How to measure angle of laths with respect to vertical axis

Hi all,

I have images like below, where I am interested to measure angle each black needle makes with the vertical axis and to have a distribution something like needles with angle < 15 degree, 15-90 degree etc. Please give me some insights.

https://picasaweb.google.com/116243239493929305987/Ti5553Ti17#5721501025869642898 https://picasaweb.google.com/116243239493929305987/Ti5553Ti17#5721501293674326034

Thanks in advance

Shan Bala