Geography Reference
In-Depth Information
10
Analysis of grids
and surfaces
Analysis of grids
and surfaces
Introduction
Introduction
10.1
h is chapter introduces methods for processing and analysing gridded data in general
and gridded elevation data in particular. Topics include map algebra, image process-
ing, and spatial i lters. Derivatives of altitude, such as slope and other products derived
from surfaces, are also described. Image processing has a central role in GIS contexts.
h is chapter begins by outlining some key approaches to the analysis of gridded data,
of which images are a common example.
h is chapter introduces methods for processing and analysing gridded data in general
and gridded elevation data in particular. Topics include map algebra, image process-
ing, and spatial i lters. Derivatives of altitude, such as slope and other products derived
from surfaces, are also described. Image processing has a central role in GIS contexts.
h is chapter begins by outlining some key approaches to the analysis of gridded data,
of which images are a common example.
Map algebra
Map algebra
10.2
Most GIS sot ware packages include functions for map algebra. In words, using map
algebra raster layers can be combined in various ways. For example, the values in
overlapping cells may be added together using this format:
Most GIS sot ware packages include functions for map algebra. In words, using map
algebra raster layers can be combined in various ways. For example, the values in
overlapping cells may be added together using this format:
OUTPUT = MAP1 + MAP2
OUTPUT = MAP1 + MAP2
Similarly, values could be multiplied or any other arithmetic operation applied.
Obviously, in many cases such operations may be meaningless—it makes no sense to
add altitude values to precipitation values. However, addition of pollutant scores (as
opposed to concentrations), for example, may be sensible. Map algebra provides a
means of masking i les. For example, if a raster exists which has values of 1 for areas of
interest and 0 for areas that are not of interest then this map layer could be multiplied
Similarly, values could be multiplied or any other arithmetic operation applied.
Obviously, in many cases such operations may be meaningless—it makes no sense to
add altitude values to precipitation values. However, addition of pollutant scores (as
opposed to concentrations), for example, may be sensible. Map algebra provides a
means of masking i les. For example, if a raster exists which has values of 1 for areas of
interest and 0 for areas that are not of interest then this map layer could be multiplied
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