Choosing A Model For The Shape of The Earth
Projection construction is also affected by how the shape of the Earth is approximated. In the following section on projection categories, the earth is taken as a sphere in order to simplify the discussion. However, the Earth's actual shape is closer to an oblate ellipsoid. Whether spherical or ellipsoidal, the principles discussed hold without loss of generality.
Selecting a model for a shape of the Earth involves choosing between the advantages and disadvantages of a sphere versus an ellipsoid. Spherical models are useful for small-scale maps such as world atlases and globes, since the error at that scale is not usually noticeable or important enough to justify using the more complicated ellipsoid. The ellipsoidal model is commonly used to construct topographic maps and for other large- and medium-scale maps that need to accurately depict the land surface.
A third model of the shape of the Earth is the geoid, a complex and more accurate representation of the global mean sea level surface that is obtained through a combination of terrestrial and satellite gravity measurements. This model is not used for mapping because of its complexity, but rather is used for control purposes in the construction of geographic datums. (In geodesy, plural of "datum" is "datums" rather than "data".) A geoid is used to construct a datum by adding irregularities to the ellipsoid in order to better match the Earth's actual shape. It takes into account the large-scale features in the Earth's gravity field associated with mantle convection patterns, and the gravity signatures of very large geomorphic features such as mountain ranges, plateaus and plains.
Historically, datums have been based on ellipsoids that best represent the geoid within the region that the datum is intended to map. Controls (modifications) are added to the ellipsoid in order to construct the datum, which is specialized for a specific geographic region (such as the North American Datum). A few modern datums, such as WGS84 which is used in the Global Positioning System, are optimized to represent the entire earth as well as possible with a single ellipsoid, at the expense of accuracy in smaller regions.
Read more about this topic: Map Projection
Famous quotes containing the words choosing a, choosing, model, shape and/or earth:
“Let him be great, and love shall follow him. Nothing is more deeply punished than the neglect of the affinities by which alone society should be formed, and the insane levity of choosing associates by others’ eyes.”
—Ralph Waldo Emerson (1803–1882)
“Let him be great, and love shall follow him. Nothing is more deeply punished than the neglect of the affinities by which alone society should be formed, and the insane levity of choosing associates by others’ eyes.”
—Ralph Waldo Emerson (1803–1882)
“The best way to teach a child restraint and generosity is to be a model of those qualities yourself. If your child sees that you want a particular item but refrain from buying it, either because it isn’t practical or because you can’t afford it, he will begin to understand restraint. Likewise, if you donate books or clothing to charity, take him with you to distribute the items to teach him about generosity.”
—Lawrence Balter (20th century)
“Art, like Nature, has her monsters, things of bestial shape and with hideous voices.”
—Oscar Wilde (1854–1900)
“A sleeping man holds in a circle around him the thread of the hours, the order of years and of worlds. He consults them instinctively upon awaking and in one second reads in them the point of the earth that he occupies, the time past until his arousal; but their ranks can be mingled or broken.”
—Marcel Proust (1871–1922)