Spatial Thinking in Planning Practice: An Introduction to GIS

13 Figure 2.2: Latitude, longitude, and the Earth’s graticule. GIS Commons. http://giscommons.org/ earth-and-map-preprocessing/ Midway between the poles, the equator stretches around the Earth, and it de!nes the line of zero degrees latitude (le$ image in Figure 2.2). Relative to the equator, latitude is measured from 90 degrees at the North Pole to -90 degrees at the South Pole. "e Prime Meridian is the line of zero degrees longitude (center image in Figure 2.2), and in most coordinate systems, it passes through Greenwich, England. Longitude runs from -180 degrees west of the Prime Meridian to 180 degrees east of the same meridian. Because the globe is 360 degrees in circumfer- ence, -180 and 180 degrees is the same location. PROJECTION ( TRANSFORMATION OF GEOGRAPHICAL COORDINATES TO CAR( TESIAN COORDINATE SYSTEMS While the system of latitude and longitude provides a consistent referencing system for anywhere on the earth, in order to portray our information on maps or for making calculations, we need to transform these angular mea- sures to Cartesian coordinates. "ese transformations amount to a mapping of geometric relationships expressed on the shell of a globe to a 'atten-able surface -- a mathematical problem that is !guratively referred to as Projec- tion. Globes do not need projections, and even though they are the best way to depict the Earth’s shape and to un- derstand latitude and longitude, they are not practical for most applications that require maps. We need 'at maps. "is requires a reshaping of the Earth’s 3-dimensions into a 2-dimensional surface. To illustrate the concept of a map projection, imagine that we place a light bulb in the center of a translucent globe (Figure 2.3). On the globe are outlines of the continents and the lines of longitude and latitude called the graticule. When we turn the light bulb on, the outline of the continents and the graticule will be “projected” as shadows on the wall, ceiling, or any other nearby surface. "is is what is meant by map “projection.” "e term “projection” implies that the ball-shaped net of parallels and meridians is transformed by casting its shadow upon some 'at, or 'atten-able, surface. In fact, almost all map projection methods are mathematical equations. Chapter 2: Coordinate Systems and Projecting GIS Data