As a result of those changes, the accuracy of the National Oceanic and Atmospheric Administration's World Magnetic Model (WMM) - a mathematical representation of the magnetic field - slowly deteriorates in the five-year periods between updates. The next update was due in 2020. But "unplanned variations" have degraded the quality of the WMM so greatly that NOAA published an out-of-cycle update Monday. It was delayed from January by the partial government shutdown.