Most plot data does not start as GeoJSON. The checker reads the formats it usually starts in, converts them in your browser, and then checks the result exactly as it would a GeoJSON file.

FormatFrom
KML, KMZGoogle Earth and most mapping apps
Shapefile (zipped, or the parts together)QGIS, ArcGIS, survey and forestry software
GPXGPS devices and tracking apps
CSVExcel and other spreadsheets: points, one row per corner, or WKT

Coordinate systems

The coordinate system is taken from the file when it says (a .prj, for example). When it does not, and the numbers are metres, it is guessed only if exactly one UTM zone puts every plot in the declared country, and you are told. Otherwise you choose, from a short list of likely systems or by searching all 6,983 in the EPSG dataset.

Nothing silent

Every decision (a path closed into a polygon, a column renamed, a zone guessed, altitude dropped) is listed after conversion and recorded in the change log and the PDF report.