You will find references to three different norths in the margin of OS maps.\u00a0 True north is just that – a direct line to the North Pole and the Earth’s spin axis. You will be very interested in this if aligning an astronomical telecope. \u00a0 \u00a0 Grid north is as defined by the grid lines on the map which cannot match true north because the map is a flat representation of a curved surface. \u00a0The north-south grid lines point west of true north for areas west of the true origin at 2 degrees west, and east for those to the east but this difference is only about 0.75 degrees per degree of longitude.<\/p>\n
Magnetic North is the direction indicated by a magnetic compass; it does not coincide with true or grid north and wanders from year to year. \u00a0Its\u00a0variation differs\u00a0across the country but with localised anomalies due to geological factors so any calculations can only be approximate. \u00a0Furthermore, magnetic storms, often coinciding with the 11 year solar sunspot cycle, can produce large short lived disturbances (eg an 8\u02da\u00a0change in less than an hour was recorded in 1989 at Lerwick).\u00a0\u00a0 Variation in the UK is small compared with that in many parts of the world but not quite small enough to be ignored – it was 4 degrees west of grid in Wales in 1996, decreasing by a degree every 8 years.\u00a0 I hope to be still walking when it’s zero!\u00a0\u00a0 Whitaker’s Almanack documents this variation in\u00a0Britain, is published annually and should be found in the reference section of any public library.\u00a0\u00a0\u00a0Alternatively, the\u00a0World Magnetic Model Calculator<\/a>\u00a0hosted by the British Geological Survey site is a useful resource.<\/p>\n This difference between magnetic and grid bearings is called magnetic variation in\u00a0Britain\u00a0and in nautical circles but declination in US.\u00a0 Neither term is actually printed in the margins of the OS maps. \u00a0To add to the confusion, declination is used to describe the latitude of stars (right ascension for longitude). \u00a0In\u00a0Britain, it also describes the vertical angle between the horizontal and the earth\u2019s magnetic field (magnetic dip in the\u00a0US?). \u00a0We are divided by a common language!<\/p>\n","protected":false},"excerpt":{"rendered":" You will find references to three different norths in the margin of OS maps.\u00a0 True north is just that – a direct line to the North Pole and the Earth’s spin axis. You will be very interested in this if aligning an astronomical telecope. \u00a0 \u00a0 Grid north is as defined by the grid lines […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":40,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-31","page","type-page","status-publish","hentry"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/pages\/31","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/comments?post=31"}],"version-history":[{"count":4,"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/pages\/31\/revisions"}],"predecessor-version":[{"id":139,"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/pages\/31\/revisions\/139"}],"wp:attachment":[{"href":"https:\/\/www.haroldstreet.org.uk\/osgb\/wp-json\/wp\/v2\/media?parent=31"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}