lwgeom
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r-spatial
Result of st_split depends on the order of input points
Dear all, I was working on a project with Open Street Map data and I found a weird behaviour with st_split. In a few words, I think I found that the result of st_split depends on the order of points that are passed as input (i.e. the y argument). More precisely,
# packages
library(osmdata)
#> Data (c) OpenStreetMap contributors, ODbL 1.0. https://www.openstreetmap.org/copyright
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1
library(lwgeom)
#> Linking to liblwgeom 3.0.0beta1 r16016, GEOS 3.8.0, PROJ 6.3.1
# get data
osmdata_by_id <- opq_osm_id(c(133096444, 133097023), type = "way") %>%
osmdata_sf()
osm_lines <- osmdata_by_id$osm_lines
# plot
par(mar = rep(0, 4))
plot(st_geometry(osm_lines), reset = FALSE)
# calculate duplicated points
osm_points <- st_coordinates(osm_lines)[, 1:2]
unique_osm_points <- st_sfc(st_multipoint(osm_points[duplicated(osm_points), ]), crs = 4326)
# plot
plot(unique_osm_points, pch = 16, col = "red", add = TRUE)
Here I would like to get 4 different geometries, obtained by splitting the input data at the two red dots but this is the result:
# split
osm_lines_split <- lwgeom::st_split(osm_lines, unique_osm_points) %>%
st_collection_extract("LINESTRING")
osm_lines_split
#> Simple feature collection with 3 features and 1 field
#> geometry type: LINESTRING
#> dimension: XY
#> bbox: xmin: -1.153723 ymin: 50.7263 xmax: -1.15313 ymax: 50.72732
#> geographic CRS: WGS 84
#> highway geometry
#> 133096444 service LINESTRING (-1.153265 50.72...
#> 133096444.1 service LINESTRING (-1.153622 50.72...
#> 133097023 service LINESTRING (-1.153717 50.72...
plot(osm_lines_split, col = 1:3, lwd = 2)
Nevertheless, if I use the same input but in different order:
unique_osm_points <- osm_points[duplicated(osm_points), ]
unique_osm_points <- unique_osm_points[2:1, ]
unique_osm_points <- st_sfc(st_multipoint(unique_osm_points), crs = 4326)
osm_lines_split <- lwgeom::st_split(osm_lines, unique_osm_points) %>%
st_collection_extract("LINESTRING")
plot(osm_lines_split, col = 1:4, lwd = 2)
Created on 2020-11-10 by the reprex package (v0.3.0)
Session info
devtools::session_info()
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#> date 2020-11-10
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I really don't understand what's going on since I would expect obtaining the same results in both scenarios.
