How to "project" a measure Given a borelian measure in $\mathbb{R}^2$, there is a canonical way or simply a way to obtain a measure on a line, for example $x=0$? (a measure with support in the line I'm considering). 
The question is very general, I explain this with this example because is clearly easy to understand.

Given $\mu$ a Borel measure on $\Bbb R^2$, define $\nu:{\cal B}_{\Bbb R}\to [0,\infty]$ by
$$\nu(E)=\mu(E\times [0,1]).$$
Then $\nu$ is a Borel measure on $\Bbb R$.
