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, current.projection = NULL, projection = NULL, as.extent = FALSE)

Arguments

One of the following:

A shape (from class Spatial or Raster)
A bounding box (either 2 by 2 matrix or an Extent object).
Open Street Map search query. The bounding is automatically generated by querying q from Open Street Map Nominatim. See geocode_OSM and See http://wiki.openstreetmap.org/wiki/Nominatim.

If x is not specified, a bounding box can be created from scratch (see details.

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.

center x coordinate

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".

current.projection

projection that corresponds to the bounding box specified by x. Either a CRS object or a character value. If it is a character, it can either be a PROJ.4 character string or a shortcut. See get_proj4 for a list of shortcut values.

projection

projection to transform the bounding box to. Either a CRS object or a character value. If it is a character, it can either be a PROJ.4 character string or a shortcut. See get_proj4 for a list of shortcut values.

as.extent

should the bounding box be returned as extent? If FALSE (default) then a matrix is returned

Value

bounding box (see argument as.extent)

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

## 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)

## convert to longlat
bb(NLD_muni, projection="longlat")

## change existing bounding box
bb(NLD_muni, ext=1.5)
bb(NLD_muni, width=2, relative = TRUE)
bb(NLD_muni, xlim=c(.25, .75), ylim=c(.25, .75), relative = TRUE)

## Not run: 
# bb("Limburg", projection = "rd")
# bb_italy <- bb("Italy", projection = "eck4")
# 
# tm_shape(World, bbox=bb_italy) + tm_polygons()
# # shorter alternative: tm_shape(World, bbox="Italy") + tm_polygons()
# ## End(Not run)