ABSTRACT
Almost all current GIS functionality assumes that constituent spatial entities in the system are referenced to the coordinatized Euclidean plane - that is, they all have coordinates positioning them with respect to a predetermined reference frame. In addition, the usual Euclidean properties and relationships, such as distance and bearing between locations, lengths and angles of line segments, and area and perimeter of regions, may be calculated in the normal way. However, there are many situations for which representation in a Euclidean space is not the most appropriate model, nor may even be derivable. Examples include travel time spaces, qualitative distances and flow spaces .
