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9 Working with raster data

From simple queries to complicated algebraic functions and logical conditions practically any analysis and modeling are possible with raster data in GRASS GIS.

Due to the large number of raster data analysis modules currently available (more than 100) this chapter will provide a general intoduction to raster data processing. The topic of remote sensing, which means the processing and analysis of satellite and aerial images, is not part of this book, and plays a subordinated role. Some literature concerning remote sensing analysis is specified in the bibliography.

As described in chapter 8 most of the analyses can be executed using the graphical user interfaces TclTkGRASS and GIS Manager. Here we will explain each step of the anaysis as a GUI operation as well as a command-line instruction. The use of GRASS at the command-line prompt is highly recommended in order to familiarize the user with individual modules, their usage, and the parameters they require.

General information concerning raster data

A raster value is defined by its localization coordinates x,y (cell center) and a z-value, which corresponds to a measurement or object value which is usually assigned a color or grey value when displayed. Two basic operation types are available for creating thematic maps based on raster data:

1. pointwise, raster cell-oriented or pixel-oriented operation (neighborhood operations)
2. matrix-oriented or pixel window supported operation (moving-window-operations)

Besides using the "ready-to-go" GRASS modules, which each provide a particular operation, solution-oriented operations of both types can be defined and applied via the arithmetic module r.mapcalc (see chapter 15).

Raster data management

The basic management for the components of raster data like the spatial reference of maps, attribute and color assignment of the pixel is directly conducted by each raster modules when executed. Thus, the module r.support used in former versions (GRASS 5.0 and 5.3) is no longer necessary. In addition, the creation of map statistics with the module r.support -r is only calculated for the selected map cutout, which in most cases was not intended and led to unintentional effects.

Help functions to the GRASS modules

A help file is available for nearly all of the more than 400 GRASS modules. In this file the module is described and command syntax explained. A short version of the help can be displayed by entering the module command with the parameter -help at the prompt:

d.rast -help

A detailed help with a module description and examples, which is equivalent to the help pages on the GRASS homepage, can be displayed by the command g.manual module name. For this purpose, a standard browser is automatically opened and displays the corresponding help file:

g.manual d.rast &