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) 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 < n...@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]] > > > > _______________________________________________ > > R-sig-Geo mailing list > > R-sig-Geo@r-project.org > > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > > > -- > Nick Bearman > +44 (0) 7717745715 > n...@geospatialtrainingsolutions.co.uk > > Please let me know if I can make any adjustments related to disability or > neurodivergence to improve how we interact. > > Due to my own life/work balance, you may get emails from me outside of > normal working hours. Please do not feel any pressure to respond outside of > your own working pattern. > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo
