#30DayMapChallenge Day 3: Polygons

What are Madison’s most complexly shaped tax parcels?

I’m a little behind with the #30DayMapChallenge. Here’s what I have for day 3, polygons. I wanted to do something with the City of Madison tax parcel dataset but it took me a while to figure out what to do. Madison’s largest parcels? The most valuable ones? The densest? Those all lend themselves to a choropleth map (which is not a theme in this year’s challenge!). Instead I looked at the shape of parcels. I wanted to find weirdly shaped ones. First I tried doing something with the LotWidth and LotDepth variables, but that didn’t yield anything interesting. Eventually I settled on the number of vertices of the parcels. Parcels with more vertices generally should have a more “complex” shape. The mapview package has a function to count the number of points/vertices/nodes of a geometry.

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library(sf)
library(tidyverse)
library(tmap)

Let’s look at the top 10 of “complex” shapes:

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parcels <- read_rds("data/parcels.RDS")

parcels2 <- parcels |>
  rowwise() |>
  mutate(nodes = mapview::npts(geometry),
         nodes_label = paste0(nodes, " vertices")) |>
  select(nodes, Parcel, nodes_label) |>
  arrange(desc(nodes)) |>
  head(10)

tm_shape(parcels2) +
  # tm_text(text = "nodes") +
  tm_polygons(fill = "nodes", title = "Vertices") +
  tm_facets(by = "nodes") +
  tm_title("The most complex tax parcels in Madison, Wisconsin by number of vertices")

Some of them look complex whereas some just have somewhat rounded shapes that are approximated with a lot of vertices.

Let’s see where the “complex” parcels are located in the city:

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ttm()
tm_shape(parcels2) +
  # tm_text(text = "nodes") +
  tm_polygons(fill = "nodes", title = "Vertices") +
  tm_title("The most complex tax parcels in Madison, Wisconsin")

Several of them are adjacent to each other, Makes sense: One complexly shaped parcel makes it likely that a parcel it shares a border with also has a complex shape.