Any measurement task requires a fixed reference base (the datum) from which measurements can be made and calculated. The linac was the datum to which all SLC components were aligned; although this reference existed as a physical object, the actual establishment of the datum and its transferral to datums that were more useful for the SLC installation challenged the technology and computational ability of the survey group. Once established, the maintenance of datums is not to be taken for granted, as demonstrated by the 1989 earthquake which destroyed all SLAC's survey datums. 7 refs., 6 figs.
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