Spatial Thinking in Planning Practice: An Introduction to GIS

16 has helped with measurement and geoid projections are now more common. Projections are abstractions, and they introduce distortions to either the Earth’s shape, area, distance, or direc- tion (and sometimes to all of these properties). Di#erent map projections cause di#erent map distortions. One way to classify map projections is to describe them by the characteristic they do not distort. Usually only one property is preserved in a projection. Map projections classi!ed based on the preserved properties include: % Conformal - Preserves: Shape, Distorts: Area % Equal Area - Preserves: Area; Distorts: Shape, Scale or Angle (bearing) % Equidistant - Preserves: Distances between certain points (but not all points); Distorts: Other distances % Azimuthal (True Direction) - Preserves: Angles (bearings); Distorts: Area and shape Map projections that accurately represent distances are referred to as equidistant projections. Note that distances are only correct in one direction, usually running north–south, and are not correct everywhere across the map. Equidistant maps are frequently used for small-scale maps that cover large areas because they do a good job of preserving the shape of geographic features such as continent. Maps that represent angles between locations, also referred to as bearings, are called conformal. Conformal map projections are used for navigational purposes due to the importance of maintaining a bearing or heading when traveling great distances. "e cost of preserving bearings is that areas tend to be quite distorted in conformal map projections. "ough shapes are more or less preserved over small areas, at small scales areas become wildly distorted. "e Mercator projection is an example of a conformal projection and is famous for distorting Green- land. As the name indicates, equal area or equivalent projections preserve the quality of area. Such projections are of particular use when accurate measures or comparisons of geographical distributions are necessary (e.g., defor- estation, wetlands). In an e#ort to maintain true proportions in the surface of the earth, features sometimes be- come compressed or stretched depending on the orientation of the projection. Moreover, such projections distort distances as well as angular relationships. As noted earlier, there are theoretically an in!nite number of map projections to choose from. One of the key considerations behind the choice of map projection is to reduce the amount of distortion. "e geographical object being mapped and the respective scale at which the map will be constructed are also important factors to think about. For instance, maps of the North and South Poles usually use planar or azimuthal projections, and conical projections are best suited for the middle latitude areas of the earth. Features that stretch east–west, such as the country of Russia, are represented well with the standard cylindrical projection, while countries oriented north–south (e.g., Chile, Norway) are better represented using a transverse projection. If a map projection is unknown, sometimes it can be identi!ed by working backward and examining closely the nature and orientation of the graticule (i.e., grid of latitude and longitude), as well as the varying degrees of distortion. Clearly, there are trade-o#s made with regard to distortion on every map. "ere are no hard-and-fast rules as to which distortions are more preferred over others. "erefore, the selection of map projection largely depends on the purpose of the map. Within the scope of GISs, knowing and understanding map projections are critical. For instance, in order to per- form an overlay analysis, all map layers need to be in the same projection. If they are not, geographical features will not be aligned properly, and any analyses performed will be inaccurate and incorrect. If you want to con- Chapter 2: Coordinate Systems and Projecting GIS Data