Spatial Models of Parliamentary Voting (Analytical Methods for Social Research)
Keith T. Poole
This book presents a geometric voting model for analyzing parliamentary roll call data.  Each legislator is represented by one point and each roll call is represented by two points that correspond to the policy consequences of voting Yea or Nay.  On every roll call each legislator votes for the closer outcome point, at least probabilistically. These points form a spatial map that summarizes the roll calls.  In this sense a spatial map is much like a road map because it visually depicts the political world of a legislature.  These maps can be used to study a wide variety of topics related to legislative voting.