2.7.1 Data acquisition and representation
GIS data represents real world objects (roads, land use,
elevation) with digital data. Real world objects can be divided into two
abstractions: discrete objects (a house) and continuous fields (rain fall
amount or elevation). There are two broad methods used to store data in a GIS
for both abstractions: Raster and Vector.
a) Raster data
A raster data type is, in essence, any type of digital image
represented in grids. While a digital image is concerned with the output as
representation of reality, in a photograph or art transferred to computer, the
raster data type will reflect an abstraction of reality. Aerial photos are one
commonly used form of raster data, with only one purpose, to display a detailed
image on a map or for the purposes of digitization. Other raster data sets will
contain information regarding elevation or reflectance of a particular
wavelength of light.
Raster data type consists of rows and columns of cells, with
each cell storing a single value. Raster data can be images (raster images)
with each pixel (or cell) containing a color value. Additional values recorded
for each cell may be a discrete value, such as land
use, a continuous value, such as temperature, or a null value
if no data is available. While a raster cell stores a single value, it can be
extended by using raster bands to represent RGB (red, green, blue) colors,
color maps (a mapping between a thematic code and RGB value), or an extended
attribute table with one row for each unique cell value. The resolution of the
raster data set is its cell width in ground units. Raster data is stored in
various formats; from a standard file-based structure of Tagged Image File
(TIF), Joint Photographic Experts Group (JPEG), etc. to binary large object
(BLOB) data stored directly in a relational database management system (RDBMS)
similar to other vector-based feature classes. Database storage, when properly
indexed, allows for quicker retrieval of the raster data but can require
storage of millions of significantly-sized records.
b) Vector data
In a GIS, geographical features are often expressed as
vectors, by considering those features as geometrical shapes. Different
geographical features are expressed by different types of geometry:
· Points
Zero-dimensional points are used for geographical features
that can best be expressed by a single point reference; in other words, simple
location. Points can also be used to represent areas when displayed at a small
scale. For example, cities on a map of the world would be represented by points
rather than polygons. No measurements are possible with point features.
· Lines or polylines
One-dimensional lines or polylines are used for linear
features such as rivers, roads, railroads, trails, and topographic lines.
Again, as with point features, linear features displayed at a small scale will
be represented as linear features rather than as a polygon. Line features can
measure distance.
· Polygons
Two-dimensional polygons are used for geographical features
that cover a particular area of the earth's surface. Such features may include
lakes, park boundaries, buildings, city boundaries, or land uses. Polygons
convey the most amount of information of the file types. Polygon features can
measure perimeter and area.
Each of these geometries is linked to a row in a database
that describes their attributes. For example, a database that describes water
resource may contain its depth, water quality, pollution level. This
information can be used to make a map to describe a particular attribute of the
dataset. For example, water resources could be colored depending on level of
pollution. Different geometries can also be compared.
Vector features can be made to respect spatial integrity
through the application of topology rules such as 'polygons must not overlap'.
Vector data can also be used to represent continuously varying phenomena.
Contour lines and triangulated irregular networks (TIN) are used to represent
elevation or other continuously changing values. TINs record values at point
locations, which are connected by lines to form an irregular mesh of triangles.
The faces of the triangles represent the terrain surface.
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