Buffer of a polygon does not fully contain a buffer around the centroid of this polygon

[roberto-amaralsantos]

OS: Ubuntu 22.04
R: 4.4.0
Terra: 1.7.78
terra::gdal(lib="all"): gdal: "3.4.1" proj: "8.2.1" geos: "3.10.2"

Dear Robert, I am using the terra package in R to get a buffer around polygons. However, I have noticed that terra::buffer() is not behaving as expected. In particular, for some polygons and buffer widths the buffered polygon does not fully contain a buffer with the same width of its centroid. Is there a bug or am I doing something wrong? I provide reproducible code below. A picture of the nonintuitive area difference can be obtained from running the code below as well. Thanks for your help!

library(terra)

# Define the vertices of the triangle (right-angled at point (0, 0))
vertices <- rbind(
  c(-93.746, 46.99),     # Vertex 1 (right angle)
  c(-93.736, 46.99),    # Vertex 2 (height of 10 units)
  c(-93.746, 46.9886),     # Vertex 3 (base of 5 units)
  c(-93.746, 46.99)      # Close the polygon by returning to Vertex 1
)

# Create the triangle polygon - EPSG:4269 is lat/lon
triangle_polygon <- vect(vertices, type = "polygons", crs = "EPSG:4269")

# Plot the rectangular triangle
plot(triangle_polygon)

# Get centroids
centroid <- terra::centroids(triangle_polygon)

# Buffer centroid (5000m)
centroid_buffer <- terra::buffer(centroid, 5000)

# Buffer polygon (5000m)
buffer <- terra::buffer(triangle_polygon, 5000)

# Do buffered centroid minus buffered polygon
diff <- terra::erase(centroid_buffer, buffer)

# The picture shows area contained in the buffered centroid, but not in the buffered polygon
plot(diff)

[roberto-amaralsantos]

There was a reply in my stackoverflow post about this that solved the issue. Apparently terra was struggling with geographic coordinate system (GCS) spheroid coordinates. To get buffer working correctly I should be using the appropriate UTM Zone CRS. My suggestion would be to add a note on that to the buffer command instructions, because it was not clear to me and may not be to other users as well.

[rhijmans]

Thank you, this has now been fixed.