Spatial Thinking in Planning Practice: An Introduction to GIS

15 Figure 2.5. "e projected graticules produced by projection equations in each category – plane, cone, and cylinder. http://2012books.lardbucket.org/books/geographic-information-system-basics/s06-02-map-scale-co- ordinate-systems-a.html Referring again to the previous example of a light bulb in the center of a globe, note that during the projection process, we can situate each surface in any number of ways. For example, surfaces can be tangential to the globe along the equator or poles, they can pass through or intersect the surface, and they can be oriented at any num- ber of angles. "e following !gures shows how these projections can vary. PROJECTION AND DISTORTION Flattening the globe cannot be done without introducing some error, and some distortion is unavoidable. Any projection has its area of least distortion. Projections can be shi$ed around in order to put this area of least dis- tortion over the topographer’s area of interest. "us any projection can have an unlimited number of variations or cases that determined by standard parallels or meridians that adjust the location of the high-accuracy part of the projection. If the geographic extent of your project area was small, like a neighborhood or a portion of a city, you could assume that the Earth is 'at and use no projection. "is is referred to as a planar surface or even a planar “projec- tion,” but with the understanding that it does not use a projection. Planar representation does not signi!cantly a#ect a map’s accuracy when scales are larger than 1:10,000. In other words, small areas do not need a projection because the statistical di#erences between locations on a 'at plane and a 3-dimensional surface are not signi!- cant. For small-scale maps one must consider the Earth’s shape. Our assumption that the Earth is round or spheri- cal does not accurately represent it. "e Earth’s constant spinning causes it to bulge slightly along the equator, ruining its perfect spherical shape. "e slightly oval nature of the Earth’s geometric surface makes the terms ellip- soid and spheroid more accurate in describing its shape, but they are not perfect terms either since di#erences in material weights (for instance iron is denser than sedimentary deposits) and the movement of tectonic plates makes the Earth dynamic and constantly changing. "e Earth is a geoid with a slight pear shape; it is a little larg- er in the southern hemisphere and includes other bulges. "e di#erence, however, between the ellipsoid and the geoid is minor enough that it does not a#ect most mapping. Until recently, projections based on geoids were rare because of the complexity and cost of collecting the necessary data to create the projection, but satellite imagery Chapter 2: Coordinate Systems and Projecting GIS Data