Invariant measure on Graff(2,1) What is the invariant measure on Graff$(2,1)$ the set of all straight lines in $\mathbb R^2$? I tried looking at it this way: One is aware that Graff$(2,1)$ can be identified with the canonical bundle over $\mathbb RP^1$. Is this measure something to do with measures on vector bundles? I need an answer or a reference to look for an answer in.

The answer can be found here. Also there is the disscussion of the general questions on invariant measures of this type