... These clusters of calibrator observations are called geodetic segments or DELZN segments. Constructing a geodetic segment can be tedious given that one wants low elevation observations at all stations. ... It is also best to have sources that are high at some stations and low at others to give robust SecZ fits. ... There is an example among the SCHED examples called egdelzn.key that shows how to construct a file with automatic geodetic segment insertion. ... One is Mark Reid's original 60 sources. ...