Unit of parameters in the expandBB argument

On Wed, 28 Feb 2024, Edzer Pebesma wrote:

 On 28/02/2024 06:04, Xiang Ye via R-sig-Geo wrote:

  Dear community,

  I recently revisited the expandBB argument in the plot() function for sf
  objects. It conveys a numeric vector of length 4 to expand the default
  canvas (in the order of bottom, left, top, right) when drawing an sf
  object. A quick refresh is here:

  https://r.geocompx.org/spatial-class#:~:text=to%20geographic%20data.-,expandBB,-%2C%20for%20example%2C%20can
  [https://r.geocompx.org/images/cover.png]<https://r.geocompx.org/spatial-class#:~:text=to%20geographic%20data.-,expandBB,-%2C%20for%20example%2C%20can>
  Chapter 2 Geographic data in R | Geocomputation with
  R<https://r.geocompx.org/spatial-class#:~:text=to%20geographic%20data.-,expandBB,-%2C%20for%20example%2C%20can>
  Prerequisites This is the first practical chapter of the book, and
  therefore it comes with some software requirements. You need access to a
  computer with a recent version of R installed (R 4.3.2...
  r.geocompx.org

  I am curious about the unit for the four values of the expandBB
  argument.
  In the help document, it only says the values are "fraction values to
  expand the bounding box with":

  https://r-spatial.github.io/sf/reference/plot.html#:~:text=fractional%20values%20to%20expand%20the%20bounding%20box%20with
  [https://r-spatial.github.io/sf/logo.png]<https://r-spatial.github.io/sf/reference/plot.html#:~:text=fractional%20values%20to%20expand%20the%20bounding%20box%20with>
  plot sf object ??
  plot<https://r-spatial.github.io/sf/reference/plot.html#:~:text=fractional%20values%20to%20expand%20the%20bounding%20box%20with>
  plot one or more attributes of an sf object on a map Plot sf object
  r-spatial.github.io

  However, it did not explicitly mention the unit adopted for the values.

  My guess:
  For the 1st and 3rd values in the argument, the unit is the height of
  the
  bounding box of the sf object; therefore a value of 0.2 expands
  (downward
  and upward, respectively) the canvas by 20% of the height of the
  bounding
  box.
  For the 2nd and 4th values in the argument, the unit is the width of the
  bounding box of the sf object; therefore a value of 0.2 expands
  (leftward
  and rightward, respectively) the canvas by 20% of the width of the
  bounding box.

  Is my understanding correct?

 Yes. It takes 4 values for each of the sides (bottom, left, top, right).
 You can use xlim and ylim to set the plot region in spatial coordinates.

The code is at: 
https://github.com/r-spatial/sf/blob/bd1940f68dd7607b7e9098ccc32c99e273c30a60/R/plot.R#L598-L611

based on the equivalent in sp: 
https://github.com/edzer/sp/blob/e60dedf3ff3d06d9ebd9d38818b072e4475758cf/R/Spatial-methods.R#L211-L226

and added in a commit 8 years ago: 
https://github.com/edzer/sp/commit/d65045951bfebf28fbd5d0ad56bfff3cb48d57a8.

So the units are implicitly those of sp::bbox() and sf::st_bbox().

Roger

  Thank you in advance!

  ??? YE, Xiang
  THINKING
  SPATIALLY<http://www.linkedin.com/in/spatialyexiang>.
  Ph.D. in Spatial Statistics

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