Here is where this expression came into being:
Prior to the age of electronic surveying, accuracies of 1 part in a million were achieved on numerous surveys while the most accurate baseline ever measured by the Coast and Geodetic Survey with classical techniques had a final accuracy of better than 1 part in 5.5 million. To put that in perspective, that is a measurement error of less than one inch in 90 statute miles. Such accuracies led to surveyors coining a saying that in its various forms stated: "Good enough for Government work". Loosely translated, that saying refers to a very accurate survey and has its roots in the science of geodesy as developed by the United States Coast and Geodetic Survey, an organization that became legendary for the accuracy and precision of its results. Such accuracies were not achieved in sterile laboratory settings.

This comes from http://www.photolib.noaa.gov/historic/c&gs/moregeo.html, a fascinating website! If you go, you have GOT to check out the photographs. See you in a week or so.