Spatial Thinking in Planning Practice: An Introduction to GIS

12 CHAPTER 2: COORDINATE SYSTEMS AND PROJECTING GIS DATA A coordinate system is a way to reference, or locate, everything on the Earth’s surface in x and y space. "e meth- od used to portray a part of the spherical Earth on a 'at surface, whether a paper map or a computer screen, is called a map projection. Each map projection used on a paper map or in a GIS is associated with a coordinate system. To simplify the use of maps and to avoid pinpointing locations on curved latitude-longitude reference lines, cartographers superimpose a rectangular grid on maps. Such grids use coordinate systems to determine the x and y position of any spot on the map. Coordinate systems are o$en identi!ed by the name of the particular projection for which they are designed. Because no single map projection is suitable for all purposes, many dif- ferent coordinate systems have been developed. Some are worldwide or nearly so, while others cover individual countries (such as the United Kingdom’s Ordnance Survey’s coordinate system), and others cover states or parts of states in the U.S. "is chapter begins with concepts that de!ne the geographical referencing standards of the Earth. Topics include latitude and longitude, projections, coordinate systems, and datums. GEOGRAPHIC COORDINATE SYSTEM( LATITUDE AND LONGITUDE Any feature can be referenced by its latitude and longitude, which are angles measured in degrees from the Earth’s center to a point on the Earth’s surface (see Figure 2.1). Across the spherical Earth, latitude lines stretch horizontally from east to west (le$ image in Figure 2.2), and they are parallel to each other, hence their alter- native name, parallels. Longitude lines, also called meridians, stand vertically and stretch from the North Pole to the South Pole (center image in Figure 2.2). Together these “north to south” and “east to west” lines meet at perpendicular angles to form a graticule, a grid that encompasses the Earth (right image in Figure 2.2). Latitude can be thought of as the lines that intersect the y-axis, and longitude as lines that intersect the x-axis. "ink of the equator as the x axis; the y axis is the prime meridian, which is a line running from pole to pole through Greenwich, England. Just as the upper right quarter in the Cartesian coordinate system is positive for both x and y, latitude and longitude east of the prime meridian and north of the equator are both positive. Europe, Asia, and part of Africa – which have positive latitudes and longitudes – correspond to the upper right quarter of the Cartesian coordinate system. With the exception of some U.S. territories in the Paci!c and the westernmost Aleutian islands, all of the United States is north of the equator and west of the prime meridian, so all latitudes in the U.S are positive (or north) while almost all longitudes are negative (or west). Figure 2.1: Latitude and longitude are angles measured in degrees from the Earth’s center to a point on the Earth’s surface. GIS Commons. http://giscommons.org/earth-and-map-preprocessing/