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Shvoong Home>Science>Astronomy>Cosmographia Summary

Cosmographia

Book Summary   by:joseju     Original Author: Nicolaus Mercator
This abstract was translated from Cosmographia

2. THE REPRESENTATION DE THE EARTH: The MAP PROJECTIONS
the perfect knowledge of the form and volume of the Earth is immediate goal to obtain by geodesy.
the measurement of the dimensions of the Earth was tried either in the antiquity and was or Erastótales de Cirene that in century III a.C established the terrestrial dimensions calculating the value of the arc of which it goes from Alexandria to the present Asuan in the valley of the Nile. At the end of century XVIII the Cossini brothers measured the distance between Dunkerque and Perpiñan.
the measurements of arcs of meridians conducted in different latitudes have stood out that the Earth exactly spherical but is not slightly flattened by the poles. Recently the study of the orbits followed by the artificial satellites has allowed to know with greater exactitude the form and dimensions the Earth. In order to arrive at the old measurements a triangulation network was used that is the first work of the cartographer. This network is denominated “of triangulation” because the zone to map is covered virtually with a network of triangles, the vertices of these triangles denominate geodesic vertices. The geographic engineers place signals and install sights in each one of these points and with the aid of a called apparatus theodolite sees other vertices and calculates the value of each triangle.
On this triangular plot leans all the later cartographic work.
the elaboration of a map demands to choose a projection system previously and is different types from projections:

Cylindrical P.: The deformations that undergo the maps drawn up with this type of projections are little important in the zone of the equator, but very great in the zones of the polar circles because they appear very separated of this a the west.

Conical P.: .En this type of projection the error is greater in the low latitudes that in the polar caps.

Flat P.: this type of projection implies to represent the Earth sphericity on a flat surface, in this projection the network of meridians and parallels projects on a drying or tangent plane the terrestrial surface.
Each type of projection takes with himself important deformations, these affect the angles, to the surface and the linear length. It is competition of the geographers finding by mathematical calculation the more advisable layout of the meridians and parallels for the endings of the map that interests to prepare. No system can simultaneously conserve the proportion of the angles and the surfaces; if the map sees being used by a navigator, is fundamental to respect the value of the angles. The projections that consider this fact denominate as. But if the map is destined to represent the distribution of the population, then he is essential to respect the surfaces; and to this type of projection it is denominated equivalent.

2,1 the cartographic representation.
Once established the triangulation network, and drawn the network of meridians and parallels is necessary to take to the map the geographic data that are tried to map. The preparation of a map requires the election of a scale, the terrain reconnaissance and meticulous works of drawings.
the maps on small scale represent very extensive regions in a surface very reduced the maps or air-navigation charts usually have a scale of 1:500000, whereas the scholastic rural maps usually are 1:5000000 fact.
the modern Spanish cartography fundamentally makes the Geographic and Cadastral Institute and the Geographic Service of the Army; both work united in the national topographic map preparation on 1:50000 scale. it is the basic map of our cartography and this formed by 1130 leaves and began to make in 1870 and it was finished in 1964. It is constructed in polyhedral projection and each leaf corresponds to a grid destined to know the land that delimits two separated meridians by 20· and two parallelos with a 10 separation of ·.
the relief imagines by contours with an equidistance of 20 meters.the old maps were made by means of topographical surveys on the land and with the help of the theodolite. But at the present time the aerial photography is the base of all the maps, thanks to this technique, the old topographical surveys are being bought up to date. In almost all the countries photographic missions in charge exist to observe and to photograph the terrestrial surface. The photogrammetry is the technique of the obtaining of maps and planes from the aerial photography, observed with estereoscopio, the observer perfectly sees single an image but in relief.
Published: November 06, 2005
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