NAME¶
g.proj - Prints and manipulates GRASS projection information files
(in various co-ordinate system descriptions).
Can also be used to create new GRASS locations.
KEYWORDS¶
general, projection
SYNOPSIS¶
g.proj
g.proj help
g.proj [-
pgdjfwetc] [
georef=
file]
[
wkt=
file] [
proj4=
params]
[
epsg=
code] [
datum=
name]
[
datumtrans=
index] [
location=
name]
[--
verbose] [--
quiet]
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
-
Create new projection files (modifies current location)
- --verbose
-
Verbose module output
- --quiet
-
Quiet module output
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
- proj4=params
-
PROJ.4 projection description
’-' for standard input
- epsg=code
-
EPSG projection code
Options: 1-1000000
- datum=name
-
Datum (overrides any datum specified in input co-ordinate system)
Accepts standard GRASS datum codes, or "list" to list and
exit
- datumtrans=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. 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.4 (-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.4 format,
enclosed in quotation marks if there are any spaces. If - is given
for description, the PROJ.4 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.4 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.4 projection descriptions is limited to
8000 bytes.
EXAMPLES¶
Print the projection information for the current location:
g.proj -p
Create a '.prj' file in ESRI format corresponding to the current location:
g.proj -wef > irish_grid.prj
Read the projection information from a geotiff file and print it in PROJ.4
format:
g.proj -jf georef=ASTER_DEM20020508161837.tif
Convert the PROJ.4 projection description contained in a text file to WKT
format:
cat proj4.description | g.proj -w proj4=-
Create a new location with the co-ordinate system referred to by EPSG code 4326
(Latitude-Longitude/WGS84), without explicitly specifiying 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 loc=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:
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
List the possible datum transformation parameters for the current location:
g.proj -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.4 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
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 4: 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 (PROJ4)
SEE ALSO¶
g.setproj
r.in.gdal
v.in.ogr
AUTHOR¶
Paul Kelly
Last changed: $Date: 2012-10-17 22:15:48 +0200 (Wed, 17 Oct 2012) $
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