Magnetic, Grid True NORTH SHURDINGTON （磁、网格与真北 SHURDINGTON）.pdf

Magnetic, Grid True NORTH We need a way of expressing direction that is accurate, is adaptable to any part of the world, and has a common unit of measure. Directions are expressed as units of angular measure. Degree. The most common unit of measure is the degree (°) with its subdivisions of minutes () and seconds (). A full circle contains 360°. 1 degree = 60 minutes. 1 minute = 60 seconds. BASE LINES In order to measure something, there must always be a starting point or zero measurement. To express direction as a unit of angular measure, there must be a starting point or zero measure and a point of reference. These two points designate the base or reference line. There are three base lines— true north, magnetic north, and grid north. The most commonly used are magnetic and grid. a. True North. A line from any point on the earths surface to the north pole. All lines of longitude are true north lines. True north is usually represented by a star (see Picture). b. Magnetic North. The direction to the north magnetic pole, as indicated by the north-seeking needle of a magnetic instrument. The magnetic north is usually symbolised by a line ending with half of an arrowhead (see Picture). Magnetic readings are obtained with magnetic instruments, (such as magnetic compass or lensatic sighting compass). c. Grid North. The north that is established by using the vertical grid lines on the map. Grid north may be symbolised by the letters GN. th 15 Cheltenham (SHURDINGTON) Scouts - 2004