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NAME

       g.proj   -  Prints  or modifies GRASS projection information files (in various co-ordinate
       system descriptions).
       Can also be used to create new GRASS locations.

KEYWORDS

       general, projection, create location

SYNOPSIS

       g.proj
       g.proj --help
       g.proj   [-pgdjfwetc]    [georef=file]     [wkt=file]     [srid=params]     [proj4=params]
       [epsg=code]    [list_codes=string]    [datum=name]   [datum_trans=index]   [location=name]
       [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -p
           Print projection information in conventional GRASS format

       -g
           Print projection information in shell script style

       -d
           Verify datum information and print transformation parameters

       -j
           Print projection information in PROJ.4 format

       -f
           Print ’flat’ output with no linebreaks (applies to WKT and PROJ.4 output)

       -w
           Print projection information in WKT format

       -e
           Use ESRI-style format (applies to WKT output only)

       -t
           Force override of datum transformation information in input co-ordinate system

       -c
           Modify current location projection files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       georef=file
           Name of georeferenced data file to read projection information from

       wkt=file
           Name of ASCII file containing a WKT projection description
           ’-’ for standard input

       srid=params
           Spatial reference ID with authority name and code
           E.g. EPSG:4326 or urn:ogc:def:crs:EPSG::4326

       proj4=params
           PROJ.4 projection description
           ’-’ for standard input

       epsg=code
           EPSG projection code
           Options: 1-1000000

       list_codes=string
           List codes for given authority, e.g. EPSG, and exit
           Options: EPSG, ESRI, IAU_2015, IGNF, NKG, OGC, PROJ

       datum=name
           Datum (overrides any datum specified in input co-ordinate system)
           Accepts standard GRASS datum codes, or "list" to list and exit

       datum_trans=index
           Index number of datum transform parameters
           "0" for unspecified or "-1" to list and exit
           Options: -1-100
           Default: 0

       location=name
           Name of new location to create

DESCRIPTION

       g.proj provides a means of converting a co-ordinate system  description  (i.e.  projection
       information) between various formats.

       For an introduction to map projections (with PROJ),see the manual page of r.proj.

       If compiled without OGR present, the functionality is limited to:

           •   Reporting   the  projection  information  for  the  current  location,  either  in
               conventional GRASS (-p flag) or PROJ (-j flag) format

           •   Changing  the  datum,  or  reporting  and  modifying  the   datum   transformation
               parameters, for the current location

       When  compiled  with  OGR,  functionality is increased and allows output of the projection
       information in the Well-Known  Text  (WKT)  format  popularised  by  proprietary  GIS.  In
       addition,  if  one  of the parameters georef, wkt, proj4 or epsg is specified, rather than
       the projection information being read from the current location it  is  imported  from  an
       external source as follows:

       georef=filename
           g.proj  attempts to invoke GDAL and OGR in turn to read a georeferenced file filename.
           The projection  information  will  be  read  from  this  file.  If  the  file  is  not
           georeferenced or cannot be read, XY (unprojected) will be used.

       wkt=filename or -
           The  file  filename  should  contain  a  projection  description in WKT format with or
           without line-breaks (e.g. a ’.prj’ file). If - is given  for  the  filename,  the  WKT
           description will be read from stdin rather than a file.

       proj4=description or -
           description  should  be a projection description in PROJ format, enclosed in quotation
           marks if there are any spaces. If - is given for  description,  the  PROJ  description
           will be read from stdin rather than as a directly-supplied command-line parameter.

       epsg=number
           number should correspond to the index number of a valid co-ordinate system in the EPSG
           database. EPSG code support is based upon a local copy of  the  GDAL  CSV  co-ordinate
           system and datum information files, stored in the directory $GISBASE/etc/proj/ogr_csv.
           These can be updated if necessary to support future revisions of the EPSG database.

       If datum information is incorrect or missing in the input  co-ordinate  system  definition
       (e.g.  PROJ  descriptions  have  very limited support for specifying datum names), a GRASS
       datum abbreviation can instead be supplied using the datum parameter. This  will  override
       any  datum contained in the input co-ordinate system, and discard any datum transformation
       parameters. Enter datum=list to return a list of all the datums supported by GRASS.  Since
       any  existing  datum  transformation  parameters  will have been discarded, the datumtrans
       parameter should in general always be used in conjunction with datum.

       The -p, -j, -w, etc. flags are all functional when importing projection  information  from
       an  external source, meaning that g.proj can be used to convert between representations of
       the information. It is not required that either the input or output be in GRASS format.

       In addition however, if the -c flag is specified, g.proj will create new GRASS  projection
       files (PROJ_INFO, PROJ_UNITS, WIND and DEFAULT_WIND) based on the imported information. If
       the location parameter is specified in addition  to  -c,  then  a  new  location  will  be
       created.   Otherwise  the  projection  information  files  in the current location will be
       overwritten. The program will not warn before doing this.

       The final mode of operation of g.proj is to report on  the  datum  information  and  datum
       transformation  parameters associated with the co-ordinate system. The -d flag will report
       a human-readable summary of this.

NOTES

       If the input co-ordinate system contains a datum name but  no  transformation  parameters,
       and  there  is  more  than  one  suitable  parameter set available (according to the files
       datum.table and datumtransform.table in $GISBASE/etc/proj), g.proj will check the value of
       the datumtrans option and act according to the following:
       -1:  List  available parameter sets in a GUI-parsable (but also human-readable) format and
       exit.
       0 (default): Continue without specifying parameters - if used when  creating  a  location,
       other  GRASS modules will use the "default" (likely non-optimum) parameters for this datum
       if necessary in the future.
       Any other number less than or equal to the number of parameter  sets  available  for  this
       datum: Choose this parameter set and add it to the co-ordinate system description.
       If  the  -t  flag is specified, the module will attempt to change the datum transformation
       parameters using one of the above two methods even if a valid  parameter  set  is  already
       specified  in  the  input  co-ordinate  system.  This  can  be  useful to change the datum
       information for an existing location.

       Output is simply based on the input projection information. g.proj  does  not  attempt  to
       verify that the co-ordinate system thus described matches an existing system in use in the
       world. In particular, this means there are no EPSG Authority codes in the WKT output.

       WKT format shows the false eastings and northings in  the  projected  unit  (e.g.  meters,
       feet) but in PROJ format it should always be given in meters.

       The maximum size of input WKT or PROJ projection descriptions is limited to 8000 bytes.

EXAMPLES

   Print information
       Print the projection information for the current location:
       g.proj -p

       List the possible datum transformation parameters for the current location:
       g.proj -t datumtrans=-1

   Create projection (PRJ) file
       Create a ’.prj’ file in ESRI format corresponding to the current location:
       g.proj -wef > irish_grid.prj

   Read projection from file
       Read the projection information from a GeoTIFF file and print it in PROJ format:
       g.proj -jf georef=ASTER_DEM20020508161837.tif

       Convert the PROJ projection description contained in a text file to WKT format:
       cat proj4.description | g.proj -w proj4=-

   Create new location
       Create  a  new  location  with  the  co-ordinate  system  referred  to  by  EPSG code 4326
       (Latitude-Longitude/WGS84), without explicitly specifying datum transformation parameters:
       g.proj -c epsg=4326 location=latlong

       Create a new location with the co-ordinate system referred to  by  ESRI-EPSG  code  900913
       (Google Mercator Projection)
       g.proj -c epsg=900913 location=google

       Create  a  new  location with the co-ordinate system referred to by EPSG code 29900 (Irish
       Grid), selecting datum transformation parameter set no. 2:
       # list available datums for EPSG code 29900
       g.proj -t datumtrans=-1 epsg=29900
       g.proj -c epsg=29900 datumtrans=2 location=irish_grid

       Create a new location with the same  co-ordinate  system  as  the  current  location,  but
       forcing a change to datum transformation parameter set no. 1:
       g.proj -c location=newloc -t datumtrans=1

       Create  a  new location with the co-ordinate system from a WKT definition stored in a text
       file:
       g.proj -c wkt=irish_grid.prj location=irish_grid

       Create a new location from a PROJ description, explicitly specifying a datum and using the
       default datum transformation parameters:
       g.proj -c location=spain proj4="+proj=utm +zone=30 +ellps=intl" datum=eur50 datumtrans=0

   Using g.proj output for GDAL/OGR tools
       Reproject  external  raster  map to current GRASS projection (does not always make sense!)
       using the GDAL ’gdalwarp’ tool. We recommend to use the ERDAS/Img format and  not  to  use
       the ESRI style of WKT:
       # example for 30x30 pixel resolution (enforce with -tr to avoid odd values)
       gdalwarp -of HFA -tr 30 30 -t_srs "`g.proj -wf`" aster.img aster_tmerc.img

       Reproject external vector map to current GRASS projection using the OGR ’ogr2ogr’ tool:
       ogr2ogr -t_srs "`g.proj -wf`" polbnda_italy_GB_ovest.shp polbnda_italy_LL.shp

REFERENCES

       PROJ: Projection/datum support library
       GDAL raster library and toolset
       OGR vector library and toolset

       Further reading

           •   ASPRS Grids and Datum

           •   MapRef - The Collection of Map Projections and Reference Systems for Europe

           •   Projections Transform List (PROJ)

SEE ALSO

        m.proj, r.proj, v.proj, r.import, r.in.gdal, v.import, v.in.ogr

AUTHOR

       Paul Kelly

SOURCE CODE

       Available at: g.proj source code (history)

       Accessed: Tuesday Jun 27 11:12:39 2023

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       © 2003-2023 GRASS Development Team, GRASS GIS 8.3.0 Reference Manual