Polygons with holes are not accepted: the two-halves workaround
A GeoJSON polygon can describe a hole, such as a farm that surrounds a house compound or a protected patch. The EU Information System does not accept polygons with holes.
What the EU documentation says
The GeoJSON File Description says: "Polygons with holes (i.e., doughnut shapes) ... are not supported and will not be processed. If a doughnut shape is needed, it can be defined by combining two half-doughnut shaped polygons." Its list of common errors mentions coordinate shapes with holes, with the workaround "two half-moon shapes", and again "polygons with holes inside", where the user is shown an error message.
How a hole looks in GeoJSON
A GeoJSON Polygon is an array of rings. The first ring is the outer boundary; any further rings are holes. So this is a polygon with a hole:
{"type":"Polygon","coordinates":[
[[-4.010,5.300],[-4.000,5.300],[-4.000,5.310],[-4.010,5.310],[-4.010,5.300]],
[[-4.006,5.304],[-4.004,5.304],[-4.004,5.306],[-4.006,5.306],[-4.006,5.304]]
]}
(Shown with 3 decimals for readability; a real file needs at least 6.) Many GIS tools create holes automatically when you subtract one area from another, for example removing a village or a river from a field.
How to fix it
Option 1: two halves. Split the shape along a line through the hole into two polygons without holes, and put them in a MultiPolygon (or two features with the same producer details). Together they cover the same land as the doughnut. In QGIS, the "Split features" tool on an editable layer does this; draw the split line across the whole shape, through the hole.
Option 2: ask whether the hole matters. If the excluded area is tiny, or was removed for convenience, check with whoever is responsible for the declaration whether the plot should simply be described by its outer boundary. That's a substantive choice about what land is declared, not a formatting decision, so don't make it silently.
What the Plot File Checker does
A polygon with holes is reported as an error and isn't changed. Splitting the shape correctly needs a decision about where to cut, and dropping the hole changes the declared area, so neither is done automatically.
Related checks after splitting
- Each half must be a valid, closed polygon on its own; see unclosed polygon.
- The two halves will share an edge. The checker reports overlapping plots as warnings; touching along an edge is normal for a split plot, but true overlaps should be investigated.
- If you give an Area property, it should match the total of both halves.