NoteTwitter location data is available only if a Twitter user chooses to activate it. For each individual Tweet, they can choose whether or not to include location data and which location they want to specify, regardless of their real location. For example, a user in New York City could select Edinburgh, Scotland as the purported origin of a Tweet. If they choose not to include location data, all the twitter.retweeted.place.* targets are unpopulated for that Tweet.
The two-character country code that corresponds to the location for an original Tweet that is being Retweeted.
When a Tweet is Retweeted, twitter.place.country_code is copied to twitter.retweeted.place.country_code.
You can learn more about country codes at this Wikipedia page.
1. Filter for Retweets of Tweets for which the user-supplied location information is in Canada: