Screen Capture in Mac using Terminal

I figured, there should be some kind of terminal script out there which enables us to take the screen capture of the desktop. This will become real handy when we need to produce consistent screenshots for the Google Earth Images for my results. Furthermore, in order to crop it nicely, there is a code too!
http://usefulcodes.blogspot.com/2011/11/using-imagemagick-shell-command-to-crop.html

screencapture ~/Desktop/test.png