Skip to contents

Put spatial model output into the form the maps draw from

Source: R/map-data.R
as_map_data.Rd

Called for you by map_surface() and friends. Call it directly when you want to inspect what they will draw, or to normalise something once and draw it several times.

Usage

as_map_data(
  x,
  value = NULL,
  by = NULL,
  coords = NULL,
  crs = NULL,
  label = NULL
)

Arguments

x

The geometry: an sf object of polygons, points or lines, a terra SpatRaster, or a data frame with coordinate columns.

value

What to colour by. One of:

  • a string naming a column of x;

  • an unnamed numeric vector, one element per feature of x, in the same order;

  • a named numeric vector, matched against x[[by]] by name – the form a posterior summary or a lookup table arrives in;

  • NULL, for geometry drawn without a colour, such as tracklines.

by

The column of x holding the identifier that a named value is keyed by. Guessed from grid_id, id or cell when not given.

coords

For a data frame, the two coordinate columns, as c("x", "y"). Guessed from the usual spellings when not given.

crs

The coordinate reference system the coordinates are in. Required for a data frame, since nothing about two numeric columns says whether they are degrees or metres. Ignored when x already carries one.

label

What to call the value in a legend. Defaults to the column name, or to the name of the expression passed as value.

Value

An object of class map_data: a list with geometry (an sfc), value (numeric or NULL), kind ("polygon", "point", "line" or "raster"), label, and data, a data frame of the non-geometry columns.

Checking the join

When value is named, every name must be found in x[[by]], and the function stops if any is not, naming the first few missing keys. It does not quietly drop them: a partial join produces a map with holes in it that looks exactly like a region where the model had nothing to say.

Keys are compared as character. An integer grid_id and a vector named "1", "2", "3" are the common case and they match.

Rasters

A SpatRaster is read into cell centres and drawn through a raster path. It is deliberately not polygonised: terra::as.polygons() on a grid of any size is slow enough to be the dominant cost of drawing the figure, and the result is a polygon per cell that is then drawn as a rectangle anyway.

Examples

grid <- sf::st_sf(
  grid_id = 1:2,
  geometry = sf::st_sfc(
    sf::st_polygon(list(cbind(c(0, 1, 1, 0, 0), c(0, 0, 1, 1, 0)))),
    sf::st_polygon(list(cbind(c(1, 2, 2, 1, 1), c(0, 0, 1, 1, 0)))),
    crs = 4326))

# a column that is already there
grid$depth <- c(40, 120)
as_map_data(grid, "depth")
#> <map_data> polygon | 2 features
#>   crs:   EPSG:4326 
#>   value: depth [40, 120] 

# values that arrived separately, keyed by id
occupancy <- c("2" = 0.8, "1" = 0.3)
as_map_data(grid, occupancy, by = "grid_id")
#> <map_data> polygon | 2 features
#>   crs:   EPSG:4326 
#>   value: <dbl> [0.3, 0.8]