Gross errors, or "outliers", are errors other than random errors or systematic errors.
They are often large and, by definition, unpredictable.
They are typically caused by sudden changes in the prevailing physical circumstances, by system faults or by operator errors.
Source: Nick Ward, Vicechair, IALA e-Nav Committee, March 2009
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM