Maps and Map Data
What Is a Map?
Mapping Toolbox software manipulates electronic representations
of geographic data. It lets you import, create, use, and present geographic
data in a variety of forms and to a variety of ends. In the digital
network era, it is easy to think of geospatial data as maps and maps
as data, but you should take care to note the differences between
these concepts.
The simplest (although perhaps not the most general) definition
of a map is a representation of geographic
data. Most people today generally think of maps as two-dimensional;
to the ancient Egyptians, however, maps first took the form of lists
of place names in the order they would be encountered when following
a given road. Today such a list would be considered as map
data rather than as a map. When most people hear the word
"map" they tend to visualize two-dimensional renditions
such as printed road, political, and topographic maps, but even classroom
globes and computer graphic flight simulation scenes are maps under
this definition.
In this toolbox, map data is any variable or set of variables
representing a set of geographic locations, properties of a region,
or features on a planet's surface, regardless of how large or complex
the data is, or how it is formatted. Such data can be rendered as
maps in a variety of ways using the functions and user interfaces
provided.
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What Is Geospatial Data?
Geospatial data comes in many forms and formats, and its structure
is more complicated than tabular or even nongeographic geometric data.
It is, in fact, a subset of spatial data, which is simply data that
indicates where things are within a given coordinate system.
Mileposts on a highway, an engineering drawing of an automobile part,
and a rendering of a building elevation all have coordinate systems,
and can be represented as spatial data when properly quantified (digitized).
Such coordinate systems, however, are local and not explicitly tied
or oriented to the Earth's surface; thus, most digital representations
of mileposts, machine parts, and buildings do not qualify as geospatial
data (also called geodata).
What sets geospatial data apart from other spatial data is that
it is absolutely or relatively positioned on a planet, or georeferenced.
That is, it has a terrestrial coordinate system that
can be shared by other geospatial data. There are many ways to define
a terrestrial coordinate system and also to transform it to any number
of local coordinate systems, for example, to create a map projection.
However, most are based on a framework that represents a planet as
a sphere or spheroid that spins on a north-south axis, and which is
girded by an equator (an imaginary plane midway
between the poles and perpendicular to the rotational axis).
Geodata is coded for computer storage and applications in two
principal ways: vector and raster representations.
It has been said that "raster is faster but vector is corrector."
There is truth to this, but the situation is more complex. The following
discussions explore these two representations: how they differ, what
data structures support them, why you would choose one over the other,
and how they can work together in the toolbox. The conclude by summarizing
the functions available for importing and exporting geospatial data
formats.
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 | Understanding Map Data | | Types of Map Data Handled by the Toolbox |  |
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