[R-sig-Geo] Unit of parameters in the expandBB argument

[Roger Bivand]

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!
>>
>>  Ҷ�� YE, Xiang
>>  THINKING
>>  SPATIALLY<http://www.linkedin.com/in/spatialyexiang>.
>>  Ph.D. in Spatial Statistics
>>
>>   [[alternative HTML version deleted]]
>>
>>
>>  _______________________________________________
>>  R-sig-Geo mailing list
>>  R-sig-Geo using r-project.org
>>  https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
>

-- 
Roger Bivand
Emeritus Professor
Department of Economics, Norwegian School of Economics,
Postboks 3490 Ytre Sandviken, 5045 Bergen, Norway.
e-mail: Roger.Bivand using nhh.no