Editing Projected coordinate system (section)

===Grid north===
'''Grid north''' ('''GN''') is a [[navigation]]al term referring to the direction northwards along the grid lines of a [[map projection]]. It is contrasted with [[true north]] (the direction of the [[North Pole]]) and [[magnetic north]] (the direction in which a compass needle points). Many [[topographic map]]s, including those of the [[United States Geological Survey]] and Great Britain's [[Ordnance Survey]], indicate the difference between grid north, true north, and magnetic north.<ref>{{Cite book|last=Estopinal|first=Stephen V.|url=https://books.google.com/books?id=g_Jvc0zBVwgC&dq=Grid+north&pg=PA35|title=A Guide to Understanding Land Surveys|date=2009|publisher=John Wiley & Sons|isbn=978-0-470-23058-9|pages=35|language=en}}</ref>

The grid lines on Ordnance Survey maps divide the UK into one-kilometre squares, east of an imaginary zero point in the Atlantic Ocean, west of Cornwall. The grid lines point to a Grid North, varying slightly from True North. This variation is zero on the central meridian (north-south line) of the map, which is at two degrees west of the [[Prime Meridian]], and greatest at the map edges. The difference between grid north and true north is very small and can be ignored for most navigation purposes. The difference exists because the correspondence between a flat map and the round Earth is necessarily imperfect.

At the [[South Pole]], grid north conventionally points northwards along the [[Prime Meridian]].<ref>[http://quest.nasa.gov/antarctica/background/cara/movpole.html "Moving the South Pole"]. {{webarchive|url=https://web.archive.org/web/20110716140440/http://quest.nasa.gov/antarctica/background/cara/movpole.html |date=2011-07-16 }}, NASA Quest</ref>  Since the meridians converge at the poles, true east and west directions change rapidly in a condition similar to [[gimbal lock]]. Grid north solves this problem.