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Plot centerline with ggplot2

Source: R/geom_cnt.R
geom_cnt.Rd

Binding for ggplot2::geom_sf(), therefore it supports only sf objects.

Usage

geom_cnt(
  mapping = ggplot2::aes(),
  data = NULL,
  stat = "sf",
  position = "identity",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE,
  keep = 0.5,
  method = c("voronoi", "straight"),
  simplify = TRUE,
  ...
)

Arguments

mapping

Set of aesthetic mappings created by ggplot2::aes(). See ggplot2::geom_sf() for details.

data

The data to be displayed in this layer. See ggplot2::geom_sf() for details.

stat

The statistical transformation to use on the data for this layer. See ggplot2::geom_sf() for details.

position

A position adjustment to use on the data for this layer. See ggplot2::geom_sf() for details.

na.rm

If FALSE, the default, missing values are removed with a warning. If TRUE, missing values are silently removed.

show.legend

logical. Should this layer be included in the legends? See ggplot2::geom_sf() for details.

inherit.aes

If FALSE, overrides the default aesthetics, rather than combining with them. See ggplot2::geom_sf() for details.

keep

numeric, proportion of points to retain (0.05-5.0; default 0.5). See Details.

method

character, either "voronoi" (default) or "straight", or just the first letter "v" or "s". See Details.

simplify

logical, if TRUE (default) then the centerline will be smoothed with smoothr::smooth_ksmooth()

...

Other arguments passed on to ggplot2::layer(). See ggplot2::geom_sf() for details.

Value

A Layer ggproto object that can be added to a plot.

CRS

coord_sf() ensures that all layers use a common CRS. You can either specify it using the crs param, or coord_sf() will take it from the first layer that defines a CRS.

Combining sf layers and regular geoms

Most regular geoms, such as geom_point(), geom_path(), geom_text(), geom_polygon() etc. will work fine with coord_sf(). However when using these geoms, two problems arise. First, what CRS should be used for the x and y coordinates used by these non-sf geoms? The CRS applied to non-sf geoms is set by the default_crs parameter, and it defaults to NULL, which means positions for non-sf geoms are interpreted as projected coordinates in the coordinate system set by the crs parameter. This setting allows you complete control over where exactly items are placed on the plot canvas, but it may require some understanding of how projections work and how to generate data in projected coordinates. As an alternative, you can set default_crs = sf::st_crs(4326), the World Geodetic System 1984 (WGS84). This means that x and y positions are interpreted as longitude and latitude, respectively. You can also specify any other valid CRS as the default CRS for non-sf geoms.

The second problem that arises for non-sf geoms is how straight lines should be interpreted in projected space when default_crs is not set to NULL. The approach coord_sf() takes is to break straight lines into small pieces (i.e., segmentize them) and then transform the pieces into projected coordinates. For the default setting where x and y are interpreted as longitude and latitude, this approach means that horizontal lines follow the parallels and vertical lines follow the meridians. If you need a different approach to handling straight lines, then you should manually segmentize and project coordinates and generate the plot in projected coordinates.

See also

geom_cnt_text(), geom_cnt_label(), ggplot2::geom_sf()

Examples

library(sf)
library(ggplot2)

lake <-
  sf::st_read(
    system.file("extdata/example.gpkg", package = "centerline"),
    layer = "lake",
    quiet = TRUE
  )

ggplot() +
  geom_sf(data = lake) +
  geom_cnt(
    data = lake,
    keep = 1,
    simplify = TRUE
  ) +
  theme_void()