Medir Area Con Google Maps
Katja Gains
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No tem segredo: visite google.com/earth, selecione a opo Medir distncia da barra lateral esquerda, e v clicando nas fronteiras do pas, estado ou terreno escolhido. Quando voc fechar o polgono, o Google Earth vai informar o permetro e a rea.
O Google tambm sugere medir distncias entre duas cidades (de onde voc mora at Nova York, por exemplo) e a rea dos estdios da Copa do Mundo, como a arena Zenit, onde foi realizada a partida Brasil x Costa Rica.
V em google.com/maps, clique com o boto direito no ponto inicial, escolha a opo Medir distncia e depois clique com o boto esquerdo ao longo do permetro. Quando voc terminar, o Maps vai informar a rea e tambm a distncia total.
A coordinate reference system (CRS) then defines, with the help of coordinates,how the two-dimensional, projected map in your GIS is related to real places on the earth.The decision as to which map projection and coordinate reference system to use, depends onthe regional extent of the area you want to work in, on the analysis you want todo and often on the availability of data.
Most of the thematic map data commonly used in GIS applications are ofconsiderably larger scale. Typical GIS datasets have scales of 1:250 000 orgreater, depending on the level of detail. A globe of this size would be difficultand expensive to produce and even more difficult to carry around. As a result,cartographers have developed a set of techniques called map projectionsdesigned to show, with reasonable accuracy, the spherical earth in two-dimensions.
When viewed at close range the earth appears to be relatively flat. However whenviewed from space, we can see that the earth is relatively spherical. Maps, aswe will see in the upcoming map production topic, are representations of reality.They are designed to not only represent features, but also their shape and spatialarrangement. Each map projection has advantages and disadvantages. Thebest projection for a map depends on the scale of the map, and on the purposesfor which it will be used. For example, a projection may have unacceptabledistortions if used to map the entire African continent, but may be an excellentchoice for a large-scale (detailed) map of your country. The properties of amap projection may also influence some of the design features of the map. Someprojections are good for small areas, some are good for mapping areas with a largeEast-West extent, and some are better for mapping areas with a large North-Southextent.
The process of creating map projections is best illustrated by positioning a lightsource inside a transparent globe on which opaque earth features are placed. Thenproject the feature outlines onto a two-dimensional flat piece of paper.Different ways of projecting can be produced by surrounding the globe in acylindrical fashion, as a cone, or even as a flat surface. Each ofthese methods produces what is called a map projection family. Therefore,there is a family of planar projections, a family of cylindricalprojections, and another called conical projections (seefigure_projection_families)
Today, of course, the process of projecting the spherical earth onto a flat pieceof paper is done using the mathematical principles of geometry and trigonometry.This recreates the physical projection of light through the globe.
Map projections are never absolutely accurate representations of the sphericalearth. As a result of the map projection process, every map shows distortionsof angular conformity, distance and area. A map projection may combine severalof these characteristics, or may be a compromise that distorts all the propertiesof area, distance and angular conformity, within some acceptable limit. Examplesof compromise projections are the Winkel Tripel projection and the Robinsonprojection (see figure_robinson_projection), which are often used for producingand visualizing world maps.
It is usually impossible to preserve all characteristics at the same time in amap projection. This means that when you want to carry out accurate analyticaloperations, you need to use a map projection that provides the bestcharacteristics for your analyses. For example, if you need to measure distanceson your map, you should try to use a map projection for your data that provideshigh accuracy for distances.
When working with a globe, the main directions of the compass rose (North, East,South and West) will always occur at 90 degrees to one another. In other words,East will always occur at a 90 degree angle to North. Maintaining correct angularproperties can be preserved on a map projection as well. A map projection thatretains this property of angular conformity is called a conformal ororthomorphic projection.
These projections are used when the preservation of angular relationships isimportant. They are commonly used for navigational or meteorological tasks. Itis important to remember that maintaining true angles on a map is difficult forlarge areas and should be attempted only for small portions of the earth. Theconformal type of projection results in distortions of areas, meaning that ifarea measurements are made on the map, they will be incorrect. The larger thearea the less accurate the area measurements will be. Examples are the Mercatorprojection (as shown in figure_mercator_projection) and the Lambert ConformalConic projection. The U.S. Geological Survey uses a conformal projection formany of its topographic maps.
If your goal in projecting a map is to accurately measure distances, you shouldselect a projection that is designed to preserve distances well. Such projections,called equidistant projections, require that the scale of the map iskept constant. A map is equidistant when it correctly represents distancesfrom the centre of the projection to any other place on the map. Equidistantprojections maintain accurate distances from the centre of the projection oralong given lines. These projections are used for radio and seismic mapping, andfor navigation. The Plate Carree Equidistant Cylindrical (seefigure_plate_caree_projection) and the Equirectangular projection are twogood examples of equidistant projections. The Azimuthal Equidistant projectionis the projection used for the emblem of the United Nations (seefigure_azimuthal_equidistant_projection).
With the help of coordinate reference systems (CRS) every place on the earth canbe specified by a set of three numbers, called coordinates. In general CRS can bedivided into projected coordinate reference systems (also called Cartesianor rectangular coordinate reference systems) and geographic coordinate referencesystems.
Lines of longitude, on the other hand, do not stand up so well to thestandard of uniformity. Lines of longitude run perpendicular to the equator andconverge at the poles. The reference line for longitude (the prime meridian) runsfrom the North pole to the South pole through Greenwich, England. Subsequentlines of longitude are measured from zero to 180 degrees East or West of the primemeridian. Note that values West of the prime meridian are assigned negative valuesfor use in digital mapping applications. See figure_geographic_crs for a pictorialview.
A two-dimensional coordinate reference system is commonly defined by two axes.At right angles to each other, they form a so called XY-plane (seefigure_projected_crs on the left side). The horizontal axis is normally labelledX, and the vertical axis is normally labelled Y. In a three-dimensionalcoordinate reference system, another axis, normally labelled Z, is added. Itis also at right angles to the X and Y axes. The Z axis provides thethird dimension of space (see figure_projected_crs on the right side). Everypoint that is expressed in spherical coordinates can be expressed as an X Y Zcoordinate.
A projected coordinate reference system in the southern hemisphere (south of theequator) normally has its origin on the equator at a specific Longitude. Thismeans that the Y-values increase southwards and the X-values increase to the West.In the northern hemisphere (north of the equator) the origin is also the equatorat a specific Longitude. However, now the Y-values increase northwards andthe X-values increase to the East. In the following section, we describe aprojected coordinate reference system, called Universal Transverse Mercator (UTM)often used for South Africa.
Say, for example, that we want to define a two-dimensional coordinate within theArea of Interest (AOI) marked with a red cross in figure_utm_for_sa. You cansee, that the area is located within the UTM zone 35S. This means, to minimizedistortion and to get accurate analysis results, we should use UTM zone 35Sas the coordinate reference system.
As you can probably imagine, there might be a situation where the data you wantto use in a GIS are projected in different coordinate reference systems. Forexample, you might get a vector layer showing the boundaries of South Africaprojected in UTM 35S and another vector layer with point information aboutrainfall provided in the geographic coordinate system WGS 84. In GIS these twovector layers are placed in totally different areas of the map window, becausethey have different projections.
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