g.proj(1grass) GRASS GIS User's Manual g.proj(1grass)
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-or-
dinate 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 trans-
formation 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 popu-
larised by proprietary GIS. In addition, if one of the parameters geo-
ref, wkt, proj4 or epsg is specified, rather than the projection infor-
mation being read from the current location it is imported from an ex-
ternal source as follows:
georef=filename
g.proj attempts to invoke GDAL and OGR in turn to read a georefer-
enced 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, en-
closed 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 lo-
cal 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 sup-
plied 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 sup-
ported 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 DE-
FAULT_WIND) based on the imported information. If the location parame-
ter is specified in addition to -c, then a new location will be cre-
ated. Otherwise the projection information files in the current loca-
tion 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 infor-
mation and datum transformation parameters associated with the co-ordi-
nate 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 transfor-
mation 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 hu-
man-readable) format and exit.
0 (default): Continue without specifying parameters - if used when cre-
ating 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 da-
tum 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 ex-
isting 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 me-
ters.
The maximum size of input WKT or PROJ projection descriptions is lim-
ited 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 loca-
tion:
g.proj -t datumtrans=-1
Create projection (PRJ) file
Create a ’.prj’ file in ESRI format corresponding to the current loca-
tion:
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 da-
tum 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 al-
ways 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 Sys-
tems 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: unknown
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GRASS 7.8.7 g.proj(1grass)
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