使用 R 中的简单要素库识别多边形的公共边界

Question

I'm trying to identify the common borders of two different polygons using the sf_intersection() function from the sf package.

I tried this simple approach in my data, which comes from a shapefile, but it's not working exactly as I expected.

My data is the shapefile names "zones" from this repository , and this is what I've tried:

library(sf)
library(ggplot2)

zones <- st_read('./Data/zones.shp')
zones$id <- seq(nrow(zones))
borders <- st_intersection(zones, zones)
borders <- borders[borders$id != borders$id.1, ]

ggplot() +
  geom_sf(data = zones, color='red', fill=NA) +
  geom_sf(data = borders, color = 'navy')

The final plot yields this result:

If you look carefully, you'll note that there are some portions of the inner line of the polygons that are not part of the line in borders (they are red and not blue). I don't know why this is happening. Any hint or advice will be much appreciated. Thanks!

Answer 1

It is local imprecision in the borders. With most vector data formats, shared POLYGON borders are duplicated in each of the neighbours. It doesn't take very much for slight differences in the coordinates to make the intersection of the two borders incomplete.

That's not a solution, I'm afraid.

This section shows the kind of problem. The view window is:

> par('usr')
[1]  764968.2  765650.8 2945266.2 2945890.9

That sliver is only about 3 metres wide.

EDIT: Just to add my attempt at a solution using st_snap . This seems to do the trick in some places but not consistently. It doesn't feel like it is working as intended. Also, just to note the projection uses US feet as units, which confused me.

z1 <- st_geometry(zones[1,])
z2 <- st_geometry(zones[2,])

z1 <- st_cast(z1, 'LINESTRING')
z2 <- st_cast(z2, 'LINESTRING')

z1s <- st_snap(z1, z2, 1000)

border <- st_intersection(z1s, z2)

That snap tolerance is way over the top - the gaps between the zone seem to be < 10 feet - but even with this huge tolerance the actual border has missing sections. More oddly, the result has a totally unexpected extension that heads off >6500 feet from the actual intersection.

Answer 2

@David_O identifies the issue - the POLYGON borders don't actually touch throughout the shared boundary so st_intersection won't identify them as such.

One workaround may be to st_buffer your zones object before intersecting, although this is admittedly a crude workaround:

borders <- st_intersection(st_buffer(zones, 5), st_buffer(zones, 5))

borders <- borders[borders$id != borders$id.1, ]

ggplot() +
  geom_sf(data = zones, color='red', fill="transparent") +
  geom_sf(data = borders, color = 'navy')