A number of files containing preconfigured transformations and default parameters for certain projections are bundled with the PROJ distribution. Init files contain preconfigured proj-strings for various coordinate reference systems and the defaults file contains default values for parameters of select projections.
In addition to the bundled init-files the PROJ project also distributes a number of packages containing transformation grids and additional init-files not included in the main PROJ package.
Where are PROJ resource files looked for ?¶
PROJ will attempt to locate its resource files - database, transformation grids or init-files - from several dictionaries. The following paths are checked in order:
For transformation grids that have an explict relative or absolute path, the directory specified in the grid filename.
Path resolved by the callback function set with the
proj_context_set_file_finder(). If it is set, the next tests will not be run.
Path(s) set with the
proj_context_set_search_paths(). If set, the next tests will not be run.
Path(s) set with by the environment variable
PROJ_LIB. On Linux/MacOSX/Unix, use
:to separate paths. On Windows,
On Windows, the ..\share\proj\ and its contents are found automatically at run-time if the installation respects the build structure. That is, the binaries and proj.dll are installed under ..\bin\, and resource files are in ..\share\proj\.
A path built into PROJ as its resource installation directory (whose value is $(pkgdatadir)), for builds using the Makefile build system. Note, however, that since this is a hard-wired path setting, it only works if the whole PROJ installation is not moved somewhere else.
The current directory
A proj installation includes a SQLite database of transformation information that must be accessible for the library to work properly. The library will print an error if the database can’t be found.
Grid files are important for shifting and transforming between datums.
PROJ supports CTable2, NTv1 and NTv2 files for horizontal grid corrections and the GTX file format for vertical corrections. Details about the formats can be found in the GDAL documentation. GDAL reads and writes all formats. Using GDAL for construction of new grids is recommended.
External resources and packaged grids¶
For a functioning PROJ, installation of the proj-datumgrid is needed. If you have installed PROJ from a package system chances are that this will already be done for you. The proj-datumgrid package provides transformation grids that are essential for many of the predefined transformations in PROJ. Which grids are included in the package can be seen on the proj-datumgrid repository as well as descriptions of those grids. This is the main grid package and the only one that is required. It includes various older grids that is mostly needed for legacy reasons. Without this package, the test suite fails miserably.
In addition to the default proj-datumgrid package regional packages are also distributed. These include grids and init-files that are valid within the given region. The packages are divided into geographical regions in order to keep the needed disk space by PROJ at a minimum. Some users may have a use for resource files covering several regions in which case they can download more than one.
At the moment three regional resource file packages are distributed:
If someone supplies grids relevant for Africa, South-America, Asia or Antarctica we will create new regional packages.
Click the links to jump to the relevant README files for each package. Details on the content of the packages maintained there.
To download the various datumgrid packages head to the download section.
The world package includes grids that have global extent, e.g. the global geoid model EGM08.
All packages above come in different versions, e.g proj-datumgrid-1.8 or proj-datumgrid-europe-1.4. The -latest packages are symbolic links to the latest version of a given packages. That means that the link https://download.osgeo.org/proj/proj-datumgrid-north-america-latest.zip is equivalent to https://download.osgeo.org/proj/proj-datumgrid-north-america-1.2.zip (as of the time of writing this).
Other transformation grids¶
Below is a list of grid resources for various countries which are not included in the grid distributions mentioned above.
The following is a list of grids distributed under a free and open license.
Not all grid shift files have licensing that allows them to be freely distributed, but can be obtained by users through free and legal methods.
This section describes the use of the crs2crs2grid.py script and the HTDP (Horizontal Time Dependent Positioning) grid shift modelling program from NGS/NOAA to produce PROJ compatible grid shift files for fine grade conversions between various NAD83 epochs and WGS84. Traditionally PROJ has treated NAD83 and WGS84 as equivalent and failed to distinguish between different epochs or realizations of those datums. At the scales of much mapping this is adequate but as interest grows in high resolution imagery and other high resolution mapping this is inadequate. Also, as the North American crust drifts over time the displacement between NAD83 and WGS84 grows (more than one foot over the last two decades).
Getting and building HTDP¶
The HTDP modelling program is in written FORTRAN. The source and documentation can be found on the HTDP page at http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml
On linux systems it will be necessary to install gfortran or some FORTRAN compiler. For ubuntu something like the following should work.
apt-get install gfortran
To compile the program do something like the following to produce the binary “htdp” from the source code.
gfortran htdp.for -o htdp
The crs2crs2grid.py script can be found at https://github.com/OSGeo/gdal/tree/trunk/gdal/swig/python/samples/crs2crs2grid.py
The script depends on having the GDAL Python bindings operational; if they are not you will get an error such as:
Traceback (most recent call last): File "./crs2crs2grid.py", line 37, in <module> from osgeo import gdal, gdal_array, osr ImportError: No module named osgeo
crs2crs2grid.py <src_crs_id> <src_crs_date> <dst_crs_id> <dst_crs_year> [-griddef <ul_lon> <ul_lat> <ll_lon> <ll_lat> <lon_count> <lat_count>] [-htdp <path_to_exe>] [-wrkdir <dirpath>] [-kwf] -o <output_grid_name> -griddef: by default the following values for roughly the continental USA at a six minute step size are used: -127 50 -66 25 251 611 -kwf: keep working files in the working directory for review.
crs2crs2grid.py 29 2002.0 8 2002.0 -o nad83_2002.ct2
The goal of crs2crs2grid.py is to produce a grid shift file for a designated region. The region is defined using the -griddef switch. When missing a continental US region is used. The script creates a set of sample points for the grid definition, runs the “htdp” program against it and then parses the resulting points and computes a point by point shift to encode into the final grid shift file. By default it is assumed the htdp program will be in the executable path. If not, please provide the path to the executable using the -htdp switch.
The htdp program supports transformations between many CRSes and for each (or most?) of them you need to provide a date at which the CRS is fixed. The full set of CRS Ids available in the HTDP program are:
1...NAD_83(2011) (North America tectonic plate fixed) 29...NAD_83(CORS96) (NAD_83(2011) will be used) 30...NAD_83(2007) (NAD_83(2011) will be used) 2...NAD_83(PA11) (Pacific tectonic plate fixed) 31...NAD_83(PACP00) (NAD 83(PA11) will be used) 3...NAD_83(MA11) (Mariana tectonic plate fixed) 32...NAD_83(MARP00) (NAD_83(MA11) will be used) 4...WGS_72 16...ITRF92 5...WGS_84(transit) = NAD_83(2011) 17...ITRF93 6...WGS_84(G730) = ITRF92 18...ITRF94 = ITRF96 7...WGS_84(G873) = ITRF96 19...ITRF96 8...WGS_84(G1150) = ITRF2000 20...ITRF97 9...PNEOS_90 = ITRF90 21...IGS97 = ITRF97 10...NEOS_90 = ITRF90 22...ITRF2000 11...SIO/MIT_92 = ITRF91 23...IGS00 = ITRF2000 12...ITRF88 24...IGb00 = ITRF2000 13...ITRF89 25...ITRF2005 14...ITRF90 26...IGS05 = ITRF2005 15...ITRF91 27...ITRF2008 28...IGS08 = ITRF2008
The typical use case is mapping from NAD83 on a particular date to WGS84 on some date. In this case the source CRS Id “29” (NAD_83(CORS96)) and the destination CRS Id is “8 (WGS_84(G1150)). It is also necessary to select the source and destination date (epoch). For example:
crs2crs2grid.py 29 2002.0 8 2002.0 -o nad83_2002.ct2
The output is a CTable2 format grid shift file suitable for use with PROJ (4.8.0 or newer). It might be utilized something like:
cs2cs +proj=latlong +ellps=GRS80 +nadgrids=./nad83_2002.ct2 +to +proj=latlong +datum=WGS84
Init files are used for preconfiguring proj-strings for often used
transformations, such as those found in the EPSG database. Most init files contain
transformations from a given coordinate reference system to WGS84. This makes
it easy to transform between any two coordinate reference systems with
cs2cs. Init files can however contain any proj-string and don’t necessarily
have to follow the cs2cs paradigm where WGS84 is used as a pivot datum. The
ITRF init file is a good example of that.
A number of init files come pre-bundled with PROJ but it is also possible to
add your own custom init files. PROJ looks for the init files in the directory
listed in the
PROJ_LIB environment variable.
The format of init files is an identifier in angled brackets and a proj-string:
<3819> +proj=longlat +ellps=bessel +towgs84=595.48,121.69,515.35,4.115,-2.9383,0.853,-3.408 +no_defs <>
The above example is the first entry from the
epsg init file. So, this is the
coordinate reference system with ID 3819 in the EPSG database. Comments can be
inserted by prefixing them with a “#”. With version 4.10.0 a new special metadata
entry is now accepted in init files. It can be parsed with a function from the public
API. The metadata entry in the epsg init file looks like this at the time of writing:
<metadata> +version=9.0.0 +origin=EPSG +lastupdate=2017-01-10
Pre-configured proj-strings from init files are used in the following way:
$ cs2cs -v +proj=latlong +to +init=epsg:3819 # ---- From Coordinate System ---- #Lat/long (Geodetic alias) # # +proj=latlong +ellps=WGS84 # ---- To Coordinate System ---- #Lat/long (Geodetic alias) # # +init=epsg:3819 +proj=longlat +ellps=bessel # +towgs84=595.48,121.69,515.35,4.115,-2.9383,0.853,-3.408 +no_defs
It is possible to override parameters when using
+init. Just add the parameter
to the proj-string alongside the
+init parameter. For instance by overriding
the ellipsoid as in the following example
where the Hayford ellipsoid is used instead of the predefined GRS80 ellipsoid. It is also possible to add additional parameters not specified in the init file, for instance by adding an observation epoch when transforming from ITRF2000 to ITRF2005:
which then expands to
+proj=helmert +x=-0.0001 +y=0.0008 +z=0.0058 +s=-0.0004 +dx=0.0002 +dy=-0.0001 +dz=0.0018 +ds=-0.000008 +t_epoch=2000.0 +convention=position_vector +t_obs=2010.5
Below is a list of the init files that are packaged with PROJ.
Great Lakes Grids
Full set of transformation parameters between ITRF2000 and other ITRF’s
Full set of transformation parameters between ITRF2008 and other ITRF’s
Full set of transformation parameters between ITRF2014 and other ITRF’s
State plane coordinate systems, North American Datum 1927
State plane coordinate systems, North American Datum 1983