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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