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:
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.
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.
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
Further reading
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