Meaning of column headers in file polygon_markers.csv
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Please note: we cannot give the actual participant labels in this file for data privacy reasons (participants have frequently chosen to 
disclose their personal attachment to a certain POI).

- polygon_id 		::= the id of the area polygon which participants have drawn and inside which the POI marker is located; this is a foreign key from the users_polygons file (i.e. see this file to determine who of the users set which POI markers).
- marker_id			::= the id of the POI marker
- marker_geometry	::= the location of the POI marker as hex EWBK in EPSG:4326
- class				::= the assigned label; possible values are: Other, Building, Square, Educational Institution, Park, Restaurant/Bar/Cafe, Supermarket/Shop, Public Transport Stop, Church
- has_label			::= "labelled" if participants have given a label, "unlabelled" otherwise