Abstract
Some problems that must be resolved by the analyst in connection with the treatment of flows across system boundaries to and from external zones are discussed in this paper. These problems make it necessary to make modifications to the conventional formulations both of the doubly and of the singly constrained members of the family of spatial-interaction models. None of the possible modifications wholly satisfies theoretical requirements in terms of the doubly constrained model, and the advantages and limitations of various approaches can only be assessed in an operational situation. For this reason some of the findings from a study of the Amersfoort region are presented which help to throw light on this problem.
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