Geography Reference
In-Depth Information
is defined as the surface of the oceans, after removing tidal and weather effects, and its
imaginary continuation to create a surface of equal gravity under the continents. Global
mapping approximates the geoid by an ellipse rotated about the Earth's axis, and since the
mid 1980s has settled on the particular parameters known as the World Geodetic System of
1984 (WGS84). However, many countries still maintain their own systems, with the result
that maps of adjacent countries may not fit along their borders. GPS receivers will often
provide a number of options, including WGS84 and local variants, and the differences can
amount to hundreds of metres of displacement in some cases. Thus data exported from a
mapping package or GIS may well not fit perfectly over the Google Earth base. Many Google
Earth users have commented on the fact that the line of zero longitude misses the Greenwich
Observatory by roughly 100 m, a shift that occurred with the adoption of WGS84.
Google Earth also includes various facilities for converting other forms of georeference.
Placenames are handled by a gazetteer, which converts recognizable names to coordinates,
and queries the user about ambiguous names that might refer to any one of a number of
locations. Street addresses require a geocoding service, which is only available for the most
developed countries.
2.3.2 Features
The images captured by the human retina or by the imaging sensors of Earth-orbiting
satellites are composed of continuous gradations of colour. The human brain and, to a
lesser extent, the image processing and pattern recognition systems of today's computers
are capable of extracting meaning from such images, primarily by identifying the scene's
discrete features. Images are partitioned into trees, vehicles, buildings, mountains, lakes and
many other feature types, and perhaps named and otherwise characterized.
In the digital world such features are likely to be represented as points, lines, areas or
volumes, with associated characteristics or attributes. When displayed visually, as in Google
Earth, points will be converted to symbols, lines will be given some appropriate thickness,
and areas or volumes will be coloured or shaded. However, this concept of discrete features
is only one of two fundamentally different ways in which the geographic world is repre-
sented digitally. The base imagery and topography of Google Earth are both conceptually
continuous, and stored and processed as such. Base imagery is rendered more or less as it
was obtained from the original acquisition system, though the basic picture elements may
have been transformed. Topography is rendered by varying the third spatial dimension, and
by computing perspective based on the assumed location of the user's eye.
This duality between what are known as the discrete-object and continuous-field concep-
tualizations of geographic reality pervades the world of GIS. In Google Earth only the base
imagery and topography are conceptualized as fields, although additional imagery can be
mashed onto the base. Other layers are almost entirely composed of discrete objects, and it
would be difficult to render other field-like phenomena, such as atmospheric temperature
or soil moisture content, within the current environment.
2.3.3 Layers
A fundamental concept in GIS is the layer, based on the principle that geographic features
can be separated into distinct classes or sets, and that the Earth's surface can be characterized
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