SUBCOURSE EDITION - Modern Prepper

d. Although Greenwich, England, is commonly used as a prime meridian of longitude, otherprime meridians of longitude are also used in various countries. Since the Meridian Conference of 1884,Greenwich has been used by almost every nation in the world. FM 21-26 lists the locations of some ofthe other prime meridians with reference to the Greenwich meridian.e. The network of meridians and parallels is called the graticule.4-3. Properties of ProjectionsDespite their inherent distortions, every projection is designed to present accurately certain relationshipswhich exist on the earth's surface. To preserve the desired property, however, it becomes necessary tosacrifice other features. Among the properties of the many projection systems in common use are thefollowing:a. Equal area. Projections of this type maintain a correct proportion between the sizes of actualland areas and the sizes of their map images, but at the expense of correct shape of the areas.b. Conformal. In this type of projection, the shapes of land areas are correctly represented,because at any given point the scale is the same in all directions, and meridians and parallels intersect atright angles. The scale may vary, however, from one part of the projection to another, and the relativesizes of areas are not correctly shown.c. Azimuthal. Certain projections have been designed so that all azimuths are true whenmeasured from one point, usually the center of the projection. Maps with this property are generallyused for polar areas, and for air distance charts.d. Rhumb lines (loxodromes). For centuries, mariners have set their courses to follow rhumblines, or lines of constant bearing, because of the ease of navigation, although a rhumb line, except fordue N-S, and due E-W, actually forms a spiral curve on the earth's surface. Mercator designed thisprojection to depict rhumb lines as straight lines, a property invaluable to marine navigators in plottingcourses. The Mercator projection is the only one with this property.e. Great circles. The shortest distance between any two points on the earth's surface alwaysfalls along a segment of a great circle. Only one projection, the Gnomonic, depicts great circle distancesas straight lines. This property is important in aeronautical navigation, which often uses radio signalsrather than compass bearings, and so follows a great circle instead of a rhumb line.4-4. Projections Used in Military Mappinga. The map maker must select, from among the many projection systems available, the onewhose properties are most suitable for the intended use of the map. For military maps, the essentialproperty is conformality, and all standard military maps are based on projections which are classed asconformal. These projections are as follows:127