1. An inspection apparatus for measuring a property of a lithographic process using a composite target structure including a plurality of component structures that have been formed by the lithographic process on a substrate, the apparatus comprising: a support for the substrate having the composite target structure formed thereon; an optical system configured to illuminate the composite target structure under predetermined illumination conditions and for forming and detecting an image of the composite target structure using a predetermined portion of radiation diffracted by the plurality of component structures under the illumination conditions; a processor configured to: identify one or more regions in the detected image, each region corresponding to a respective one of the plurality of component structures, process pixel values within the one or more regions to obtain a measurement of the property of the lithographic process, and identify the one or more regions such that their boundaries fall within image regions corresponding to separation zones between the plurality of component structures within the composite target structure.