Intersection of polygons in R using sf

Question

I want to assess the degree of spatial proximity of each point to other equivalent points by looking at the number of others within 400m (5 minute walk).

I have some poi

Answer 1

How can I : asess the degree of spatial proximity of each point to other equivalent points by looking at the number of others within 400m (5 minute walk).

Here is how to add a column to your initial sfc of pollings stations that tells you how many polling stations are within 400m of each feature in that sfc.

Note that the minimum value is 1 because a polling station is always within 400m of itself.

# n_neighbors shows how many polling stations are within 400m
polls %>% 
  mutate(n_neighbors = lengths(st_is_within_distance(polls, dist = 400)))

Similarly, for your sfc collection of intersecting polygons, you could add a column that counts the number of buffer polygons that contain each intersection polygon:

polls_intersection %>% 
  mutate(n_overlaps = lengths(st_within(geometry, polls_buffer_400)))

And this is the bit I'm not sure about, to get to the output I want (which will show "Hotspots" of polling stations in this case) how do I colour things?

If you want to plot these things I highly recommend using ggplot2. It makes it very clear how you associate an attribute like colour with a specific variable.

For example, here is an example mapping the alpha (transparency) of each polygon to a scaled version of the n_overlaps column:

library(ggplot2)
polls_intersection %>% 
  mutate(n_overlaps = lengths(st_covered_by(geometry, polls_buffer_400))) %>% 
  ggplot() + 
  geom_sf(aes(alpha = 0.2*n_overlaps), fill = "red") 

Lastly, there should be a better way to generate your intersecting polygons that already counts overlaps. This is built in to the st_intersection function for finding intersections of sfc objects with themselves.

However, your data in particular generates an error when you try to do this:

st_intersection(polls_buffer_400)

# > Error in CPL_nary_intersection(x) : 
#>  Evaluation error: TopologyException: side location conflict at 315321.69159061194 199694.6971799387.

I don't know what a "side location conflict" is. Maybe @edzer could help with that. However, most subsets of your data do not contain that conflict. For example:

# this version adds an n.overlaps column automatically:
st_intersection(polls_buffer_400[1:10,]) %>% 
  ggplot() + geom_sf(aes(alpha = 0.2*n.overlaps), fill = "red")