VIEWING AND MANAGING RASTER MAP LAYERS

GRASS is a multipurpose GIS software, it support vector and raster data format. In GRASS, raster data can be stored as a 2D integer grid, 2D floating point grid (single or double precision), or a 3D floating point grid (single or double precision). To display a raster map into 'Map Display' window, first we have to click on 'Add raster layer' button under pulldown menu on 'GRASS GIS Manager' window. And if we take a look at GRASS GIS Manager, we can notice that this window is divided into three main section. First upper layer consist of pulldown menu and graphical buttons where we can manage various kind of GRASS command. In the middle of the windows, there's a list of Map layers that we want to manage. The lower part of this window is Display map management. For raster map, we can manage to display only particular value and set the background color. We can try to display a geology map, only the value that resemble sandstone and limestone and aspect map that show us the southern exposure slope (Spearfish dataset can be downloaded HERE).

Every time GRASS GIS display a raster map, a color scheme or known as color table from each map was loaded to distinguish each raster map value. But when a map has no color table file, the rainbow color coding will be used. If we are not satisfied with the default color table, we can assign new color table using r.colors module ( Raster -> Manage map colors -> Colors table).
When we want to know the meaning of each color displayed, just dislay map legend by activate the 'add raster legend layer'. This 'layer' can be managed so we'll be able to show all legend or just what we need and also the size and the position. Another information that help us to understand the map better is scale bar and north arrow.

To acquire more specific value at a certain location, for example if we confused with the color table, we can use d.what.rast module. This module is integrated in the 'Map Display' window, a graphical interface name 'Query' in the right hand side of Zoom button. Every time we click on the map, the coordinate and the value will appear on output – GIS window. On the right hand side of Query button there are Measure and Create profile buttons. Distance between two point on the map can be calculate using Measure button. Profile of a transect on the raster map can be draw using Create profile button, just try to create elevation profile of spearfish area.

r.info module (Raster → Reports and statistics → Report basic map information) is used to show the metadata of a map. All important information about the map can be retrieve from this module. r.report module ( Raster → Reports and statistics → sum area by map and category) is used to retrieve category information of a raster map. We can also retrieve general statistic information, such as category label, total area for each category, number of cell using r.stat module ( Raster → Reports and statistics → General statistics).

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GRASS GIS: Understanding the GRASS Database

One of main different between GRASS and others GIS software is on its geodata management. Other GIS software allow us to store the geodata in any file directory of our computer, and we can also rename, copy, remove or erase the data using file manager. GRASS in the other hand, require us to store it under certain directory called DATABASE or GISDBASE. This directory has to be created with a file manager (or mkdir command), before starting to work with GRASS. After that, all of internal organization and management of geodata should be left to GRASS. Operation such us renaming, copying or deleting data are highly recommended to be done through GRASS command.
When we enter GRASS for the first time, we are asked to create new LOCATION, a subdirectory under DATABASE where the projects are orginezed. A LOCATION is defined by its coordinate system, map projection and geographical boundaries. Each LOCATION can have several MAPSETs. There are two purposes to maintain different mapsets. First to store maps related to project issues or subregions, and second is to support simultaneous access of several users to the map layers stored within the same LOCATION.
When creating a new LOCATION, a special MAPSET called PERMANENT is automatically creates. In this Mapset the core data for the project are stored. Data in the PERMANENT MAPSET can only be added, modified or removed by the owner of the PERMANENT MAPSET; however, they can be accessed, analyzed, and copied into their own MAPSET by the users. To import data into PERMANENT, just start GRASS with the relevant LOCATION and the PERMANENT MAPSET. This mapset also contains the DEFAULT_WIND file which holds the default region boundary coordinate values. In all mapsets additionally a WIND file is kept for storing the current boundary coordinate values and the currently selected raster resolution.
The internal organization and management of LOCATION, MAPSETs and map layers should be left to GRASS. Operations such as renaming or copying map layers involve several internal files and should always be done through GRASS commands. Non-GRASS interventions are acceptable only in exceptional situations and when one has a good understanding of GRASS internal structure.
The most important thing to understand for first user of GRASS, is that a map layer is represented by several files which include the data, categories, header, and other information. To simplify procedures such as listing, copying, renaming and deleting map layers, a set of file management tools is available. These commands must be used to maintain the consistency in the GRASS DATABASE. It is not recommended to directly modify the files in the LOCATION or MAPSET directories, using file manager.
If there is no maps (core data) in PERMANENT, it will only contains files with information about the project area:

DEFAULT_WIND Specifications of the edge coordinates, extension and resolution of the environment PERMANENT

MYNAME Name of the project -> e.g.: spearfish

PROJ_INFO Specifications of the projection -> e.g. tmerc (Transverse Mercator
Projection), bessel (ellipsoid), potsdam (date)

PROJ_UNITS Specifications of the units used e.g.: meter

WIND Specifications of the current REGION and of the MAPSET projection

VAR Specifications of the database driver and path
After we Input our project with vector map, sattelite image and another data, our work will be saved in the mapset of the users current GRASS session.

When we work with GRASS, the resulting maps of any analysis are saved in the mapset of the users current GRASS session.
Mapset file structure:

cats/ Category values (e.g. color or temperature values) and attributes
(classes with caption) of the individual raster maps
cell/ Individual raster maps
cellhd/ Header rows of the individual raster maps
cell_misc/ Statistical data of the individual raster maps
colr/ Color information of the individual raster maps
dbf/ Contains the internal vector attributes in DBASE format
fcell/ Raster maps with floating point numbers (f: floating point)
hist/ 'Developing history' of the individual raster maps
vector/ Contains the individual vector data (geometry, topology, etc.)
WIND Data of the current REGION and the MAPSET projection

For a first GRASS user, this arrangement is quite confusing and sometime frustating. After several time using GRASS, I reallized that this is one of thing that make GRASS better than any other GIS software. It is saver and easier to orginezed so many map layer with GRASS without worry that I wil forgotten where I saved my data like when I used other GIS Software.

When starting a new project, we need to define a new LOCATION and its projection and coordinate system. To create a Location the following parameters must be known:
1. Coordinate system (xy environment or environment with projection, ellipsoid and datum)
2. Area of interest (minimum and maximum coordinates of the work area)
3. Raster resolution (usually the most frequently occurring raster resolution)

After starting GRASS click on the button Create New Location for creating a new location. Now, the text-based mode is opened. When a session is ended, the start mode is saved and become the default start mode. Please remember: The mapset PERMANENT in which the general information of the project region is saved in individual files will automatically be created by GRASS, even if you have entered an other name in the sector mapset.
Subsequently, the keystrokes , that is typical for GRASS, leads you to the screen
for defining the project region.

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