<P> Political borders are imposed on the world through human agency . That means that although a political border may follow a river or mountain range, such a feature does not automatically define the political border, even though it may be a major physical barrier to crossing . </P> <P> Political borders are often classified by whether or not they follow conspicuous physical features on the earth . </P> <P> Natural borders are geographical features that present natural obstacles to communication and transport . Existing political borders are often a formalization of such historical, natural obstacles . </P> <P> Some geographical features that often constitute natural borders are: </P>

How many state boundaries are completely defined by longitude and latitude