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A grid, as a plain list

Source: R/grid.R
grid_spec.Rd

Describe a regular grid: how many cells across and down, what region of the plane it covers, and optionally what coordinate system that plane is in.

Usage

grid_spec(
  x = NULL,
  dimension = c(60L, 50L),
  extent = NULL,
  crs = NULL,
  pad = 0
)

Arguments

x

coordinates to cover: anything with two or more columns, such as a matrix or data frame. Ignored when extent is given.

dimension

number of columns and rows, in that order

extent

outer edges c(xmin, xmax, ymin, ymax); taken from x if not given

crs

coordinate reference system, or NULL (the default) for none

pad

fraction of each axis range to add on every side, so the outermost input coordinates do not sit exactly on the boundary. 0 by default.

Value

A guerrilla_grid, which is a list of dimension, extent, crs and values.

Details

The value returned is a list. That is deliberate. Every raster in every package is some version of these few numbers, and a reader who can print the whole thing has actually been told what a raster is:

  • dimension, the number of columns and rows, in that order;

  • extent, the outer edges as xmin, xmax, ymin, ymax;

  • crs, a coordinate reference system, or NULL for none;

  • values, one number per cell, or NULL for a grid that is only a target.

dimension and extent are exactly the first two arguments of the vaster functions, so grid$dimension and grid$extent can be handed straight to vaster::xy_from_cell() and friends. Cells are numbered left to right along the top row first, the same order raster and GDAL use.

Nothing here assumes the plane is the Earth. crs stays NULL unless you say otherwise.

Examples

xy <- cbind(c(0, 4, 10), c(0, 2, 5))
grid_spec(xy)
#> <guerrilla grid>
#> dimension : 60, 50  (ncol, nrow) = 3000 cells
#> extent    : 0, 10, 0, 5  (xmin, xmax, ymin, ymax)
#> resolution: 0.1666667, 0.1000000
#> crs       : <none>
#> values    : <none>

## it really is just a list
str(unclass(grid_spec(xy)))
#> List of 4
#>  $ dimension: int [1:2] 60 50
#>  $ extent   : num [1:4] 0 10 0 5
#>  $ crs      : NULL
#>  $ values   : NULL

## give the points some room
grid_spec(xy, pad = 0.05)$extent
#> [1] -0.50 10.50 -0.25  5.25

## or skip the points entirely
grid_spec(dimension = c(4, 3), extent = c(0, 8, 0, 6))
#> <guerrilla grid>
#> dimension : 4, 3  (ncol, nrow) = 12 cells
#> extent    : 0, 8, 0, 6  (xmin, xmax, ymin, ymax)
#> resolution: 2, 2
#> crs       : <none>
#> values    : <none>