Resource files¶

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 directories. The following paths are checked in order:

For resource files that have an explicit relative or absolute path, the directory specified in the 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.

Added in version 7.0.

The PROJ user writable directory, which is :
- on Windows, ${LOCALAPPDATA}/proj
- on macOS, ${HOME}/Library/Application Support/proj
- on other platforms (Linux), ${XDG_DATA_HOME}/proj if XDG_DATA_HOME is defined. Else ${HOME}/.local/share/proj
Applications may also customize the user writable directory using proj_context_set_user_writable_directory() (since 9.5)
Path(s) set with by the environment variable PROJ_DATA. Prior to PROJ 9.1, this environment variable was called PROJ_LIB. This older name is still supported in PROJ 9.1 as a fallback, but support for it may be removed in future release. On Linux/macOS/Unix, use : to separate paths. On Windows, ;
Added in version 7.0.

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/libproj.so are installed under ../bin/ or ../lib/, 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 or ${CMAKE_INSTALL_PREFIX}/${DATADIR} for CMake builds). Note, however, that since this is a hard-wired path setting, it only works if the whole PROJ installation is not moved somewhere else.

Note

If PROJ is built with the PROJ_DATA_ENV_VAR_TRIED_LAST CMake option (called PROJ_LIB_ENV_VAR_TRIED_LAST before PROJ 9.1), then this hard-wired path will be tried before looking at the environment variable PROJ_DATA.
The current directory

When networking capabilities are enabled, either by API with the proj_context_set_enable_network() function or when the PROJ_NETWORK environment variable is set to ON, PROJ will attempt to use remote grids stored on CDN (Content Delivery Network) storage.

`proj.db`¶

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.

The database may be customized/reduced by deleting entries not relevant for a certain use-case. An example for a simple SQL script removing all entries not related to 'WGS 84' ellipsoid:

PRAGMA FOREIGN_KEYS=1;
DELETE FROM ellipsoid WHERE name != 'WGS 84';
-- clean up table usage
DELETE FROM usage WHERE (object_table_name, object_auth_name, object_code) IN (
  SELECT object_table_name, object_auth_name, object_code FROM usage WHERE NOT EXISTS (
    SELECT 1 FROM object_view o WHERE
        o.table_name = object_table_name AND
        o.auth_name = object_auth_name AND
        o.code = object_code));
VACUUM;
PRAGMA foreign_key_check;

`proj.ini`¶

Added in version 7.0.

proj.ini is a text configuration file, mostly dedicated at setting up network related parameters.

Its default content is:

[general]
; Lines starting by ; are commented lines.
;

; Network capabilities disabled by default.
; Can be overridden with the PROJ_NETWORK=ON environment variable.
; Cf https://proj.org/en/latest/usage/network.html
; Valid values = on, off
network = off

; Endpoint of the Content Delivery Network where remote resources might
; be accessed. Only used if network access is allowed (cf above "network"
; option)
; Can be overridden with the PROJ_NETWORK_ENDPOINT environment variable.
cdn_endpoint = https://cdn.proj.org

; Whether to enable a cache of remote resources that are accessed, on the
; local file system
; Valid values = on, off
cache_enabled = on

; Size of the cache in megabytes
cache_size_MB = 300

; Time-to-live delay in seconds before already accessed remote resources are
; acessed again to check if they have been updated.
cache_ttl_sec = 86400

; Can be set to on so that by default the lack of a known resource files needed
; for the best transformation PROJ would normally use causes an error, or off
; to accept missing resource files without errors or warnings.
; This default value itself is overriden by the PROJ_ONLY_BEST_DEFAULT environment
; variable if set, and then by the ONLY_BEST setting that can be
; passed to the proj_create_crs_to_crs() method, or with the --only-best
; option of the cs2cs program.
; (added in PROJ 9.2)
; Valid values = on, off
only_best_default = off

; Filename of the Certificate Authority (CA) bundle.
; Can be overriden with the PROJ_CURL_CA_BUNDLE / CURL_CA_BUNDLE environment variable.
; (added in PROJ 9.0)
; ca_bundle_path = /path/to/cabundle.pem

; Transverse Mercator (and UTM)  default algorithm: auto, evenden_snyder or poder_engsager
; * evenden_snyder is the fastest, but less accurate far from central meridian
; * poder_engsager is slower, but more accurate far from central meridian
; * default will auto-select between the two above depending on the coordinate
;   to transform and will use evenden_snyder if the error in doing so is below
;   0.1 mm (for an ellipsoid of the size of Earth)
tmerc_default_algo = poder_engsager

Transformation grids¶

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¶

proj-data¶

The proj-data package is a collection of all the resource files that are freely available for use with PROJ. The package is maintained on GitHub and the contents of the package are show-cased on the PROJ CDN. The contents of the package can be installed using the projsync package or by downloading the zip archive of the package and unpacking in the PROJ_DATA directory.

proj-datumgrid¶

Note

The packages described below can be used with PROJ 7 and later but are primarily meant to be used with PROJ 6 and earlier versions. The proj-datumgrid series of packages are not maintained anymore and are only kept available for legacy purposes.

For a functioning build of PROJ prior to version 7, 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.

Regional packages¶

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.

Tip

To download the various datumgrid packages head to the download section.

World package¶

The world package includes grids that have global extent, e.g. the global geoid model EGM08.

-latest packages¶

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 package. 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.

Free grids¶

The following is a list of grids distributed under a free and open license.

Hungary¶

Hungarian grid ETRS89 - HD72/EOV (epsg:23700), both horizontal and elevation grids

Non-Free Grids¶

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.

Austria¶

Overview of Austrian grids and other resources related to the local geodetic reference.

Brazil¶

Brazilian grids for datums Corrego Alegre 1961, Corrego Alegre 1970-72, SAD69 and SAD69(96)

Netherlands¶

Dutch grid (Registration required before download)

Portugal¶

Portuguese grids for ED50, Lisbon 1890, Lisbon 1937 and Datum 73

HTDP¶

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 written in 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

Getting `crs2crs2grid.py`¶

The crs2crs2grid.py script can be found at https://github.com/OSGeo/gdal/blob/master/swig/python/gdal-utils/osgeo_utils/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

Usage¶

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 htdp 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 that htdp is 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:

..NAD_83(2011) (North America tectonic plate fixed)
..NAD_83(CORS96)  (NAD_83(2011) will be used)
..NAD_83(2007)    (NAD_83(2011) will be used)
..NAD_83(PA11) (Pacific tectonic plate fixed)
..NAD_83(PACP00)  (NAD 83(PA11) will be used)
..NAD_83(MA11) (Mariana tectonic plate fixed)
..NAD_83(MARP00)  (NAD_83(MA11) will be used)

..WGS_72                             16...ITRF92
..WGS_84(transit) = NAD_83(2011)     17...ITRF93
..WGS_84(G730) = ITRF92              18...ITRF94 = ITRF96
..WGS_84(G873) = ITRF96              19...ITRF96
..WGS_84(G1150) = ITRF2000           20...ITRF97
..PNEOS_90 = ITRF90                  21...IGS97 = ITRF97
..NEOS_90 = ITRF90                   22...ITRF2000
..SIO/MIT_92 = ITRF91                23...IGS00 = ITRF2000
..ITRF88                             24...IGb00 = ITRF2000
..ITRF89                             25...ITRF2005
..ITRF90                             26...IGS05 = ITRF2005
..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¶

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_DATA 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

+init=epsg:25832 +ellps=intl

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 a central epoch when applying the ITRF2014:NOAM plate motion model:

+init=ITRF2014:NOAM +t_epoch=2010.0

which then expands to

+proj=helmert +drx=0.000024 +dry=-0.000694 +drz=-0.000063 +convention=position_vector +t_epoch=2010.0

Below is a list of the init files that are packaged with PROJ.

Name

Description

GL27

Great Lakes Grids

ITRF2000

Full set of transformation parameters between ITRF2000 and other ITRF's

ITRF2008

Full set of transformation parameters between ITRF2008 and other ITRF's

ITRF2014

Full set of transformation parameters between ITRF2014 and other ITRF's

nad27

State plane coordinate systems, North American Datum 1927

nad83

State plane coordinate systems, North American Datum 1983