extract coastline data from etopo20

[JIANG Mingshun <jiang@xxxxxxxxxxxxxxxxxxxxxxx>]

hi,

I am trying to extract coastline data from etopo20. What I need is a series of pairs longitude 
and latitude to form a cloased domain or several closed domain(including islands). Is there any 
ways to get this with Ferret? 

I know there is a web site of USGS providing an online extraction but their data is just too 
much detailed and the segments are too many. 

Any suggestions appreciated.

Jiang