g.proj — Prints or modifies GRASS projection information files (in various co-ordinate system descriptions). Can also be used to create new GRASS locations. g.projg.proj --helpg.proj [-pgdjfwetc] [georef=file] [wkt=file] [proj4=params] [epsg=code] [datum=name] [datum_trans=index] [location=name] [--help] [--verbose] [--quiet] [--ui] 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: 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. Create a new location with the co-ordinate system referred to by EPSG code 4326 (Latitude-Longitude/WGS84), without explicitly specifiying datum transformation parameters: Create a new location with the co-ordinate system referred to by ESRI-EPSG code 900913 (Google Mercator Projection) Create a new location with the co-ordinate system referred to by EPSG code 29900 (Irish Grid), selecting datum transformation parameter set no. 2: 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: Create a new location from a PROJ.4 description, explicitly specifying a datum and using the default datum transformation parameters: 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: PROJ 4: Projection/datum support library GDAL raster library and toolset OGR vector library and toolset Source.