Self-intersecting polygon: crossing lines, figure-eights and spikes
A plot polygon whose boundary crosses itself, like a figure eight, is not accepted by the EU Information System. Neither are shapes whose sides overlap or fold back into a spike.
What the EU documentation says
The GeoJSON File Description says that "shapes with crossing lines (like a figure eight for example) are not supported and will not be processed". Its list of common errors starts with "coordinate lines crossing (i.e. figure eight shapes or intersecting polygon lines)" and "overlapping sides: internal overlap or holes where part of the polygon folds inward ... are not accepted by the system". The Validation Rules add, more generally, that geolocation data cannot contain invalid geometry such as invalid polygons.
How these shapes happen
- Corners out of order. Points recorded clockwise for half the field and counter-clockwise for the rest, or sorted by ID instead of walking order, make edges cross.
- GPS jitter. Standing still while the GPS logs several points can produce a tiny loop or a spike where the boundary goes out and back on itself.
- Closing an unfinished boundary. A straight closing line across a half-walked plot can cut through another edge.
- Truncation. Rarely, cutting coordinates to 6 decimals makes two nearby corners collide; see decimal places.
Why it shouldn't be fixed automatically
Software can "repair" a self-crossing polygon, for example by splitting it into two pieces or rebuilding it around its outer edge. But each repair is a guess about where the real boundary is, and the result may include land that isn't part of the farm or leave out land that is. For a deforestation declaration, a plausible-looking wrong boundary is worse than an error message. The safe fix is to correct the point order from the field notes, or re-map the plot.
How to find and fix it
- Open the file in QGIS and run "Check validity" (Vector, Geometry Tools). It lists the location of each self-intersection.
- Zoom to the location. If two corners are simply in the wrong order, reorder them; if a spike comes from GPS jitter at one corner, delete the extra points.
- If you can't tell what the boundary should be, mark the plot for re-mapping.
- Run the check again before exporting.
What the Plot File Checker does
Self-intersections, figure-eight pinches and spikes are reported as errors and are never fixed by the checker; the plot is flagged for re-mapping. A self-crossing ring is never rewound or rebuilt. You can download the fixed file with plots that still have errors left out, so the clean plots aren't held up while the problem ones are re-mapped.