If you have a cover story ready then you will have less problems with nosey people who ask questions when out with your camera. The more common ones I use are..

I am making a detailed study of Scottish town/city development for a book.
 
I am looking at the distribution of Turdus Turdus(Starling) which is notable for its ability to utilise higher temperatures in built up areas and flocking desires.

I am collecting location details for a potential film project which is confidential.

I am making a survey of foot traffic in realation to future development.Also confidential.

By being specific instead of saying its my hobby or job I find that people often loose interest quickly because they dont want to waste my time especially i f i say I am working and dont have time to chat about a confidential matter. And i am not joking, several of my clients do not want me to discuss possible projects in public. Luckily when doing the aerial views no one is pestering me !