add argument to ukp_geo_chull to widen polygon

[njtierney]

The case I think of here is that you might have long/lat points for a region, and if you want to find the crime that occurs within the long/lat polygon, you might want a bit of padding/space around the edges.

Extra bonus would be if we could specify this extra padding in metres or kilometres.

Here's a working example as a starting point, thanks @samclifford!

library(mvtnorm)
library(rgeos)
library(sp)
library(tidyverse)
xy <- rmvnorm(n = 100, mean=c(0,0), sigma = diag(c(1,1)))

hull.ind <- chull(xy)

hull <- xy[hull.ind, ]

plot(xy)
polygon(hull, asp=1)

centroid <- Polygon(hull) %>%
  SpatialPoints %>%
  gCentroid %>%
  data.frame

points(centroid, col="red")

z <- 1

newhull <- cbind(hull, centroid) %>%
  mutate(dx = `1` - x,
         dy = `2` - y) %>%
  # euclidean distance
  mutate(dr = sqrt(dx^2 + dy^2),
         dx = dx/dr,
         dy = dy/dr) %>%
  mutate(newx = `1` + z*dx,
         newy = `2` + z*dy)
#> Warning in data.frame(..., check.names = FALSE): row names were found from
#> a short variable and have been discarded

ggplot(data=newhull, aes(x=newx, y=newy)) +
  geom_polygon(fill="grey", alpha=0.5) +
  geom_polygon(fill="grey", alpha=0.5,
               aes(x=`1`, y=`2`)) +
  geom_point(data=as.data.frame(xy),
             aes(x=V1, y=V2)) +
  coord_equal()

# expand by mean/median distance across all points