Single sided buffer QGIS strange behaviour

Question

I'm creating a single sided buffer in QGIS based on line segments. In the project the line segments represent river shore lines. Therefore, they are often of meandering shape.

I used the algorithms 'native:singlesidedbuffer' and 'gdal:onesidebuffer' from the toolbox to generate the buffer encountering the same unsatisfying result as parts of the buffer also appear on the wrong side of the line:

In my understanding the buffer should not exceed the line segment, when generated on one side only. I could work around this issue with trimming the buffer but would rather generate a valid buffer geometry in the first place.

Data download link: https://www.dropbox.com/s/tyim9iid40ke000/baseline.zip?dl=0; EPSG:31287;

PS: I stumbled across this thread adressing the same issue over 3 years ago (Single Sided Buffer Error QGIS) which got closed and does not provide an answer.

Answer 1

I made some tests with PostGIS and OpenJUMP and they give identical results so I conclude that the algorithm that does the job comes from the JTS and GEOS libraries. Algorithm seem to compute the buffer and trim the full buffer by the original geometry for getting the one sided buffer. Algorithm obviously do not make a second pass and trim also the single sided buffer by the original geometry but I am not sure if this is an error. After all the 40 m left side buffer in the last image covers just that area. If farmer is spraying herbicides with 40 m working width the result is very useful, some of the chemicals are spread into the nature.

select ST_Buffer(ST_GeomFromText('LINESTRING ( 295 381, 272 321, 261 275, 289 248, 319 297, 331 351, 352 373, 381 363, 376 327, 350 283, 340 241, 368 233, 383 286, 421 355, 443 341, 420 291, 413 275 )'),
40,'side=left');

The geometry:

With 20 m buffer

With 40 m buffer