The sf::st_perimeter() function doesn't seem to work properly when trying to determine the perimeter of an sf object on the ellipsoid
Dear Micha, Josiah and Mike, First of all, a very big thank you to all three of you for taking the time to share your thoughts on the problem I raised. Mike, an even bigger thank you for the detailed explanations. It's really appreciated.? The details of the calculations on your Github post are particularly interesting and confirm the doubt I was expressing about the sf::st_perimeter() function when sf::sf_use_s2() is set as FALSE (i.e. the case where the function returns 129.09815 [m] as shown in my reprex). I understand from your Github post that you validate the fact that there is an error somewhere in the function's code, which will most probably be corrected soon (by you or Edzer, I guess). Many thanks again for your particularly enlightening and helpful contribution... and thanks in advance to the person who will make the correction in the code. Cheers, Lo?c ________________________________________ De :?Michael Sumner <mdsumner at gmail.com> Envoy? :?samedi 17 mai 2025 14:13 ? :?Josiah Parry <josiah.parry at gmail.com> Cc?:?Lo?c Val?ry <lvalery at outlook.fr>; r-sig-geo <r-sig-geo at r-project.org> Objet :?Re: [R-sig-Geo] The sf::st_perimeter() function doesn't seem to work properly when trying to determine the perimeter of an sf object on the ellipsoid ? s2 is actually less accurate, because s2 is spherical (terra always uses ellipsoidal Karney when crs is set, and sf+longlat-s2 uses Karney, as does geodist, but sf uses "the map projection" when crs is nonlonlat?or NA, because this is an appropriate coordinate system in which to calculate distance (within this region)). It's a fun complex to grok, not helpful for inexpert use sadly, or with a lack of deep knowledge of the inter-package history.? Fun stuff.? There are three numbers in play in this code (I've worked on illustrating this in different ways over the years, trying to get attention on it to no avail, this is a great example and here is some code as a start at unpicking it:? The numbers in play are?129.285384,?129.248935,?129.09815 https://gist.github.com/mdsumner/5daedfcbee851ca31a7aa9006dbf74cf Choose your tools very carefully, it's? pretty?treacherous space here.? Best, Mike ?
On Sat, May 17, 2025 at 8:29?AM Josiah Parry <josiah.parry at gmail.com> wrote:
I believe that using s2 is calculating a *more accurate** perimeter because it is used _only_ with spherical geometries. It uses a spherical calculation because you?re using a spherical projection. Otherwise you?re calculating the perimeter of a geography in 2D ignoring the fact that the data are not themselves projected onto a 2D plane.
On Fri, May 16, 2025 at 15:18 Lo?c Val?ry <lvalery at outlook.fr> wrote:
Dear Micha, First of all, many thanks for exploring my request so quickly. However, the results you obtained don't remove the doubt I have when trying to determine the perimeter on the ellipsoid with the sf::st_perimeter() function. There area two main reasons for this: ? ?1 - When I focus on the last part of your e-mail (i.e. from where you transform the crs from EPSG:2154 to EPSG:4326), I get the same results as you do (namely, 129.0982 [m] whether I use the st_perimeter approach or the other two approaches, i.e., sf::st_boundary() and sf::st_cast()), but if, and only if, I leave the following default setting: sf::sf_use_s2(TRUE). ? ? ? ?The problem is that if this setting is left as default, I understand that the perimeter is calculated based on a sphere and not an ellipsoid (at least, if I've correctly understood the following explanations here: https://r-spatial.github.io/sf/reference/geos_measures.html). ? ? ? ?To compute the perimeter on the ellipsoid, it seems that we need to use sf::sf_use_s2(FALSE) before any calculation. And in doing so, I always end up with the same values as those indicated in my initial e-mail, namely 129.0982 [m] with the sf::st_perimeter() function and 129.2854 [m] for the other two approaches (i.e. sf::st_boundary() and sf::st_cast()). ? ?2 - This value of 129.2854 [m] is, moreover, precisely the one found when the calculation of the perimeter based on the ellipsoid is performed in QGIS [$perimeter] or POSTGIS [ST_Perimeter(ST_Transform(geom,4326)::geography)]. ? ? ? ?This leads me to guess that it must be the right value and that, consequently, there may be a problem with the sf::st_perimeter() function when sf::sf_use_s2() is turned to FALSE. Thank you very much again for your tests... and for your comment about the use of "plot" in my REPREX (sorry for that) For members who would also like to do the tests, please find below the REPREX modified accordingly (using "plt" instead of "plot") I have also added the session information at the bottom of this message. Many thanks in advance. Cheers, Lo?c ---------------------------- REPREX # Generate the land plot as an sf object (french projection: EPSG:2154) plt <- structure(list(GEOM = structure(list(structure(list(structure(c(320102.67, 320111.85, 320131.82, 320134.33, 320138.5, 320141.93, 320135.33, 320118.58, 320113.45, 320105.18, 320102.67, 6845954.56, 6845962.15, 6845971.55, 6845971.9, 6845953.21, 6845936.12, 6845935.07, 6845931.54, 6845929.91, 6845950.57, 6845954.56), dim = c(11L, 2L))), class = c("XY", ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? "POLYGON", "sfg"))), n_empty = 0L, class = c("sfc_POLYGON", "sfc" ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ), precision = 0, bbox = structure(c(xmin = 320102.67, ymin = 6845929.91, ? ? ? ? ? ? ? ? ? ? ? ? ? ?xmax = 320141.93, ymax = 6845971.9), class = "bbox"), crs = structure(list( ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?input = "EPSG:2154", wkt = "PROJCRS[\"RGF93 v1 / Lambert-93\",\n? ? BASEGEOGCRS[\"RGF93 v1\",\n? ? ? ? DATUM[\"Reseau Geodesique Francais 1993 v1\",\n? ? ? ? ? ? ELLIPSOID[\"GRS 1980\",6378137,298.257222101,\n LENGTHUNIT[\"metre\",1]]],\n? ? ? ? PRIMEM[\"Greenwich\",0,\n ANGLEUNIT[\"degree\",0.0174532925199433]],\n? ? ? ? ID[\"EPSG\",4171]],\n ? ?CONVERSION[\"Lambert-93\",\n? ? ? ? METHOD[\"Lambert Conic Conformal (2SP)\",\n? ? ? ? ? ? ID[\"EPSG\",9802]],\n? ? ? ? PARAMETER[\"Latitude of false origin\",46.5,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8821]],\n? ? ? ? PARAMETER[\"Longitude of false origin\",3,\n ? ? ? ? ? ?ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8822]],\n? ? ? ? PARAMETER[\"Latitude of 1st standard parallel\",49,\n? ? ? ? ? ? ANGLEUNIT[\"degree\",0.0174532925199433],\n ? ? ? ? ?ID[\"EPSG\",8823]],\n? ? ? ? PARAMETER[\"Latitude of 2nd standard parallel\",44,\n? ? ? ? ? ? ANGLEUNIT[\"degree\",0.0174532925199433],\n ? ? ? ? ?ID[\"EPSG\",8824]],\n? ? ? ? PARAMETER[\"Easting at false origin\",700000,\n? ? ? ? ? ? LENGTHUNIT[\"metre\",1],\n ID[\"EPSG\",8826]],\n? ? ? ? PARAMETER[\"Northing at false origin\",6600000,\n? ? ? ? ? ? LENGTHUNIT[\"metre\",1],\n ID[\"EPSG\",8827]]],\n? ? CS[Cartesian,2],\n? ? ? ? AXIS[\"easting (X)\",east,\n? ? ? ? ? ? ORDER[1],\n LENGTHUNIT[\"metre\",1]],\n? ? ? ? AXIS[\"northing (Y)\",north,\n ? ?ORDER[2],\n? ? ? ? ? ? LENGTHUNIT[\"metre\",1]],\n? ? USAGE[\n SCOPE[\"Engineering survey, topographic mapping.\"],\n? ? ? ? AREA[\"France - onshore and offshore, mainland and Corsica (France m?tropolitaine including Corsica).\"],\n? ? ? ? BBOX[41.15,-9.86,51.56,10.38]],\n ID[\"EPSG\",2154]]"), class = "crs"))), row.names = 1L, class = c("sf", ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?"data.frame"), sf_column = "GEOM", agr = structure(integer(0), levels = c("constant", ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?"aggregate", "identity"), class = "factor", names = character(0))) # What the plot looks like plot(plt) # The geom of the plot seems O.K.: sf::st_is_valid(plt) # To compute ellipsoidal metrics sf::sf_use_s2(FALSE) # Calculating ellipsoidal perimeter with st_perimeter() --> 129.0982 [m] plt |> ? ?sf::st_transform(4326) |> ? ?sf::st_perimeter() # Calculating ellipsoidal perimeter with st_boundary()? --> 129.2854 [m] plt |> ? ?sf::st_transform(4326) |> ? ?sf::st_boundary() |> ? ?sf::st_length() # Calculating ellipsoidal perimeter with st_cast()? ? ? --> 129.2854 [m] plt |> ? ?sf::st_transform(4326) |> ? ?sf::st_cast("MULTILINESTRING") |> ? ?sf::st_length() --------------------------------------------------------------------------------------
sessionInfo()
R version 4.3.1 (2023-06-16 ucrt) Platform: x86_64-w64-mingw32/x64 (64-bit) Running under: Windows 10 x64 (build 19045) Matrix products: default locale: [1] LC_COLLATE=French_France.utf8? LC_CTYPE=French_France.utf8 LC_MONETARY=French_France.utf8 LC_NUMERIC=C ? LC_TIME=French_France.utf8 time zone: Europe/Paris tzcode source: internal attached base packages: [1] stats? ? ?graphics? grDevices utils? ? ?datasets? methods? ?base other attached packages: [1] sf_1.0-21 loaded via a namespace (and not attached): ? ?[1] RColorBrewer_1.1-3? ? wk_0.9.4? ? ? ? ? ? ? sys_3.4.3 ? rstudioapi_0.17.1? ? ?jsonlite_2.0.0? ? ? ? happign_0.3.3 ? magrittr_2.0.3? ? ? ? rmarkdown_2.29? ? ? ? farver_2.1.2 ? [10] fs_1.6.6? ? ? ? ? ? ? vctrs_0.6.5? ? ? ? ? ?memoise_2.0.1 ? base64enc_0.1-3? ? ? ?terra_1.8-50? ? ? ? ? htmltools_0.5.8.1 ? usethis_3.1.0? ? ? ? ?s2_1.1.8? ? ? ? ? ? ? raster_3.6-32 ? [19] cellranger_1.1.0? ? ? pins_1.4.1? ? ? ? ? ? sass_0.4.10 ? shinyFiles_0.9.3? ? ? KernSmooth_2.23-21? ? bslib_0.9.0 ? htmlwidgets_1.6.4? ? ?keyring_1.3.2? ? ? ? ?httr2_1.1.2 ? [28] stars_0.6-8? ? ? ? ? ?cachem_1.1.0? ? ? ? ? igraph_2.1.4 mime_0.13? ? ? ? ? ? ?lifecycle_1.0.4? ? ? ?pkgconfig_2.0.3? ? ? ?R6_2.6.1 ? ? ? ? ? ? ?fastmap_1.2.0? ? ? ? ?shiny_1.10.0 ? [37] connections_0.2.0? ? ?digest_0.6.37? ? ? ? ?colorspace_2.1-1 mapview_2.11.2? ? ? ? lobstr_1.1.2.9000? ? ?shinycssloaders_1.1.0 leafem_0.2.4? ? ? ? ? pkgload_1.4.0? ? ? ? ?nngeo_0.4.8 ? [46] crosstalk_1.2.1? ? ? ?lwgeom_0.2-14? ? ? ? ?abind_1.4-8 ? RPostgreSQL_0.7-8? ? ?compiler_4.3.1? ? ? ? proxy_0.4-27 remotes_2.5.0? ? ? ? ?bit64_4.6.0-1? ? ? ? ?backports_1.5.0 ? [55] viridis_0.6.5? ? ? ? ?DBI_1.2.3? ? ? ? ? ? ?pkgbuild_1.4.7 rappdirs_0.3.3? ? ? ? sessioninfo_1.2.3? ? ?leaflet_2.2.2 ? waiter_0.2.5? ? ? ? ? classInt_0.4-11? ? ? ?tools_4.3.1 ? [64] formattable_0.2.1? ? ?units_0.8-7? ? ? ? ? ?rpostgis_1.4.4 odbc_1.6.1? ? ? ? ? ? zip_2.3.3? ? ? ? ? ? ?httpuv_1.6.16 ? glue_1.8.0? ? ? ? ? ? satellite_1.0.5? ? ? ?promises_1.3.2 ? [73] grid_4.3.1? ? ? ? ? ? generics_0.1.4? ? ? ? gtable_0.3.6 tzdb_0.5.0? ? ? ? ? ? class_7.3-22? ? ? ? ? RPostgres_1.4.8 ? tidyr_1.3.1? ? ? ? ? ?data.table_1.17.2? ? ?hms_1.1.3 ? [82] sp_2.2-0? ? ? ? ? ? ? xml2_1.3.8? ? ? ? ? ? pillar_1.10.2 ? stringr_1.5.1? ? ? ? ?later_1.4.2? ? ? ? ? ?dplyr_1.1.4 ? lattice_0.21-8? ? ? ? bit_4.6.0? ? ? ? ? ? ?tidyselect_1.2.1 ? [91] miniUI_0.1.2? ? ? ? ? knitr_1.50? ? ? ? ? ? gridExtra_2.3 ? xfun_0.52? ? ? ? ? ? ?stats4_4.3.1? ? ? ? ? devtools_2.4.5 dm_1.0.11? ? ? ? ? ? ?stringi_1.8.7? ? ? ? ?pacman_0.5.1 [100] geojsonsf_2.0.3? ? ? ?evaluate_1.0.3? ? ? ? shinyWidgets_0.9.0 codetools_0.2-19? ? ? archive_1.1.12? ? ? ? tibble_3.2.1 cli_3.6.5? ? ? ? ? ? ?xtable_1.8-4? ? ? ? ? rscontract_0.1.2 [109] jquerylib_0.1.4? ? ? ?dichromat_2.0-0.1? ? ?Rcpp_1.0.14 ? readxl_1.4.5? ? ? ? ? dbplyr_2.5.0? ? ? ? ? png_0.1-8 ? parallel_4.3.1? ? ? ? yyjsonr_0.1.20? ? ? ? ellipsis_0.3.2 [118] ggplot2_3.5.2? ? ? ? ?readr_2.1.5? ? ? ? ? ?assertthat_0.2.1 blob_1.2.4? ? ? ? ? ? profvis_0.4.0? ? ? ? ?urlchecker_1.0.1 viridisLite_0.4.2? ? ?scales_1.4.0? ? ? ? ? e1071_1.7-16 [127] purrr_1.0.4? ? ? ? ? ?rlang_1.1.6? ? ? ? ? ?shinyjs_2.1.0 ? rgeos_0.6-4
________________________________________
De : Micha Silver <micha_silver+DEV at proton.me>
Envoy? : vendredi 16 mai 2025 18:14
? : Lo?c Val?ry <lvalery at outlook.fr>
Cc : r-sig-geo <r-sig-geo at r-project.org>
Objet : Re: [R-sig-Geo] The sf::st_perimeter() function doesn't seem to
work properly when trying to determine the perimeter of an sf object on the
ellipsoid
Interesting. Thanks for the MWE!
Obviously, when you transform to WGS84 you'll get different
length/perimeter values than the original projection.
Here's what I see (I used plt as variable name to avoid conflicts with the
'plot` func):
st_crs(plt)[1]
$input
[1] "EPSG:2154"
# Calculating ellipsoidal perimeter with st_perimeter()
plt |>? sf::st_perimeter()
129.0982 [m]
# Calculating ellipsoidal perimeter with st_boundary()
plt |>
? ?sf::st_boundary() |>
? ?sf::st_length()
129.0982 [m]
# Calculating ellipsoidal perimeter with st_cast()
plt |>
? ?sf::st_cast("MULTILINESTRING") |>
? ?sf::st_length()
129.0982 [m]
# Looks OK
# Now transformed
plt <- st_transform(plt, 4326)
# Calculating ellipsoidal perimeter with st_perimeter()
plt |>? sf::st_perimeter()
129.0982 [m]
# Calculating ellipsoidal perimeter with st_boundary()
plt |>
? ?sf::st_boundary() |>
? ?sf::st_length()
129.0982 [m]
# Calculating ellipsoidal perimeter with st_cast()
plt |>
? ?sf::st_cast("MULTILINESTRING") |>
? ?sf::st_length()
129.0982 [m]
# Also OK
sessionInfo()
R version 4.2.2 Patched (2022-11-10 r83330)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Debian GNU/Linux 12 (bookworm)
Matrix products: default
BLAS:? ?/usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3
LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.21.so
locale:
? [1] LC_CTYPE=en_IL.UTF-8? ? ? ? ? LC_NUMERIC=C
? [3] LC_TIME=en_IL.UTF-8? ? ? ? ? ?LC_COLLATE=en_IL.UTF-8
? [5] LC_MONETARY=en_IL.UTF-8? ? ? ?LC_MESSAGES=en_IL.UTF-8
? [7] LC_PAPER=en_IL.UTF-8? ? ? ? ? LC_NAME=en_IL.UTF-8
? [9] LC_ADDRESS=en_IL.UTF-8? ? ? ? LC_TELEPHONE=en_IL.UTF-8
[11] LC_MEASUREMENT=en_IL.UTF-8? ? LC_IDENTIFICATION=en_IL.UTF-8
attached base packages:
[1] stats? ? ?graphics? grDevices utils? ? ?datasets? methods? ?base
other attached packages:
[1] sf_1.0-20? ? ? rkward_0.7.5? ?devtools_2.4.5 usethis_3.1.0
HTH,
Micha
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? ? ? ? [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list R-sig-Geo at r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo -- Michael Sumner Research Software Engineer Australian Antarctic Division Hobart, Australia e-mail: mdsumner at gmail.com