PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

Figure 4.2: Information representation using raster data [16].
An thophoto, thophotograph or thoimage is an aerial photograph geometrically corrected such
that the scale is uniform: the photo has the same lack of distortion as a map. Unlike an uncorrected
aerial photograph, an orthophotograph can be used to measure true distances because
it is an accurate representation of the Earth’s surface, being adjusted for topographic relief 1 ,
lens distortion 2 and camera tilt 3 . When orthophotographs are displayed underneath other
layers, they provide user with confidence that map layers are spatially aligned and represent real
objects.
Figure 4.3: Basemap raster for road data [16].
• Continuous maps Due to the matrix of cells structure, rasters are well suited for representing
and storing data that changes continuously across a surface. Elevation values measured from
the Earth’s surface are the most common application of surface maps. For example, raster in
Figure 4.4 displays elevation, where green cells show lower elevation and red, pink and white cells
show higher elevation. Other values, such as rainfall, temperature, concentration and population
density, can also be used in spatial analysis as cells representing surfaces are regularly spaced.
• Thematic maps Rasters representing thematic data can be derived from other data through
analyzing operations. A common analysis application is classifying a satellite image by landcover
categories. Basically, this activity groups the values of data into classes (such as vegetation
type) and assigns a categorical value. Figure 4.5 is an example of a classified raster data that
expresses the human use of land. Thematic maps can also result from geoprocessing operations
that combine data from various sources such as vector, raster and terrain data. For example,
user can process data through a geoprocessing model to create raster data that maps suitability
for a specific activity.
• User-defined attributes Digital photography and scanner are output devices that capture real
world phenomena into digital world for long-term storage. Raster data is means by which digital
1 http://en.wikipedia.org/wiki/Topography.
2 http://en.wikipedia.org/wiki/Barrel_distortion.
3 http://en.wikipedia.org/wiki/Camera_tilt.
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