On Wed, 28 Feb 2024, Roger Bivand wrote:
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().
That is, the *output* units are map units, expandBB are proportions, as
in:
expBB = function(lim, expand) c(lim[1] - expand[1] * diff(lim), lim[2] +
expand[2] * diff(lim))
called for example as: xlim <- expBB(bbox[1,], expandBB[c(2,4)]).
Roger
Roger
Thank you in advance!
Ҷ�� YE, Xiang
THINKING
SPATIALLY<http://www.linkedin.com/in/spatialyexiang>.
Ph.D. in Spatial Statistics
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--
Roger Bivand
Emeritus Professor
Department of Economics, Norwegian School of Economics,
Postboks 3490 Ytre Sandviken, 5045 Bergen, Norway.
e-mail: roger.biv...@nhh.no
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