> need to decide some exact scale at which surveillance becomes illegal and under which it is legal
Surveillance of specified individuals should be allowed, but just random surveillance of the public should be declared illegal except for very particular events and purposes (e.g. searching people for entry to a music gig). If there is public surveillance in an area, it should be made clear with signs etc unless it's for the express purpose of locating specified individuals (e.g. tracking a criminal's movements on public transport).
Surveillance of specified individuals should be allowed, but just random surveillance of the public should be declared illegal except for very particular events and purposes (e.g. searching people for entry to a music gig). If there is public surveillance in an area, it should be made clear with signs etc unless it's for the express purpose of locating specified individuals (e.g. tracking a criminal's movements on public transport).