I think it's good move to stop encroachment in the city. please see the link below:
http://www.dailytimes.com.pk/default.asp?page=20131214story_14-12-2013_pg11_5
or
http://www.thenews.com.pk/todays-news-6-220005-satellite-imaging-of-islamabad-to-help-
The interesting part is past encroachments of last 5 years are also detectable. "By using the overlay imaging technique the authority would be able to identify encroachments made in the last five years."
I do not know how to detect previous encroachments. However, if they so what sector is likely to be found more encroached? Forum experts may please share their views on the pros and cons of this move.