Data can come come from different telescopes, filters, software and even different configurations for a single software. As a result, one of the primary things to do after generating catalogs from each of these sources (for example, with MakeCatalog), is to find which sources in one catalog correspond to which in the other(s). In other words, to ‘match’ the two catalogs with each other.
Gnuastro’s Match program is in charge of such operations. The nearest objects in the two catalogs, within the given aperture, will be found and given as output. The aperture can be a circle or an ellipse with any orientation.