How is the poverty line estimated

Hey Pavithra, All this definitions that you've asked are stated in the NCERT textbook very clearly.
In case you dont have it, Here's the link on which you can access all the NCERT textbooks :                          
http:// nce rtbooks.p rashant hellina.com/ 9_SocialS cience.html

( All this without spaces )

The poverty line is estimated normally after every 5 years by conducting sample surveys. These sample surveys are carried out by the National Sample Survey Organisation {NSSO}.