D 2019-05-08T16:20:25.702 L PROJ.6 P 70a4fcd137bc5035b00de2519bd915b178090415 U sandro W 37442 back

Introduction

PROJ is a well-known library for performing conversions between cartographic projections.
It's universally supported by almost all open source GIS-oriented applications and packages, so there is no need to waste time in futher presentations.
We just need a bit of history to fully understand the current state of the art of this library:

Past History

Timeline:

Very few users and developers do really realize how ancient is PROJ, and how far in time it started moving its first steps.
in 1980 (about 40 years ago) Gerald Evenden started working on the very first PROJ version, and at the time it was a Ratfor program.
in 1985 the code was completely rewritten in C to run on UNIX systems, and it was named PROJ.2.
in 1990 was released an updated version named PROJ.3

in 1994 a more advanced version was released, and it was obviously named PROJ.4

in 1995 Evenden stopped any further development activity and the project become inactive for several years.

in 2000 Frank Warmerdam became the new maintainer and released version 4.4

After this reborn the revitalized project continued to be regularly maintained, but no further relevant improvements were introduced.
PROJ.4 just continued its very placid evolution in a substantially conservative way.

Short conclusion: the fourth version of PROJ (aka PROJ.4) lasted for about two decades, a very uncommon situation.
And consequently a full generation of developers and users became sincerely convinced that PROJ.4 was the real name of the library.
The revolution comes
Timeline:

in 2018 Even Rouault, Kristian Evers and others start developing a revolutionized PROJ supporting many relevant innovations.
PROJ.5 is intended to be the first preliminary step of a more complex evolution schema.

on March 2019 a more mature version is released, and it's PROJ.6

Next year (2020) PROJ.7 is expected to be released, and it will fully complete the transition between the old and new architectures.

Note: the whole transition implies many relevant changes, so that a deeply revised API will be required. In other words, the old PROJ.4 and the new PROJ.7 will support two different APIs, thus abruptly breaking cross-version compatibility.
This is an umpleasant new, because it practically means that all software modules depending on PROJ (this including SpatiaLite) will require a not at all trivial rewrite in order to fulfill the new API requirements.
But when you consider that's the first time in its very long life that PROJ requires an extra effort in order to introduce so many useful innovations, this unexpected API breakage looks fully justified and absolutely reasonable.

More details about the API breakage:

PROJ.5 started introducing the new API, but was still able to support the old traditional API without any complaint.

PROJ.6 have depreceted the old traditional API.
It still continues to be reluctanctly supported, but the library requires to be compiled by explicitly defining a -DACCEPT_USE_OF_DEPRECATED_PROJ_API_H=1 directive in order to effectively enable this option.

and finally the next-to-come PROJ.7 will completely get rid of the old API.

What's new in PROJ.6

Fully supporting ISO-19111:2019 WKT
Old versions of PROJ (including PROJ.4) required to define each CRS (Coordinate Reference System) by a corresponding proj-string. The following table exemplifies the case of few CRSes:

SRID CRS Name proj-string

3003 Monte Mario / Italy zone 1 +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68 +units=m +no_defs

4326 WGS 84 +proj=longlat +datum=WGS84 +no_defs

32632 WGS 84 / UTM zone 32N +proj=utm +zone=32 +datum=WGS84 +units=m +no_defs

New versions of PROJ (starting since PROJ.6) still continue to support the old proj-strings, but the preferred notation for defining any CRS is now conformant to the ISO-19111:2019 international standard
(OGC Abstract Specification Topic 2: “Referencing By Coordinates”).
The following table exemplifies the same CRSes as above in the more recent ISO WKT notation:

SRID 3003 4326 32632

CRS Name Monte Mario / Italy zone 1 WGS 84 WGS 84 / UTM zone 32N

ISO-2018 WKT PROJCRS["Monte Mario / Italy zone 1", BASEGEODCRS["Monte Mario", DATUM["Monte Mario",
ELLIPSOID["International 1924",6378388,297, LENGTHUNIT["metre",1]]], PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]]], CONVERSION["Italy zone 1", METHOD["Transverse Mercator", ID["EPSG",9807]], PARAMETER["Latitude of natural origin",0, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8801]], PARAMETER["Longitude of natural origin",9, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8802]], PARAMETER["Scale factor at natural origin",0.9996, SCALEUNIT["unity",1], ID["EPSG",8805]], PARAMETER["False easting",1500000, LENGTHUNIT["metre",1], ID["EPSG",8806]], PARAMETER["False northing",0, LENGTHUNIT["metre",1], ID["EPSG",8807]]], CS[Cartesian,2], AXIS["easting (X)",east, ORDER[1], LENGTHUNIT["metre",1]], AXIS["northing (Y)",north, ORDER[2],
LENGTHUNIT["metre",1]], AREA["Italy - west of 12°E"], BBOX[36.53,5.94,47.04,12], ID["EPSG",3003]] GEODCRS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84",6378137,298.257223563, LENGTHUNIT["metre",1]]], PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]], CS[ellipsoidal,2], AXIS["geodetic latitude (Lat)",north, ORDER[1], ANGLEUNIT["degree",0.0174532925199433]], AXIS["geodetic longitude (Lon)",east, ORDER[2], ANGLEUNIT["degree",0.0174532925199433]], AREA["World"], BBOX[-90,-180,90,180], ID["EPSG",4326]] PROJCRS["WGS 84 / UTM zone 32N", BASEGEODCRS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84",6378137,298.257223563, LENGTHUNIT["metre",1]]], PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]]], CONVERSION["UTM zone 32N", METHOD["Transverse Mercator", ID["EPSG",9807]], PARAMETER["Latitude of natural origin",0, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8801]], PARAMETER["Longitude of natural origin",9, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8802]], PARAMETER["Scale factor at natural origin",0.9996, SCALEUNIT["unity",1], ID["EPSG",8805]], PARAMETER["False easting",500000, LENGTHUNIT["metre",1], ID["EPSG",8806]], PARAMETER["False northing",0, LENGTHUNIT["metre",1], ID["EPSG",8807]]], CS[Cartesian,2], AXIS["(E)",east, ORDER[1], LENGTHUNIT["metre",1]], AXIS["(N)",north, ORDER[2], LENGTHUNIT["metre",1]], AREA["World - N hemisphere - 6°E to 12°E - by country"], BBOX[0,6,84,12], ID["EPSG",32632]]

As you can easily notice, the two notations are profoundly different. The old proj-string notation is extremely concise and rough, whilst the new ISO-WKT is verbose but exhaustive, detailed and precise.
There is no possible match: ISO-WKT is clearly superior and more sophisticated under any possible aspect.
Not at all surprisingly, coordinate transformations based on ISO-WKT definitions (instead of proj-strings) are usually expected to be more accurate.

And that's not all; the old proj-strings weren't formally defined by any standard, and only PROJ, GDAL and few other FLOSS / GFOSS implementations can understand them.
At the opposite, the new ISO-WKT is formally defined by an international standard, and almost all free and proprietary implementations are expected to support it.

Important notice: there are several dialects in the WKT notation describing CRSes.
All them share the same common core and mainly differ in few minor details.
PROJ.6 can understand the following dialects:

ESRI WKT: a dialect widely adopted by ESRI proprietary software.

GDAL WKT1: a dialect historically supported by the open source GDAL library.

WKT2:2015: as defined by ISO 19162:2015 standard specification (now superseded).

WKT2:2018: as defined by the more recent ISO 19162:2018 standard specification .

The private SQLite database supporting PROJ.6
Starting since PROJ.6 the PROJ library is supported by a private SQLite database containing all definitions about supported CRSes, Ellipsoids, Prime Meridians, Units of Measure, Transformations and alike.
Note: many of the new advanced PROJ.6 features will fail if such database is not available at run time.

Transformations from CRS to CRS
Differently from all previous versions, PROJ.6 can now apply any transformation from a CRS to another without requiring to specify any proj-string or WKT expression qualifying each CRS.
This is because PROJ.6 is now supported by a private SQLite database of its own containing all the CRS definitions, and consequently each CRS can be fully identified by simply specifying its symbolic name, as in:

EPSG:4326 or

urn:ogc:def:crs:EPSG::4326

azzo

Transformation pipelines
There is a last astonishing improvement supported by PROJ.6, that are transformation pipelines.
When using a pipeline you can freely define any complex geodetic transformation by chaining together many elementary steps such as conversion, transformation, projection, axis swap and so on.
A pipeline is conceptually similar to a UNIX shell script, with a dataflow regularly proceding forward from step to step until producing the final result.
The following is a practial example of a pipeline corresponding to a transformation from EPSG:4326 WGS 84 to EPSG:32632 WGS 84 / UTM zone 32N: +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=utm +zone=32 +ellps=WGS84

Useful hint: PROJ.6 supports a new CLI tool (projinfo) for inspecting in full detail the internal definitions of CRSes, Transformations etc.
The above pipeline is simply the output of the following command: projinfo -s EPSG:4326 -t EPSG:32632 -o proj projinfo is a very precious resource; don't esitate to frequently use it in order to discover what's really happening behind the scenes.
It's the best and easiest way for fully understanting how PROJ.6 do really works.

In order to learn more about the many interesting cool new features supported by PROJ.6 please consult the original documentation

SpatiaLite-5.0.0 supporting PROJ.6

Creating and populating the spatial_ref_sys metatable

Existing SQL functions affected by PROJ.6

SQL Function Extended signature Behavior under PROJ.6

ST_Transform() ST_Transform ( geom Geometry , newSrid Integer , area Geometry ) : Geometry
ST_Transform ( geom Geometry , newSrid Integer , area Geometry , method Text ) : Geometry
ST_Transform ( geom Geometry , newSrid Integer , area Geometry , method Text , proj_string Text ) : Geometry

Note: all these extended signatures are supported only when libspatialite-5.0 has been built on the top of PROJ.6
If the library has been built instead on earlier versions of PROJ any attempt to call these extended signatures will just return a wrong number of arguments to function ST_Transform() error.

Any invalid argument passed to ST_Transform() (extended signatures) will raise an SQL Exception.

the optional argument area may be NULL (default setting).
Otherwise it's expected to contain a valid Geometry in SRID=4326 (long/lat). When an area of use is explicitly supplied, then PROJ.6 may eventually use the corresponding BBOX for finely tuning the more accurate transformation.

the optional argument method is expected to contain one the following values (auth_name:auth_strid will be always assumed as the default setting):

proj4text: both CRSes (origin and destination) will be defined by passing to PROJ.6 the traditionals proj-string definitions for the corresponding SRIDs retrieved from column proj4text in Table spatial_ref_sys
This exactly corresponds to the well extablished traditional behavior of all previous versions of libspatialite and still continues to be supported, but isn't any longer the best way for taking full profit of the more advanced capabilities of PROJ.6

srtext: both CRSes (origin and destination) will be defined by passing to PROJ.6 the WKT definitions for the corresponding SRIDs retrieved from column srtext in Table spatial_ref_sys

auth_name:auth_srid: both CRSes (origin and destination) will be defined by ignoring the definitions stored in spatial_ref_sys and leaving PROJ.6 free to retrieve their corresponding definitions from its own private SQLite database.
Note: this method will always take full profit of the more advanced capabilities of PROJ.6, even when using some SpatiaLite database created by earlier versions not yet supporting PROJ.6

proj_string: an user-defined proj-string (may be one definining a transformation pipeline) will be passed to PROJ.6; this is an advanced option reserved to skilled power users.

the optional argument proj_string may be NULL (default setting).
Otherwise it's expected to contain a valid PROJ.6 transformation string, may well be corresponding to an user-defined transformation pipeline. Meaningful only in the case of method=proj_string

Notes

SpatiaLite does not constraints you in any way and fully respects users freedom carefully avoiding auto-magical assumptions.

The new extended signatures supported by ST_Transform closely mimic the most recent PROJ.6 API, so you are free to define your coordinates transformations in several different ways.

The default behavior of SpatiaLite when using PROJ.6 is to completely ignore the CRS definitions stored within the internal table spatial_ref_sys
Any required CRS definition will be always retrieved form the private SQLite database supporting PROJ.6 (method=auth_name:auth_srid).

If you have any good reason suggesting to do such a thing, you are anyway free to use the proj-strings (method=proj4text) or the WKT definitions (method=srtext) stored into the internal table spatial_ref_sys table.
But this is under your full responsibility, and you must be well self-conscious of what are you doing.

SpatiaLite encourages and promotes the most creative uses of PROJ.6, this including transformation pipelines.
Correctly using method=proj_string for computing custom transformations isn't necessarily simple and easy, but it's a big opportunity for all skilled and talented power users.

Few practical SQL examples: -- to EPSG:3003 - default settings; ignoring spatialite_ref_sys and using the PROJ.6 own private database SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056 , 4326 ) , 3003 )); ------------------------------------- SRID=3003;POINT(1732852.942716769 4816277.617690674) -- to EPSG:3003 - NULL area of use, using WKT definitions from spatial_ref_sys.srtext SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003 , NULL , 'srtext' )); ------------------------------------- SRID=3003;POINT(1732852.942716769 4816277.617690674) -- to EPSG:3003 - NULL area of use, using proj-string definitions from spatial_ref_sys.proj4text SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003 , NULL , 'proj4text' )); ------------------------------------- SRID=3003;POINT(1732852.942716769 4816277.617690674) This first test is based on a new SpatiaLite database created with full PROJ.6 support. There isn't anything worth to be noted, except that all methods return the same result.

-- to EPSG:3003 - using a transformation pipeline SELECT AsEWKT ( ST_Transform( SwapCoords ( MakePoint( 11.878056 , 43.463056, 4326 ) ) , 3003 , NULL , 'proj_string' , '+proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=WGS84 +step +inv +proj=helmert +x=-104.1 +y=-49.1 +z=-9.9 +rx=0.971 +ry=-2.917 +rz=0.714 +s=-11.68 +convention=position_vector +step +inv +proj=cart +ellps=intl +step +proj=pop +v_3 +step +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl' )); ------------------------------------- SRID=3003;POINT(1732852.942716769 4816277.617690674) -- to EPSG:3003 - using a customized transformation pipeline SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003 , NULL , 'proj_string' , '+proj=pipeline +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=WGS84 +step +inv +proj=helmert +x=-104.1 +y=-49.1 +z=-9.9 +rx=0.971 +ry=-2.917 +rz=0.714 +s=-11.68 +convention=position_vector +step +inv +proj=cart +ellps=intl +step +proj=pop +v_3 +step +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl' )); ------------------------------------- SRID=3003;POINT(1732852.942716769 4816277.617690674) This second test is based on using transformation pipelines:

the first query directly uses a proj-string representing the pipeline as defined by executing
projinfo -s EPSG:4326 -t EPSG:3003 --area "Italy - mainland" -o proj
(we'll see this topic in more depth on the next paragraph).

the second query is just a small adaptation of the first one:

As you can notice the first query requires calling SwapCoodinates() because the first step of the pipeline has an axisswap directive.

The second query just avoids to call SwapCoords() and consequently the trasformation pipeline step removes this first step so to get the same identical overall effect.

Note: SpatiaLite will always do a best effort in order to guess if swapping the coordinates before passing them to PROJ.6 is required or not.
But in the specific case of a transformation pipeline this test may easily fail, thus requiring to explicitly call SwapCoords()

using projinfo for detecting transformation pipelines and supported areas of use
projinfo -s EPSG:4326 -t EPSG:3003 -o proj --spatial-test intersect Candidate operations found: 3 ------------------------------------- Operation n°1: unknown id, Inverse of Monte Mario to WGS 84 (4) + Italy zone 1, 4 m, Italy - mainland PROJ string: +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=WGS84 +step +inv +proj=helmert +x=-104.1 +y=-49.1 +z=-9.9 +rx=0.971 +ry=-2.917 +rz=0.714 +s=-11.68 +convention=position_vector +step +inv +proj=cart +ellps=intl +step +proj=pop +v_3 +step +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl ------------------------------------- Operation n°2: unknown id, Inverse of Monte Mario to WGS 84 (2) + Italy zone 1, 4 m, Italy - Sardinia onshore PROJ string: +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=WGS84 +step +inv +proj=helmert +x=-168.6 +y=-34 +z=38.6 +rx=-0.374 +ry=-0.679 +rz=-1.379 +s=-9.48 +convention=position_vector +step +inv +proj=cart +ellps=intl +step +proj=pop +v_3 +step +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl ------------------------------------- Operation n°3: unknown id, Inverse of Monte Mario to WGS 84 (11) + Italy zone 1, 10 m, Italy - Sicily Strait west of 13°E PROJ string: +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=WGS84 +step +proj=helmert +x=230.47 +y=56.08 +z=-22.43 +step +inv +proj=cart +ellps=intl +step +proj=pop +v_3 +step +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl

-- to EPSG:3003 - default settings, NULL area of use, - ignoring spatialite_ref_sys and using the PROJ.6 own private database SELECT AsEWKT ( ST_Transform( MakePoint( 9.169464, 39.478275, 4326 ), 3003 )); ------------------------------------- SRID=3003;POINT(1514600.134321862 4369874.489269957) -- to EPSG:3003 - same as above, but explicitly defining a specific area of use SELECT AsEWKT ( ST_Transform( MakePoint( 9.169464, 39.478275, 4326 ), 3003 , BuildMBR(9.1 , 39.4 , 9.2 , 39.5 , 4326 ))); ------------------------------------- SRID=3003;POINT(1514605.982762248 4369873.004226943) Third and last example. In some cases explicitly defining a specific area of use may enable PROJ.6 to select an optimized transformation leading to more precise results.
In this example we've used a Point located in Sardinia, and PROJ.6 supports several flavors of EPSG:3003 depending on the specific area of use.
Italy - Sardinia onshore is one between them, so after specifying an explicit area of use a more precise transformation pipeline was selected.

Lesson learned:

The best mode for taking full profit from PROJ.6 is by using method=auth_name:auth_srid (that is the default setting).
This way you'll be absolutely sure that the most recent and fully detailed definitions will be always retrieved from the PROJ.6 own database completely ignoring any definition (may be incomplete or obsolete) eventually stored into the spatial_ref_sys table. This is a particulary useful option when processing older databases being created by earlier versions of SpatiaLite not yet supporting PROJ.6

SpatiaLite does not constraints you in any way and fully respects users freedom carefully avoiding auto-magical assumptions.
If you have any good reason suggesting to do such a thing you are anyway free to use the proj-strings (method=proj4text) or the WKT definitions (method=srtext) stored into the spatial_ref_sys table.
But this is under your full responsibility, and you must be well sure of what are you doing.

SpatiaLite encourages and promotes the most creative uses of PROJ.6, this including transformation pipelines.
Correctly using method=proj_string for computing custom transformations isn't necessarily simple and easy, but it's a big opportunity for all skilled and talented power users.

New auxiliary SQL functions specifically supporting PROJ.6

SQL Function Supported arguments Description

HasProj6( void ) : Boolean None Will return 1 (TRUE) if the library has been built on PROJ.6 (or any later version), otherwise 0 (FALSE).

PROJ_GetLastErrorMsg( void ) : String None Will return the most recent error message returned by PROJ (if any).
NULL will be returned if there is no curently pending PROJ error.

PROJ_GetDatabasePath( void : String) None Will return the currently set pathname leading to the private PROJ's SQLite database.
NULL will be returned if there is no private PROJ's SQLite database currently connected.

PROJ_SetDatabasePath ( new_path String ) : String

new_path: a relative or absolute pathname leading to a valid PROJ's SQLite database.

Will change the currently set pathname leading to the private PROJ's SQLite database.
NULL will be returned if the passed path is invalid, otherwise the path of the currently set private PROJ's SQLite database will be returned.

PROJ_AsWKT ( auth_name String , auth_srid Integer ) : String
PROJ_AsWKT ( auth_name String , auth_srid Integer , wkt_style String ) : String
PROJ_AsWKT ( auth_name String , auth_srid Integer , wkt_style String , indented Boolean ) : String
PROJ_AsWKT ( auth_name String , auth_srid Integer , wkt_style String , indented Boolean , indentation Integer ) : String

auth_name and auth_srid identify the intended CRS.
auth_name can be NULL, and in this case EPSG will be assumed.

the optional argument style determines which specific WKT format should be adopted, and must be one between GDAL, ESRI, ISO-2015 or ISO-2018 (this latter being the default setting).

the optional argument indented if set to TRUE will nicely format a multiline WKT expression, otherwise a single monolithic line lacking any white-space or new-line will be printed (the default is TRUE).

the optional argument indentation determines how many white-spaces are to be used for indenting (only meaningful if indented=TRUE; the default values is 4).

Will return the WKT expression corresponding to a given CRS; the definitions will be taken directly from the private PROJ's own database.
NULL will be returned on failure or on invalid arguments.

PROJ_GuessSridFromWKT ( wkt_expr String ) : Integer

wkt_expr: the WKT expression to be evaluated.

Will possibly return the SRID value corresponding to a given WKT expression defining a CRS.
-1 will be returned if no CRS supported by PROJ.6 matches the WKT expression.
NULL will be returned on invalid argument.

PROJ_GuessSridFromSHP ( filename String ) : Integer

filename: the absolute or relative path leading to some Shapefile.
Note: exactley as required by ImportSHP() filename must omit any .shp, .shx, .dbf or .prj suffix.

Will possibly return the SRID value corresponding to the CRS defined by the .PRJ member of the Shapefile.
-1 will be returned if no CRS supported by PROJ.6 matches PRJ member of the Shapefile.
NULL will be returned on invalid argument.
Please note well: this SQL function opend the door to many potential security issues, and thus is always disabled by default.
Explicitly setting the environment variable SPATIALITE_SECURITY=relaxed is absolutely required in order to enable this function.

Note: all the above SQL Functions will be available only when Spatialite-5.0.0 has been built against PROJ.6 (or any subsequent version).
If SpatiaLite-5.0.0 has been built instead against any previosus version (as e.g. PROJ.4) any attempt to call one of these SQL Functions will simply return a no such function SQL error.

Practical examples: SELECT PROJ_GetDatabasePath(); -- retrieving the currently set PROJ's own database ------------------------------ /usr/local/share/proj/proj.db SELECT PROJ_SetDatabasePath('/home/sandro/not_existing_proj.db'); -- non existing database ------------------------------ NULL SELECT PROJ_GetLastErrorMsg(); -- retrieving the failure cause ------------------------------ proj_context_set_database_path: Open of /home/sandro/not_existing_proj.db failed SELECT PROJ_SetDatabasePath('/home/sandro/valid_proj.db'); -- valid database ------------------------------ /home/sandro/valid_proj.db SELECT PROJ_AsWKT('EPSG', 4326); -- default settings: ISO-2018 style, indented with indentation 4 ------------------------------ GEODCRS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84",6378137,298.257223563, LENGTHUNIT["metre",1]]], PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]], CS[ellipsoidal,2], AXIS["geodetic latitude (Lat)",north, ORDER[1], ANGLEUNIT["degree",0.0174532925199433]], AXIS["geodetic longitude (Lon)",east, ORDER[2], ANGLEUNIT["degree",0.0174532925199433]], AREA["World"], BBOX[-90,-180,90,180], ID["EPSG",4326]] SELECT PROJ_AsWKT('EPSG', 4326, 'ESRI', 0, 0); -- ESRI style, monolithic line ------------------------------ GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]] SELECT PROJ_GuessSridFromWKT('GEOGCS["GCS_Monte_Mario_Rome",DATUM["D_Monte_Mario",SPHEROID["International_1924",6378388.0,297.0]],PRIMEM["Rome",12.4523333333333],UNIT["Degree",0.0174532925199433]]'); ------------------------------ 4806 SELECT PROJ_GuessSridFromSHP('/home/sandro/tuscany_shp/prov2011'); -- remember: requires to explicitly set SPATIALITE_SECURITY=relaxed ------------------------------ 3003

Important notice for Windows users

PROJ.6 critically depends on its own private SQLite database (proj.db) containing all relevant definitions about Ellipsoids, Prime Meridians, CRSes, Transformations and alike.
If PROJ.6 is unable to correctly establish a connection to this database it will be severely limited and will not be able to correctly behave as expected.

Short conclusion: installing the software alone is not enough.
The PROJ.6 private database proj.db must be properly installed as well, in order to ensure that anything runs smoothly.

This isn't usually a big issue on Linux and Unix-like platforms, where a rational and very clear filesystem layout exists.
On these operating systems the package manager (or make install) will automatically take care to install proj.db on the most appropriate directory (usually as /usr/share/proj/proj.db or as /usr/local/share/proj/proj.db) and that's all.

Things are unhappily a little bit more difficults on Windows platforms. The user itself is responsible for properly installing proj.db So it becomes critical understanding the basic rules adopted by libspatialite in order to properly locate proj.db on Windows.

Where proj.db is expected to be found on Windows

the first place where proj.db will be searched is the same folder form where the EXE binary was initially loaded.
Example: assuming that you are currently executing C:\myprogs\spatialite\bin\spatialite.exe

then an attempt will be made to connect C:\myprogs\spatialite\bin\proj.db

a second attempt will be made on behalf of the Public folder.
Example: an attempt will be made to connect C:\Users\Public\spatialite\proj\proj.db

a third (and last) attempt will be made on behalf of the User folder.
Example: an attempt will be made to connect C:\Users\sandro\spatialite\proj\proj.db

An useful diagnostic check
SELECT PROJ_GetDatabasePath();

if NULL is returned, then PROJ.6 is definitely unable to connect to its own private SQLite database proj.db
You have to carefully verifiy that it's really installed on one of the expected standard locations.

otherwise the path leading to the currently connected proj.db will be returned.

Using the environment variable SPATIALITE_PROJ_DB_PATH

If the environment variable SPATIALITE_PROJ_DB_PATH is set, then libspatialite will simply attempt to connect the database pointed by this variable.
Such a capability is supported not only on Windows but also on Linux (and on any other platform).

Linux example:

export "SPATIALITE_PROJ_DB_PATH=/home/sandro/aux/proj.db"
echo $SPATIALITE_PROJ_DB_PATH
spatialite

Windows example:

SET SPATIALITE_PROJ_DB_PATH=C:\Users\sandro\aux\proj.db
echo %SPATIALITE_PROJ_DB_PATH%
spatialite_gui

back Z f55e7eb6772581ffd838a6b3552246a4

SRID	CRS Name	proj-string
3003	Monte Mario / Italy zone 1	+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68 +units=m +no_defs
4326	WGS 84	+proj=longlat +datum=WGS84 +no_defs
32632	WGS 84 / UTM zone 32N	+proj=utm +zone=32 +datum=WGS84 +units=m +no_defs

SRID	3003	4326	32632
CRS Name	Monte Mario / Italy zone 1	WGS 84	WGS 84 / UTM zone 32N
ISO-2018 WKT	PROJCRS["Monte Mario / Italy zone 1", BASEGEODCRS["Monte Mario", DATUM["Monte Mario", ELLIPSOID["International 1924",6378388,297, LENGTHUNIT["metre",1]]], PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]]], CONVERSION["Italy zone 1", METHOD["Transverse Mercator", ID["EPSG",9807]], PARAMETER["Latitude of natural origin",0, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8801]], PARAMETER["Longitude of natural origin",9, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8802]], PARAMETER["Scale factor at natural origin",0.9996, SCALEUNIT["unity",1], ID["EPSG",8805]], PARAMETER["False easting",1500000, LENGTHUNIT["metre",1], ID["EPSG",8806]], PARAMETER["False northing",0, LENGTHUNIT["metre",1], ID["EPSG",8807]]], CS[Cartesian,2], AXIS["easting (X)",east, ORDER[1], LENGTHUNIT["metre",1]], AXIS["northing (Y)",north, ORDER[2], LENGTHUNIT["metre",1]], AREA["Italy - west of 12°E"], BBOX[36.53,5.94,47.04,12], ID["EPSG",3003]]	GEODCRS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84",6378137,298.257223563, LENGTHUNIT["metre",1]]], PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]], CS[ellipsoidal,2], AXIS["geodetic latitude (Lat)",north, ORDER[1], ANGLEUNIT["degree",0.0174532925199433]], AXIS["geodetic longitude (Lon)",east, ORDER[2], ANGLEUNIT["degree",0.0174532925199433]], AREA["World"], BBOX[-90,-180,90,180], ID["EPSG",4326]]	PROJCRS["WGS 84 / UTM zone 32N", BASEGEODCRS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84",6378137,298.257223563, LENGTHUNIT["metre",1]]], PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]]], CONVERSION["UTM zone 32N", METHOD["Transverse Mercator", ID["EPSG",9807]], PARAMETER["Latitude of natural origin",0, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8801]], PARAMETER["Longitude of natural origin",9, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8802]], PARAMETER["Scale factor at natural origin",0.9996, SCALEUNIT["unity",1], ID["EPSG",8805]], PARAMETER["False easting",500000, LENGTHUNIT["metre",1], ID["EPSG",8806]], PARAMETER["False northing",0, LENGTHUNIT["metre",1], ID["EPSG",8807]]], CS[Cartesian,2], AXIS["(E)",east, ORDER[1], LENGTHUNIT["metre",1]], AXIS["(N)",north, ORDER[2], LENGTHUNIT["metre",1]], AREA["World - N hemisphere - 6°E to 12°E - by country"], BBOX[0,6,84,12], ID["EPSG",32632]]

SQL Function	Supported arguments	Description
HasProj6( void ) : Boolean	None	Will return 1 (TRUE) if the library has been built on PROJ.6 (or any later version), otherwise 0 (FALSE).
PROJ_GetLastErrorMsg( void ) : String	None	Will return the most recent error message returned by PROJ (if any). NULL will be returned if there is no curently pending PROJ error.
PROJ_GetDatabasePath( void : String)	None	Will return the currently set pathname leading to the private PROJ's SQLite database. NULL will be returned if there is no private PROJ's SQLite database currently connected.
PROJ_SetDatabasePath ( new_path String ) : String	new_path: a relative or absolute pathname leading to a valid PROJ's SQLite database.	Will change the currently set pathname leading to the private PROJ's SQLite database. NULL will be returned if the passed path is invalid, otherwise the path of the currently set private PROJ's SQLite database will be returned.
PROJ_AsWKT ( auth_name String , auth_srid Integer ) : String PROJ_AsWKT ( auth_name String , auth_srid Integer , wkt_style String ) : String PROJ_AsWKT ( auth_name String , auth_srid Integer , wkt_style String , indented Boolean ) : String PROJ_AsWKT ( auth_name String , auth_srid Integer , wkt_style String , indented Boolean , indentation Integer ) : String	auth_name and auth_srid identify the intended CRS. auth_name can be NULL, and in this case EPSG will be assumed. the optional argument style determines which specific WKT format should be adopted, and must be one between GDAL, ESRI, ISO-2015 or ISO-2018 (this latter being the default setting). the optional argument indented if set to TRUE will nicely format a multiline WKT expression, otherwise a single monolithic line lacking any white-space or new-line will be printed (the default is TRUE). the optional argument indentation determines how many white-spaces are to be used for indenting (only meaningful if *indented=TRUE*; the default values is 4).	Will return the WKT expression corresponding to a given CRS; the definitions will be taken directly from the private PROJ's own database. NULL will be returned on failure or on invalid arguments.
PROJ_GuessSridFromWKT ( wkt_expr String ) : Integer	wkt_expr: the WKT expression to be evaluated.	Will possibly return the SRID value corresponding to a given WKT expression defining a CRS. -1 will be returned if no CRS supported by PROJ.6 matches the WKT expression. NULL will be returned on invalid argument.
PROJ_GuessSridFromSHP ( filename String ) : Integer	filename: the absolute or relative path leading to some Shapefile. Note: exactley as required by *ImportSHP() filename* must omit any .shp, .shx, .dbf or .prj suffix.	Will possibly return the SRID value corresponding to the CRS defined by the .PRJ member of the Shapefile. -1 will be returned if no CRS supported by PROJ.6 matches PRJ member of the Shapefile. NULL will be returned on invalid argument. Please note well: this SQL function opend the door to many potential security issues, and thus is always disabled by default. Explicitly setting the environment variable SPATIALITE_SECURITY=relaxed is absolutely required in order to enable this function.