Unclosed polygon: the first and last point must be the same
A polygon in an EUDR plot file must be a closed shape: its last coordinate pair must repeat the first. "Open" polygons are on the EU's list of common GeoJSON errors.
What the EU documentation says
The GeoJSON File Description defines a Polygon as at least four pairs of coordinates representing an enclosed area, and notes that "the first and last point's coordinates of polygons are the same (they coincide geographically to close the shape)". Its list of common errors includes "Open polygons. All polygons must represent closed shapes (i.e. the 1st coordinate pair same as the last)".
So a triangle needs four positions (three corners plus the first one again), and a four-cornered field needs five.
Why polygons come out open
- Walked boundaries. A field agent walks the plot with a GPS app and stops a few metres before the starting point. The app saves the corners as recorded.
- Spreadsheet conversion. One row per corner, converted to GeoJSON by a script that doesn't add the closing point.
- Lines instead of polygons. Some apps save a walked boundary as a LineString. That's a different error; see LineString not accepted.
How to fix it
Closing a ring is safe when the plot is otherwise sound: copy the first coordinate pair to the end of the ring. In QGIS, the "Fix geometries" tool or simply re-saving a polygon layer as GeoJSON closes rings. If you edit the file by hand, the closing pair must match the first pair exactly, to every decimal.
Before closing, look at the gap. If the last point is far from the first, closing it draws a straight line across the field and may cut off part of the plot, or cross another edge and create a self-intersection. A large gap usually means the boundary wasn't finished and should be re-mapped.
Too few corners
A ring with fewer than three distinct corners can't enclose an area; neither can corners that all lie on one line. The EU page lists "coordinate shapes representing straight lines" among the common errors. These can't be fixed by editing; the plot needs to be mapped again, or submitted as a point if it is 4 hectares or less (see points and the 4 ha rule).
What the Plot File Checker does
- Ring not closed: an error, closed automatically in the fixed file.
- Fewer than 3 distinct corners, or a flat shape: an error, not fixable automatically.
- A closed LineString: turned into a Polygon and marked "check this", because the checker can't know whether the line was meant as a boundary.
After closing, the checker re-runs the shape tests, so a ring that becomes self-crossing when closed is still reported.