The sf::st_perimeter() function doesn't seem to work properly when trying to determine the perimeter of an sf object on the ellipsoid

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

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