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MAP PROJECTIONS All of us use maps in a variety of ways – to get from one place to another without getting lost, to estimate the distance between two points, to find the location of an unfamiliar place where an important social or political event is taking place, and, in the case of biogeographers, to study the distribution of biodiversity. But have you ever wondered how maps are made, or thought about exactly what information is being conveyed by a map? A map can be defined as a two-dimensional representation of the Earth. But, as most of us are now aware, the Earth is not flat. So how can we represent the surface of a roughly spherical object on a flat surface? A map projection is the “flattening” of a curved surface into a plane. As the famed German mathematician Carl Gauss (1777–1855) proved, a curved surface cannot be represented on a plane without distortion. fte type of distortion varies with the type of map projection. ftere are many varieties of map projections, but most fall into three broad categories: cylindrical, conic, and azimuthal. ftere is no single, objectively “best” map projection; the usefulness of any given type of projection depends on the use for which it is intended. A cylindrical projection of the earth can be imagined as a projection of the earth onto a cylindrical sheet of paper, with the cylinder tangentially touching the earth at, for example, the equator. A well-known cylindrical projection is the standard Mercator projection (Fig. 3.1). In this projection, objects such as landmasses are distorted as one moves from the equator toward the poles. For instance, Greenland appears much larger than Australia in Fig. 3.1. However, comparing the true area of the two landmasses, we see that in reality Australia is over 3.5 times as large as Greenland (Fig. 3.2). Fig. 3.3 shows Tissot’s indicatrix, which indicates the distortion at various points for, in this case, the standard Mercator projection. On a spherical globe, the dots would all be equal sizes. Figure 3.1: Mercator projection of the world between 82°S and 82°N (Wikipedia ‘Mercator projection;’ Attribution: Daniel R. Strebe, August 15, 2011) Figure 3.2: Comparison of actual land areas of Australia and Greenland (Wikipedia ‘Mercator projection;’ Attribution: Benjamin Hell (User: Siebengang)) Figure 3.3: Tissot’s Indicatrices on the Mercator projection; on a spherical globe, the dots would all be equal sizes (Wikipedia ‘Mercator projection;’ Attribution: Stefan Kühn) In conic projections, we can visualize a conical sheet of paper intersecting the earth at two standard parallels, resulting in little distortion between the parallels, but increasing distortion with distance away from the parallels. For instance, in an Albers conic projection using standard parallels of 20°N and 50°N, North America is depicted with little distortion, but southern hemisphere landmasses are increasingly distorted, particularly in an east-west direction (Fig. 3.4). Azimuthal projections, which can be visualized as a flat sheet of paper contacting the globe at a central point, provide an accurate depiction of regions near the central point and distances/directions from the central point, but elsewhere shapes and sizes become distorted (Fig. 3.5). Figure 3.4: Albers projection of the world with standard parallels 20°N and 50°N (Wikipedia ‘Albers projection;’ Attribution: Daniel R. Strebe, August 15, 2011) Figure 3.5: Polar azimuthal equidistant projection (Wikipedia ‘Azimuthal equidistant projection;’ Attribution: Daniel R. Strebe, August 15, 2011) For large-scale maps, the transverse Mercator projection, a variant of the standard Mercator projection, is probably the most commonly used. For the transverse Mercator projection, the axis of the cylinder lies along the equatorial plane, and the line of tangency is any chosen meridian, which is designated as the central meridian. ftis projection allows construction of highly accurate large-scale maps anywhere on Earth. For smaller-scale maps that include whole continents or even the entire world, a variety of projections are used, depending on the purpose of the map.

Kategoria: Moje artykuły | Dodał: kolo (2019-03-11)
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