bb: Bounding box generator

Description

Swiss army knife for bounding boxes. Modify an existing bounding box or create a new bounding box from scratch. See details.

Usage

bb(
  x = NA,
  ext = NULL,
  cx = NULL,
  cy = NULL,
  width = NULL,
  height = NULL,
  xlim = NULL,
  ylim = NULL,
  relative = FALSE,
  asp.target = NULL,
  asp.limit = NULL,
  current.projection = NULL,
  projection = NULL,
  output = c("bbox", "matrix", "extent")
)

Value

bounding box (see argument output)

Arguments

x

One of the following:

A shape from class sf, stars, sp, or raster.
A bounding box (st_bbox, Extent (raster package, which will no longer be supported in the future versions), numeric vector of 4 (default order: xmin, ymin, xmax, ymax), or a 2x2 matrix).
Open Street Map search query. The bounding is automatically generated by querying x from Open Street Map Nominatim. See geocode_OSM

ext

Extension factor of the bounding box. If 1, the bounding box is unchanged. Values smaller than 1 reduces the bounding box, and values larger than 1 enlarges the bounding box. This argument is a shortcut for both width and height with relative=TRUE. If a negative value is specified, then the shortest side of the bounding box (so width or height) is extended with ext, and the longest side is extended with the same absolute value. This is especially useful for bounding boxes with very low or high aspect ratios.

cx

center x coordinate

cy

center y coordinate

width

width of the bounding box. These are either absolute or relative (depending on the argument relative).

height

height of the bounding box. These are either absolute or relative (depending on the argument relative).

xlim

limits of the x-axis. These are either absolute or relative (depending on the argument relative).

ylim

limits of the y-axis. See xlim.

relative

boolean that determines whether relative values are used for width, height, xlim and ylim or absolute. If x is unspecified, relative is set to "FALSE".

asp.target

target aspect ratio, which is width/height, of the returned bounding box.

asp.limit

maximum aspect ratio, which is width/height. Number greater than or equal to 1. For landscape bounding boxes, 1/asp.limit will be used. The returned bounding box will have an aspect ratio between 1/asp.limit and asp.limit.

current.projection

projection that corresponds to the bounding box specified by x.

projection

projection to transform the bounding box to.

output

output format of the bounding box, one of:

"bbox" a sf::bbox object, which is a numeric vector of 4: xmin, ymin, xmax, ymax. This representation used by the sf package.
"matrix" a 2 by 2 numeric matrix, where the rows correspond to x and y, and the columns to min and max. This representation used by the sp package.
"extent" an raster::extent object, which is a numeric vector of 4: xmin, xmax, ymin, ymax. This representation used by the raster package.

Details

An existing bounding box (defined by x) can be modified as follows:

Using the extension factor ext.
Changing the width and height with width and height. The argument relavitve determines whether relative or absolute values are used.
Setting the x and y limits. The argument relavitve determines whether relative or absolute values are used.

A new bounding box can be created from scratch as follows:

Using the extension factor ext.
Setting the center coorinates cx and cy, together with the width and height.
Setting the x and y limits xlim and ylim

Examples

Run this code

if (require(tmap)) {

    ## load shapes
    data(NLD_muni)
    data(World)

    ## get bounding box (similar to sp's function bbox)
    bb(NLD_muni)

    ## extent it by factor 1.10
    bb(NLD_muni, ext=1.10)

    ## double the width
    bb(NLD_muni, width=2, relative = TRUE)

    ## crop both dimensions from 0.25 to 0.75
    bb(NLD_muni, xlim=c(.25, .75), ylim=c(.25, .75), relative = TRUE)

    ## extent it such that aspect ratio is 1
    bb(NLD_muni, asp.target = 1)

    ## convert to longlat (EPSG 4326)
    bb(NLD_muni, projection=4326)
}

if (FALSE) {
if (require(tmap)) {
    bb("Limburg", projection = 28992)
    bb_italy <- bb("Italy", projection = "+proj=eck4")

    tm_shape(World, bbox=bb_italy) + tm_polygons()
    # shorter alternative: tm_shape(World, bbox="Italy") + tm_polygons()
}}