Stepping through the code the change in point locations occurs in GeometryOverlay in the JTS code base. isOverLayNG is set to true so OverlayNGRobust.overlay is used. The two input geometries are:
POLYGON ((412023.08 92158.29, 412021.68 92161.6, 412022.58 92161.96, 412021.45
92164.58, 412022.19 92165.38, 412021.43 92167.13, 412018.32 92165.86, 412016.28
92165.03, 412019.75 92156.92, 412023.08 92158.29))
and a negative (-0.5m) buffer of it:
POLYGON ((412022.4 92158.6, 412020 92157.6, 412016.9 92164.8, 412021.2 92166.5,
412021.6 92165.5, 412021.1 92164.9, 412021 92164.8, 412021 92164.7, 412021
92164.6, 412021 92164.5, 412021 92164.4, 412021.9 92162.2, 412021.5 92162.1,
412021.4 92162, 412021.3 92162, 412021.3 92161.9, 412021.2 92161.8, 412021.2
92161.7, 412021.2 92161.6, 412021.2 92161.5, 412021.2 92161.4, 412022.4
92158.6))
are differenced to give a bordered polygon. which results in:
POLYGON ((412019.8 92156.9, 412016.3 92165, 412018.3 92165.9, 412021.4 92167.1,
412022.2 92165.4, 412021.5 92164.6, 412022.6 92162, 412021.7 92161.6, 412023.1
92158.3, 412019.8 92156.9), (412021.2 92161.4, 412021.2 92161.5, 412021.2
92161.6, 412021.2 92161.7, 412021.2 92161.8, 412021.3 92161.9, 412021.3 92162,
412021.4 92162, 412021.5 92162.1, 412021.9 92162.2, 412021 92164.4, 412021
92164.5, 412021 92164.6, 412021 92164.7, 412021 92164.8, 412021.1 92164.9,
412021.6 92165.5, 412021.2 92166.5, 412016.9 92164.8, 412020 92157.6, 412022.4
92158.6, 412021.2 92161.4))
Some of the points in the outer polygon are moved for no reason.
Reverting to isOverlayNG=false solves this issue.
Stepping through the code the change in point locations occurs in
GeometryOverlayin the JTS code base.isOverLayNGis set totruesoOverlayNGRobust.overlayis used. The two input geometries are:and a negative (-0.5m) buffer of it:
are differenced to give a bordered polygon. which results in:
Some of the points in the outer polygon are moved for no reason.
Reverting to isOverlayNG=false solves this issue.