## Some questions on perspective

Hey guys ^^

I've been trying to teach myself perspective for some time... Some things just don't seem to register... or they do for a while and then after a bit of time I find they make no sense again...
So I've started over, practising all the setups and everything else, watching tutorials, reading books and I hoped I could get some help here as well.

What I'm trying to do now is to deconstruct photographs and try to see what the perspective setup would look like - ie where the station point would be, the vanishing points, what the cone of vision would encompass, but a few things confuse me :
1st thing is - http://commons.wikimedia.org/wiki/Fi...rison_Home.jpg
in this photograph, where would the center point of vision be? I understand that in 1 point perspective it would be where the VP is, so that's easy, but what about in 2 point? Is drawing a 90 degree angle from the VPs the way to do it?
2nd -
http://commons.wikimedia.org/wiki/Fi...tober_2012.jpg
This one is easy to locate the VP and CPV, but how can I locate the station point?

And one more thing - the ground line is supposed to be where the picture plane meets the ground plane - ie where the picture begins, but what if the picture is cropped? Or what if we're unsure if the image is cropped? How can we locate the ground line then?

Thank you very much!