A circle is deformed by rotation around its center by the spin() function

[jeanlobry]

Dear all,

I'm unsure whether this is a bug or or a feature, or if I'm missing something obvious. Here is my MRE:

xlon <- +5.220700 ; ylat <- +46.21010
FRBOUB <- terra::vect(cbind(xlon, ylat), crs = "EPSG:4326")
cercle <- terra::buffer(FRBOUB, width = 25*10^3, quadsegs = 100)
rotated <- terra::spin(cercle, 45, xlon, ylat)
terra::plot(cercle, las = 1, xlab = "Longitude", ylab = "Latitude")
terra::points(FRBOUB, pch = 3, cex = 2)
terra::sbar(type = "bar", below = "km")
terra::north()
terra::lines(rotated, col = "red")

The result is very confusing for me, if this is the intented behaviour I suggest that this should be documented in the spin() function.

Very best and thanks for this wonderful package,

Pr. Jean R. Lobry

P.-S. My sessionInfo()

R version 4.3.2 (2023-10-31)
Platform: x86_64-apple-darwin20 (64-bit)
Running under: macOS Sonoma 14.4.1

Matrix products: default
BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib 
LAPACK: /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/lib/libRlapack.dylib;  LAPACK version 3.11.0

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

time zone: Europe/Paris
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] leaflet_2.2.2     boot_1.3-28.1     terra_1.7-71      dichromat_2.0-0.1

loaded via a namespace (and not attached):
 [1] jsonlite_1.8.7          highr_0.10              compiler_4.3.2          crayon_1.5.2           
 [5] Rcpp_1.0.11             zip_2.3.0               jquerylib_0.1.4         yaml_2.3.7             
 [9] fastmap_1.1.1           readr_2.1.4             R6_2.5.1                knitr_1.44             
[13] htmlwidgets_1.6.4       tibble_3.2.1            leaflet.providers_2.0.0 pillar_1.9.0           
[17] tzdb_0.4.0              readODS_2.1.0           rlang_1.1.1             utf8_1.2.4             
[21] stringi_1.7.12          xfun_0.44               pkgload_1.3.4           cli_3.6.1              
[25] magrittr_2.0.3          crosstalk_1.2.1         grid_4.3.2              digest_0.6.33          
[29] rstudioapi_0.15.0       hms_1.1.3               lifecycle_1.0.3         vctrs_0.6.4            
[33] evaluate_0.22           glue_1.6.2              cellranger_1.1.0        codetools_0.2-19       
[37] fansi_1.0.5             rmarkdown_2.25          tools_4.3.2             pkgconfig_2.0.3        
[41] ellipsis_0.3.2          htmltools_0.5.7        `

[edzer]

My guess: buffer computes a geometry on a sphere, spin rotates it in R2, in a Cartesian space, not on a sphere.

[jeanlobry]

Yes but my rotated object is not in a Cartesian space...

[edzer]

It got rotated in a Cartesian space.

[edzer]

library(terra)
# terra 1.7.78
library(sf)
# Linking to GEOS 3.12.1, GDAL 3.8.4, PROJ 9.3.1; sf_use_s2() is TRUE
xlon <- 0 ; ylat <- -90
v = vect("POLYGON((0 -80,-120 -80,120 -80,0 -80))")
crs(v) = 'EPSG:4326'
spin(v, 120, xlon, ylat) |> sf::st_as_sf() |> sf::st_geometry() -> x
x[[1]]
# POLYGON ((8.660254 -95, 68.66025 8.923048, -51.33975 -198.923, 8.660254 -95))

note the latitudes outside +/- 90.

[jeanlobry]

Thanks, you example illustrates better my point. Should't spin() be renamed spin2D() or at least restricted to planar geometries?