Surveillance for certain kinds of situations is often best done when it is done covertly. After all, you could not expect people to act as they normally would if they know they are being watched. For instance, surveillance for the purpose of spying or for trying to catch criminals in the act should always necessarily be  covert surveillance . On the other hand, it might be more appropriate to conduct non-covert surveillance in situations where a deterrent is desired, such as in stores. There is a whole industry dedicated to providing people with the means to conduct covert surveillance of all kinds.