3. Methodology
3.1. Data
Hurricane Sandy, which struck the Northwestern US on 29 October 2012, is selected as a case study, and downtown New York is chosen as the study area. Since the paper aims to establish geographical situational awareness, only tweets with geo-tags are examined. To retrieve all geo-tagged messages posted on Twitter during 10 October and 27 November 2012, from Gnip (http://gnip.com/), we sent a geographic query with the boundary of the selected study area. A total of 1,763,141 tweets were collected. In addition to the message text content, each tweet includes metadata, such as the timestamp of posting, geo-tag (location), and author profile information, which includes author location, profile description, number of tweets, number of followers and friends, etc.