strange buffer using terra

[zhangzhixin1102]

I want to generate a buffer around minimum convex polygon. However, the results are quite strange.
Could you please confirm the issue?
Below is my scripts and data.

library(terra)
locs <- read.csv("opportunistic-occ2.csv", row.names = 1)
locs <- vect(locs, crs="+proj=longlat")
mcp <- convHull(locs)

# generate buffer around MCP
d <- 1000*1000 ## 1000 km 
b <- terra::buffer(mcp, width = d)

maps::map('world', col = 'grey85', fill = T, border = 'grey85')
points(locs, pch = 20, col = "red")
lines(mcp)
lines(b)

opportunistic-occ2.csv

[kadyb]

I think you should reproject your longitude and latitude data to a planar coordinate system (e.g. EPSG:4087). If you want to use coordinates in degrees, you should use the {sf} package, which supports calculations on the sphere. See this article: https://r-spatial.github.io/sf/articles/sf7.html

[zhangzhixin1102]

Thanks for the editing by @rhijmans and the suggestion from @kadyb.
I solved the issue by using two methods (terra + sf and sf only). Below are the results.
I am still wandering: can terra alone solve this issue?

library(sf)
library(terra)
locs <- read.csv("opportunistic-occ2.csv", row.names = 1)
locs <- vect(locs, crs="+proj=longlat")
mcp <- convHull(locs)

# generate buffer using terra + sf
d <- 1000*1000 ## 1000 km

## change crs first then buffer
b <- terra::buffer(project(mcp, "+init=EPSG:4087"), width = d)
b <- st_wrap_dateline(st_transform(x = st_as_sf(b), crs = 4326), options = c("WRAPDATELINE=YES", "DATELINEOFFSET=180"), quiet = TRUE)

## plot check
maps::map('world', col = 'grey85', fill = T, border = 'grey85')
points(locs, pch = 20, col = "red")
lines(mcp)
plot(b$geometry, add = T, border = 'blue', lwd = 2)

## generate bufer using sf only
sf::sf_use_s2(TRUE)
mcp.sf <- st_as_sf(mcp, crs = 4326)
b2 <- st_wrap_dateline(st_transform(x = st_buffer(x = st_transform(x = mcp.sf, crs = 4087), dist = d), crs = 4326), options = c("WRAPDATELINE=YES", "DATELINEOFFSET=180"), quiet = TRUE)
plot(b2$geometry, add = T, border = "green", lwd = 2, lty = 2)

[rhijmans]

Thank you for reporting this. This case now works (but there are a few other extreme cases that need to be addressed).

library(terra)
xy <- read.csv("opportunistic-occ2.csv", row.names = 1)
locs <- vect(xy, crs="+proj=longlat")
mcp <- convHull(locs)
b <- terra::buffer(mcp, width = 1000*1000)

maps::map('world', col = 'grey85', fill = T, border = 'grey85')
points(locs, pch = 20, col = "red")
lines(mcp)
lines(b, col="blue", lwd=2)