Abstract
Some gravity models seek to describe how the number of trips between a pair of zones changes with increasing journey cost by determining a factor for each interval of journey cost. In certain circumstances it is appropriate to find such factors by requiring that the model reproduces exactly those figures that the survey gave for 1) the number of strips generated by a zone; 2) the number of strips attracted to a zone and 3) the number of strips in each interval of journey cost. It is shown that the problem of determining a set of factors such that the above constraints are satisfied is an extension of the problem of synthesizing a gravity model distribution to given zonal trip totals.