Meaning of column headers in file users_polygons.csv
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Note: This file may contain more than one lines for a participant as they were free to draw several polygons. 

participant_id		::= the identifier of a participant 
polygon_id 			::= the identifier of a drawn polygon; this identifier is used as a foreign key in the polygons_markers file.
polygon_geometry 	::= the geometry of the polygon given as a hex EWKB in EPSG:4326.
updated_at			::= the timestamp when the polygon was finally submitted