Some embodiments are directed to a method for determining, for a set of co-ordinate records associating co-ordinates with incident values, the numbers of incidents associated with co-ordinates within a query area. Co-ordinate data is stored as data representing a binary tree comprising leaf nodes, a root node and branch nodes wherein the leaf nodes are associated with individual items of co-ordinate data identified by the set of co-ordinate records, and the root node and branch nodes are each associated with a pair of child nodes, and are each associated with split values. In response to a query identifying a query area the binary tree is then recursively processed starting with the root node.