def _csv_header(self):
        """
        Extract the expected CSV header from the exposure metadata
        """
        fields = ['id', 'number', 'taxonomy', 'lon', 'lat']
        for name in self.cost_types['name']:
            fields.append(name)
        if 'per_area' in self.cost_types['type']:
            fields.append('area')
        if self.occupancy_periods:
            fields.extend(self.occupancy_periods.split())
        fields.extend(self.tagcol.tagnames)
        return set(fields)
Extract the expected CSV header from the exposure metadata