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GRASS GIS Database

       A GRASS GIS Database is simply a set of directories and files with certain structure which
       GRASS GIS works efficiently with.  Location is  a  directory  with  data  related  to  one
       geographic  location or a project.  All data within one Location has the same cartographic
       projection.  A Location contains Mapsets and  each  Mapset  contains  data  related  to  a
       specific  task,  user  or  a smaller project.  Within each Location, a mandatory PERMANENT
       Mapset exists which can contain commonly used data within a Location such  as  base  maps.
       PERMANENT  Mapset  also  contains  metadata  related to Location such as projection.  When
       GRASS GIS is started it connects to a Database, Location and Mapset specified by the user.

       Fig. 1: GRASS GIS Database structure as visible to the user

   GRASS GIS Database
       All data for GRASS GIS must be in GRASS GIS Database which is a directory (visible on  the
       disk)  containing  subdirectories which are GRASS Locations.  User can have one or more of
       Databases on the disk. Typically users have one directory called grassdata in  their  home
       directory.   In multi-user environment users often have a grassdata directory mounted as a
       network directory (network file system).  For teams, a centralized GRASS DATABASE would be
       defined in a shared network file system (e.g. NFS).

       GRASS  GIS  Databases  can  be  safely copied or moved as any other directories.  Don’t be
       confused with (relational) databases which are used in GRASS GIS to  hold  attribute  data
       and  might be part of the GRASS GIS Database.  From user point of view, GRASS GIS Database
       with all its data in  it  is  similar  to,  e.g.  PostGIS,  database,  as  it  stores  all
       information  inside  in  a specific format and is accessible by specific tools.  GRASS GIS
       Databases is in GRASS GIS often called GISDBASE or DATABASE.

   GRASS Locations
       Location is a directory which contains GRASS Mapsets which are  its  subdirectories.   All
       data  in  one Location have the same projection (coordinate system, datum).  Each Location
       must contain Mapset called PERMANENT.  Typically, a Location contains all data related  to
       one project or a geographic area (geographic location or region).  Alternatively, Location
       can simply contain all data in a given projection.

       GRASS Locations can be safely copied  or  moved  as  any  other  directories.   Compressed
       Location is usually what GRASS users exchange between each other when they want to share a
       lot of data.  For example, GRASS GIS sample data are provided as Locations.

       Don’t be confused with location as a place (file or directory) in a file system.  The word
       location  in  GRASS  Location  refers  to  a  location  or  area  on Earth (or whatever is
       applicable).  Users and programmers familiar with relational databases such as  PostgreSQL
       can  view  Location  as  an  individual database inside the system or a storage area which
       would be equivalent to GRASS GIS Database. Mapsets in a Locations are like  namespaces  or
       schemas inside a database.

   GRASS Mapsets
       Mapsets  contains  the  actual  data, mostly geospatial data, referred to as maps in GRASS
       GIS.  Mapsets are a tool for organizing maps in a transparent way as well as  a  tool  for
       isolating different tasks to prevent data loss.

       GRASS  GIS  is  always  connected to one particular Mapset.  GRASS GIS modules can create,
       modify, change, or delete a data only in the current Mapset.  By default,  only  the  data
       from the current Mapset and PERMANENT Mapset are visible. Using g.mapsets module or in GUI
       other Mapsets can be made visible and seamlessly accessible.  All data are  available  for
       reading  when  Mapset is specified explicitly, for example to access map streets in Mapset
       new_highway user can use streets@new_highway.  For  maps  which  are  in  the  current  or
       PERMAENT  Mapsets or Mapsets sets as visible (accessible), there is no need to use @mapset
       syntax.

       Mapsets are used to store maps related to one project,  smaller  project,  specific  task,
       issue  or  subregions.   In  multi-user  environment,  when  a  team works together on one
       project, Mapsets support simultaneous access of several users to the  maps  stored  within
       the  same Location.  Besides access to his or her own Mapset, each user can also read maps
       in PERMANENT Mapsent and in other users’ Mapsets when set.  However, each user can  modify
       or remove only the maps in his or her own Mapset.

       Besides  the  geospatial  data, Mapset holds additional data such as color tables (managed
       e.g. by r.colors) and the current computational region’s extent and resolution stored in a
       file called WIND and managed by g.region.

       Mapsets  can  be  copied  and moved as directories, however only when it is clear that the
       projections of both Locations (as reported by g.proj) match  each  other.  Since  this  is
       sometimes hard to to establish, it is recommended to use r.proj or v.proj to reproject the
       data.  The files and directories should not be moved or modified directly, but only  using
       GRASS GIS tools.

   The role of the PERMANENT Mapset
       When  creating  a  new  Location,  GRASS GIS automatically creates a special Mapset called
       PERMANENT where the core data for the Location are stored.

       Since the maps in PERMANENT Mapset are visible from all the other Mapsets, it can be  used
       to  store  the  base  maps  (base  cartography), data common to all projects or needed for
       different analyses done is separate Mapsets.

       In multi-user environment, 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 other  users.  The  PERMANENT  Mapset  is  useful  for
       providing  general  spatial data (e.g. an elevation model), accessible but write-protected
       to all users who are working in the same Location as the database owner.  To manipulate or
       add  data to PERMANENT, the owner can start GRASS GIS and choose the relevant Location and
       the PERMANENT Mapset.

       The PERMANENT  Mapset  also  contains  the  DEFAULT_WIND  file  which  holds  the  default
       computational  region’s  extent  and resolution values for the Location (which all Mapsets
       will inherit when they are created).  Users have the  option  of  switching  back  to  the
       default region at any time.

   Importing, exporting and linking data
       GRASS GIS works only with data which are imported into a GRASS Database, so all data needs
       to be imported, e.g. by  r.in.gdal  or  highly  convenient  r.import,  before  the  actual
       analysis.   Data  in  GRASS Datable can be exported using for example r.in.gdal in case of
       raster maps.

       For cases when  import  is  not  desirable,  an  option  to  link  external  data  exists.
       Projection  of the linked data must match the Location’s projection otherwise the external
       data cannot be linked. (Linking data in different projection is not allowed  as  it  would
       require on-the-fly reprojection which could cause inconsistencies in the data.

       For example, module r.external links external raster data, so that the data are accessible
       in  GRASS  Database  as  standard  raster  maps.  Similarly  for   newly   created   maps,
       r.external.out setups a format and directory where the actual data will be stored, however
       in GRASS Database the data will be created as standard maps.

   Starting GRASS GIS using GUI
       After launching GRASS GIS, the startup window will open (Fig. 2).

       Fig. 2: GRASS GIS startup window The startup windows provides these functions:

       1      Selecting the GRASS GIS Database directory.

       2      Selecting the Location (e.g. a project or area).  See the Location Wizard  (4)  for
              creating new Locations.

       3      Selecting  the  Mapset (a subproject or task).  Creating a new Mapset requires only
              name.

       4      The Location Wizard for creating new  Locations  based  for  example,  on  existing
              georeferenced file or EPSG code.

       5      Download a sample Location from the Internet.

       6      Start GRASS GIS once you have selected an existing Location and Mapset or defined a
              new one. The graphical user interface wxGUI will open and provide you with  a  menu
              system, map visualization tool, digitizer, and more.

   Starting GRASS GIS using command line
       GRASS  GIS  can be started with given Database, Location and Mapset from the command line.
       For example, the following will start in a given Mapset with only command line interface:
       grass76 --text ~/grassdata/mylocation/mymapset
       And the following will create the given Location with projection given by  the  EPSG  code
       and it will start the default interface (GUI or command line):
       grass76 -c EPSG:5514:3 ~/grassdata/mylocation
       See grass command manual for more details.

   Creating a New Location with the Location Wizard
       The  wxGUI  graphical  user  interface provides a graphical Location Wizard which lets you
       easily create a new Location for your own data. You will be guided  through  a  series  of
       dialogues to browse and select predefined projections or to define custom projections.

       The  most  convenient  way  of  using  Location Wizard is creating new Location based on a
       georeferenced file, such as Shapefile or GeoTIFF, or by selecting the  corresponding  EPSG
       projection  code.   In  case  of  using georeferenced file, you are asked whether the data
       itself should be imported into the new Location.  The default region is then set to  match
       imported map.

       After defining a new Location, wxGUI starts automatically.  If data were already imported,
       you can add them into the Layer Manager now and display them.  More data can  be  imported
       into  the  Location,  e.g.  using  import  options  in  the  File menu in Layer Manager or
       r.import.

See also

        GRASS GIS 7 Reference Manual
       GRASS GIS 7 startup program manual page
       Importing data on GRASS Wiki
       r.import, v.import, r.external, v.external, r.proj, v.proj,

       Last changed: $Date: 2018-09-05 07:59:43 +0200 (Wed, 05 Sep 2018) $

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