Ensuring I'm not breaking copyright

Like everyone else, I don't want people to copy my work. Equally, I want to be sure that I am not copying other people's work, which leads to an interested question. I create pen and ink sketches of things - cars, railway engines etc. which sometimes are just side on views without any background, just the item itself, but all done free hand so it isn't a engineering drawing. The pictures need to be relatively accurate in size and what goes where as they are pictures of real cars, for example. I never use one source to produce the pictures - I use photos, and measured drawings of items etc. and combine the features of them all to make one picture. The problem for me is what is copying and what isn't. I never copy a photo as is, usually just select the car or engine from it without the background, but I obviously use the drawings to get things fairly accurate. And I guess the question is here, what is 'copying' and what isn't - I'm creating a picture of a real man made thing at the end of the day and need the drawings as source material (even though I don't follow them faithfully as they usually disagree with the photos), but also want to be sure I'm not infringing the drawings copyright too. Any thoughts would be appreciated!