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

[Xiang Ye]

Dear Nathaniel,

Thank you for suggesting a new (and more advanced) way to tackle this issue. Your solution is obviously with better flexibility. In particular I appreciate your explicit adoption of units::set_units() to remove ambiguity.
Thank you for the help!

叶翔 YE, Xiang
THINKING SPATIALLY<http://www.linkedin.com/in/spatialyexiang>.
Ph.D. in Spatial Statistics
________________________________
From: Nathaniel Henry <henry.spatial.analysis using gmail.com>
Sent: Friday, February 23, 2024 3:46
To: Xiang Ye <xiangye using buffalo.edu>
Cc: 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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Hi Xiang,

As an alternative, you could just convert the polygon boundaries to lines, buffer those lines, and remove them from the original polygons. I think this will work regardless of your CRS/s2 settings, and you don't have to worry about how the buffer function is handling negative distance internally.

Example:
bristol_polys <- sf::st_read(
  "https://martinjc.github.io/UK-GeoJSON/json/eng/wards_by_lad/topo_E06000023.json",
  crs = 'EPSG:4326'
) |> sf::st_transform(crs = 'EPSG:27700')
# Convert to polylines and buffer
buffered_lines <- sf::st_cast(bristol_polys, to = 'MULTILINESTRING') |>
  sf::st_make_valid() |>
  sf::st_buffer(dist = units::set_units(100, 'm')) |>
  sf::st_union()
# Remove the buffers from the polygons
shrunk_polygons <- sf::st_difference(x = bristol_polys, y = buffered_lines)

Yields:
[image.png]

Best,
Nat
--
Nathaniel Henry, DPhil
Henry Spatial Analysis<https://henryspatialanalysis.com/>
Cell: +1-341-226-6277

On Thu, 22 Feb 2024 at 08:28, Nick Bearman <nick using geospatialtrainingsolutions.co.uk<mailto:nick using geospatialtrainingsolutions.co.uk>> wrote:
Dear Xiang,

I've not had a chance to try your code, but your example probably won't
work with the CRS set to WGS84. The buffer will use the units of the
coordinate system, so degrees, which will be far too large for the
bristol_zones example.

Assuming bristol_zones are in the UK, you need to reproject to BNG - 27700.

Also I am not sure whether bristol_zones from spDataLarge are in sp or
sf format.

Once those are addressed - I see no reason why the example in the Stack
Exchange should work.

Best wishes,
Nick.

On 19/02/2024 10:04, Xiang Ye via R-sig-Geo wrote:
> 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://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!
>
> 叶翔 YE, Xiang
> THINKING SPATIALLY<http://www.linkedin.com/in/spatialyexiang>.
> Ph.D. in Spatial Statistics
>
>       [[alternative HTML version deleted]]
>
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--
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+44 (0) 7717745715
nick using geospatialtrainingsolutions.co.uk<mailto:nick using geospatialtrainingsolutions.co.uk>

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