NAME

KEYWORDS

SYNOPSIS

DESCRIPTION

g.proj provides a means of converting a coordinate reference system (CRS) description 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 CRS information for the current project (previously called 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 project

When compiled with OGR, functionality is increased and allows output of the CRS 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 being read from the current project, the CRS information 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 CRS 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 CRS 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 CRS 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 CRS 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 CRS files (PROJ_INFO, PROJ_UNITS, WIND and DEFAULT_WIND) based on the imported information. If the project parameter is specified in addition to -c, then a new project will be created. Otherwise the CRS information files in the current project 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 project, 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 project.

Output is simply based on the input CRS 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 CRS descriptions is limited to 8000 bytes.

EXAMPLES

Print information

g.proj -p

List the possible datum transformation parameters for the current project:

g.proj -t datumtrans=-1

Create projection (PRJ) file

g.proj -wef > irish_grid.prj

Read CRS from file

g.proj -jf georef=ASTER_DEM20020508161837.tif

Convert the PROJ CRS description contained in a text file to WKT format:

cat proj4.description | g.proj -w proj4=-

Create new project

Create a new project with the coordinate system referred to by EPSG code 4326 (Latitude-Longitude/WGS84), without explicitly specifying datum transformation parameters:

g.proj -c epsg=4326 project=latlong

Create a new project with the coordinate system referred to by EPSG code 900913 (Google Mercator Projection)

g.proj -c epsg=900913 project=google

Create a new project with the coordinate 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 project=irish_grid

Create a new project with the same coordinate system as the current project, but forcing a change to datum transformation parameter set no. 1:

g.proj -c project=newloc -t datumtrans=1

Create a new project with the coordinate system from a WKT definition stored in a text file:

g.proj -c wkt=irish_grid.prj project=irish_grid

Create a new project from a PROJ description, explicitly specifying a datum and using the default datum transformation parameters:

g.proj -c project=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 project (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 project using the OGR 'ogr2ogr' tool:

ogr2ogr -t_srs "`g.proj -wf`" polbnda_italy_GB_ovest.shp polbnda_italy_LL.shp

REFERENCES

Further reading

• ASPRS Grids and Datum
• MapRef - The Collection of Map Projections and Reference Systems for Europe
• Projections Transform List (PROJ)

SEE ALSO

AUTHOR

SOURCE CODE

Available at: g.proj source code (history)

Latest change: Saturday Sep 21 14:36:34 2024 in commit: d89ad9e3a186a6791190f25d6cbc1e093390bb37

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© 2003-2024 GRASS Development Team, GRASS GIS 8.5.0dev Reference Manual