lines between the future geometries
13. - delete them
14. - continue drawing from a corner on one of the new ways
15. - close it up
16. - continue the other
17. - close the other
18. - select whichever way got cut into 2 (because we are never lucky enough to pick a corner that is the start/end of the way)
19. - merge the cut-up way into a single way
20. - select the new ways
21. - and reapply/paste the tags back on them

Ta-daa! Barely past 20 operations.

Now, this is one way I imagine I would do this with a dedicated operation:

1. - draw an area I wish to "remove" from my ways
2. - select the area and the parent way (which would be optional for a smarter operation if there's just one "parent" way)
3. - perform a cutout operation, leaving two areas with the original tags (if the the drawn area also had tags, then it is also kept and connected)

This is from a user's perspective. I want a simple split/cutout operation. There are already tools to draw lines and areas. So, to me, it's only logical that I could produce a "negative" line or area somehow. I am sad every time I don't bother fixing areas because it takes too much effort for what should be as simple as cutting out/off an area/line. I don't expect JOSM-level of freedom, nor do I really want to - I'm okay to be told my area is not cut-off-able because I didn't draw it right or just undo a bad cut and try again.

I am not adding a lot of details here and leaving out a lot of things I would want to say and add. I think the most important point is to present the current use case and compare it to the user's proposed use case.

[RudyTheDev]

I went ahead and did some coding and here is the first result of my prototype:

Basically, if you draw a line between edges of an area (and select both for now), you can use a "splice" operation which will then split that area into two along the line.

[EvanCarroll]

That implementation looks great to me, and would totally work for my purposes, and save me lots of time. Thanks! Look forward to it landing.

[EvanCarroll]

@RudyTheDev is there a chance this lands? Did submit your patch?

[RudyTheDev]

@EvanCarroll Sorry, I've been super busy with work and such. My progress is at RudyTheDev/iD/splice_operation. There are a lot of small details to consider. I implemented it for single-way areas. I wanted to do polygons too, but it got complicated fast, so I haven't done that. I will probably submit the current version PR at some point.

[EvanCarroll]

Awesome. No rush! Just please drop a message here if you can't get it done or decide to drop it. With that link, hopefully someone else can pick it up. I may take a look it at it myself, but it's really low on my pecking order now. Look forward to seeing it land.

[jnicho02]

+1 would love to see this feature added. It would make it a lot easier to make 2 * semi-detached houses from a single outline