[R-sig-Geo] How to create inward (shrinking) buffer zones with st_buffer()

[Xiang Ye]

Dear Ákos,

Thank you so much for your quick and detailed reply!
I did not know about Google s2geometry.io library before, but now I noticed its impact on the sf package.

Have a great one!

叶翔 YE, Xiang
THINKING SPATIALLY<http://www.linkedin.com/in/spatialyexiang>.
Ph.D. in Spatial Statistics
________________________________
From: R-sig-Geo <r-sig-geo-bounces using r-project.org> on behalf of Bede-Fazekas Ákos <bfalevlist using gmail.com>
Sent: Monday, February 19, 2024 18:24
To: r-sig-geo using r-project.org <r-sig-geo using r-project.org>
Subject: Re: [R-sig-Geo] How to create inward (shrinking) buffer zones with st_buffer()

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Dear Xiang,

sure it is possible. I suggest using a CRS that is projected, e.g. 3857
(Web Mercator), or turn off s2 geometry. If you turn off s2, then
st_buffer() will interpret the dist parameter in degrees. Anyway, if you
choose a small enough number, then you'll get a non-empty geometry.

 > sf_use_s2(FALSE)
Spherical geometry (s2) switched off
 > st_buffer(a, -100)
dist is assumed to be in decimal degrees (arc_degrees).
Geometry set for 1 feature  (with 1 geometry empty)
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: NA ymin: NA xmax: NA ymax: NA
Geodetic CRS:  WGS 84
POLYGON EMPTY
Warning message:
In st_buffer.sfc(a, -100) :
   st_buffer does not correctly buffer longitude/latitude data
 > st_buffer(a, -0.001)
dist is assumed to be in decimal degrees (arc_degrees).
Geometry set for 1 feature
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: -2.533411 ymin: 51.40595 xmax: -2.489763 ymax: 51.4315
Geodetic CRS:  WGS 84
POLYGON ((-2.533083 51.41747, -2.528692 51.4204...
Warning message:
In st_buffer.sfc(a, -0.001) :
   st_buffer does not correctly buffer longitude/latitude data

(I don't know why s2 can't deal with negative distance if the CRS is
WGS-84.)

HTH,
Ákos
____________
Ákos Bede-Fazekas
Centre for Ecological Research, Hungary

2024. 02. 19. 11:04 keltezéssel, Xiang Ye via R-sig-Geo írta:
> Dear community,
>
> I am learning some basic geometry operation functions of sf package including st_buffer().
>
> It seems there should be no wonder if I provide a negative value to the dist argument in st_buffer(), I should expect an inward/shrinking buffer zone (I also followed here: https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgis.stackexchange.com%2Fquestions%2F392505%2Fcan-i-use-r-to-do-a-buffer-inside-polygons-shrink-polygons-negative-buffer&data=05%7C02%7Cxiangye%40buffalo.edu%7C3442c930a8b649158f2a08dc3135888f%7C96464a8af8ed40b199e25f6b50a20250%7C0%7C0%7C638439353229473724%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=O%2FqzCFqVDv2%2FmjZWZTjLgYx73eq%2FxYVMz5yQKUW5jWw%3D&reserved=0)<https://gis.stackexchange.com/questions/392505/can-i-use-r-to-do-a-buffer-inside-polygons-shrink-polygons-negative-buffer>. However, it turn out to be as long as I provide a negative value, the output will be an empty geometry:
>
> library(sf)
> library(spDataLarge)
> st_geometry(bristol_zones[1, ]) -> a    # a is the exemplary data set
>
>> a
> Geometry set for 1 feature
> Geometry type: MULTIPOLYGON
> Dimension:     XY
> Bounding box:  xmin: -2.534502 ymin: 51.40487 xmax: -2.488435 ymax: 51.43478
> Geodetic CRS:  WGS 84
> MULTIPOLYGON (((-2.510462 51.42878, -2.507985 5...
>> st_buffer(a, 100)
> Geometry set for 1 feature
> Geometry type: POLYGON
> Dimension:     XY
> Bounding box:  xmin: -2.536248 ymin: 51.40393 xmax: -2.486907 ymax: 51.43598
> Geodetic CRS:  WGS 84
> POLYGON ((-2.517834 51.43188, -2.518218 51.4318...
>> st_buffer(a, -100)
> Geometry set for 1 feature  (with 1 geometry empty)
> Geometry type: POLYGON
> Dimension:     XY
> Bounding box:  xmin: NA ymin: NA xmax: NA ymax: NA
> Geodetic CRS:  WGS 84
> POLYGON EMPTY
>
> So I would like to know if it is possible to create inward buffer zones with st_buffer()? If st_buffer() is not designed to perform this, what is the best alternative?
>
> Thank you, and have a great start of the week!
>
> $B3pfF(B YE, Xiang
> THINKING SPATIALLY<https://nam12.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.linkedin.com%2Fin%2Fspatialyexiang&data=05%7C02%7Cxiangye%40buffalo.edu%7C3442c930a8b649158f2a08dc3135888f%7C96464a8af8ed40b199e25f6b50a20250%7C0%7C0%7C638439353229481810%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=LI5uCDlgSC09%2BWqxLiZOTzTQs2dKFZ3wybiWz2Txp8w%3D&reserved=0<http://www.linkedin.com/in/spatialyexiang>>.
> Ph.D. in Spatial Statistics
>
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