PROJ.6
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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 much so that most don't even realize that PROJ is being used in the background.
So a short history would be appropriate to fully understand the current state of the art of this library:

Past History

Timeline
Very few users and developers realize how ancient PROJ is, and how far it has progressed since its first steps:
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:
Note: the whole transition requires 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 unpleasant news, because it practically means that all software modules depending on PROJ (including SpatiaLite) will require a not at all trivial rewrite in order to fulfill the new API requirements.
But considering that this is the first major API break in 40 years and with so many useful innovations, such a breakage is fully justified and reasonable.

More details about the API breakage:

What's new in PROJ.6

Fully supporting ISO 19162:2018 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:

SRIDCRS Nameproj-string (PROJ.4 style)proj-string (PROJ.6 style)
3003Monte 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
+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 \
+ellps=intl +units=m +no_defs +type=crs
4326WGS 84 +proj=longlat +datum=WGS84 +no_defs+proj=longlat +datum=WGS84 +no_defs +type=crs
32632WGS 84 / UTM zone 32N +proj=utm +zone=32 +datum=WGS84 +units=m +no_defs+proj=utm +zone=32 +datum=WGS84 +units=m +no_defs +type=crs

PROJ.6 still continues to support the old proj-strings, but it now adopts a slightly changed notation:
Although the newer versions of PROJ (starting with PROJ.6) will still continue to support the old proj-strings, this is no longer the preferred notation for defining a CRS.
The newer, preferred, notation is fully conforms to the ISO 19162:2018 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:

SRID3003432632632
CRS NameMonte Mario / Italy zone 1WGS 84WGS 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],<br>
            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 see, 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.
They are radically different: ISO-WKT is clearly superior and more sophisticated in every way.
Not surprisingly, the coordinate transformations based on ISO-WKT definitions (instead of proj-strings) are usually expected to be more accurate, complete and extendable.

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 could be expected to understand them.
The new ISO-WKT, on the other hand, is a formally defined international standard, and almost all free and proprietary implementations are expected to support it.
This is a very relevant change that ensures a robust compatibility between different applications.

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:
  1. ESRI WKT: a dialect widely adopted by ESRI proprietary software.
  2. GDAL WKT1: a dialect historically supported by the open source GDAL library.
  3. WKT2:2015: as defined by ISO 19162:2015 standard specification (now superseded).
  4. WKT2:2018: as defined by the more recent ISO 19162:2018 standard specification .

The private SQLite database supporting PROJ.6

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, Conversions, Transformations and alike.
Note: many of the new advanced PROJ.6 features will fail if this database is not available at run time.

Direct transformations from CRS to CRS

As apposed to all previous versions, PROJ.6 can now apply any transformation from a CRS to another without specifying the CRS definitions (proj-string or WKT).
This is because PROJ.6 is now capable to retrieve any CRS definition directly from its own private SQLite database, and consequently each CRS can be fully identified by simply specifying its symbolic name, as in: or alternatively
Another major difference: which results in more precise transformations:
  • PROJ.4: always transformed first to WSG84 and from there to the target projection.
  • PROJ.6: will transform to the target projection directly.
This will also avoid floating point accuracy problems which may occur through the extra WSG84 transformation

Supporting specific areas of use

PROJ.6 is now able to make a distinction between different areas of use within a CRS, and can eventually apply the most appropriate transformation when the intended area-of-use is explicitly specified, thus leading to more precise results.

The side figure shows the situation for the Italian peninsula.
As you can see there are two main areas (East and West of longitude 12E), but there are several smaller areas covering specific regions.
  • The two main areas correspond to EPSG:3003 Monte Mario / Italy zone 1 and EPSG:3004 Monte Mario / Italy zone 2.
  • The smaller areas correspond to the more specific areas of use now supported by PROJ.6

If your specific area of interest is contained within one of these sub-regions you can inform PROJ.6 which sub-region you want by giving either the area BBOX or name, and then PROJ.6 will attempt to use the most specific transformation parameters available.

We'll examine this topic in more depth in the following sections.
italy areas of use


Concepts to keep well in mind

  • forget the old proj-strings; they are no longer the best way for taking full profit from PROJ.6
  • the best way to utilize PROJ.6 capabilities is by allowing the library to freely choose the best possible transformation paths between the origin and the destination CRSes.
    This, however, strictly requires that the libraries private database (proj.db) can be found.

Additional concept

The traditional spatial_ref_sys table contained within any SpatiaLite DB noticeably changes its intended purpose when using PROJ.6:
  • srid, auth_name, auth_srid and ref_sys_name columns are still required in order to define all supported CRSes and to support relations based on Primary and Foreign Keys.
  • but columns proj4text and srtext are now practically useless, because PROJ.6 is now capable of retrieving the needed CRS definitions from its own private database.
  • Future Forecasts: both proj4text and srtext will probably be removed in some future version of SpatiaLite.
    For now they will continue to be needed for versions of SpatiaLite that uses, the now superseded, PROJ.4

Transformation pipelines

There is a last astonishing innovation introduced in PROJ.6: transformation pipelines.
When using a pipeline you can freely define any complex geodetic transformation by combining 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 processing each step until the final result is produced.
The following is a practical 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 usefull resource; don't hesitate to use it frequently in order to discover what's really being done in the background.
It's the best and easiest way to fully understand what PROJ.6 really does.

projinfo also gives good hints when problems exist. In this case PROJ_LIB was incorrectly set and the needed grid files could not be found:
Grid BETA2007.gsb needed but not found on the system.
Can be obtained from the proj-datumgrid package at https://download.osgeo.org/proj/proj-datumgrid-latest.zip
It not only tells you which grid-file is needed, but also where to get it.


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


PROJ.6 support on SpatiaLite-5.0.0

Creating and populating the spatial_ref_sys metatable

There is no relevant change. As in all previous versions spatial_ref_sys will be automatically created and properly populate by calling the appropriate SQL function.
SELECT InitSpatialMetaData(1);
      or
SELECT InitSpatialMetaDataFull(1);
Please remember that now in SpatiaLite-5.0.0 InitSpatialMetaDataFull represents the preferred method for creating all metadata tables required by this version.
InitSpatialMetaData is still maintained so to not break historical compatibility, but should no longer be used.

The most relevant change is that different versions of SpatiaLite/PROJ will now behave in radically different ways, as summarized in the following cross-version compatibility table.

SpatiaLite / PROJ versionConnected DBAction
SpatiaLite-5 built on PROJ.6 DB created by SpatiaLite-5 / PROJ.6 libspatialite-5 when built on PROJ.6 will always ignore both the proj-strings stored in column proj4text and the WKT definitions stored in column srtext.
It will instead allow PROJ.6 to retrieve the needed CRS definitions contained in its own private database.

Note that this applies indifferently to both legacy databases created by any previous version and most modern databases created by the latest version.
DB created by SpatiaLite-5 / PROJ.4
or
by any previous version of SpatiaLite (< 5.0.0)
SpatiaLite-5 built on PROJ.4
or
any previous version of SpatiaLite (< 5.0.0)
DB created by SpatiaLite-5 / PROJ.6 When libspatialite (any version, including 5.0.0) that was not compiled against PROJ.6, then the traditional approach will be always applied, and each CRS will be consequently identified by its corresponding proj-string stored in column proj4text

Note that even when PROJ.6 is not supported any version of SpatiaLite (including 5.0.0) can safely connect to any DB-file created by the most recent version created with full PROJ.6 support.
DB created by SpatiaLite-5 / PROJ.4
or
by any previous version of SpatiaLite (< 5.0.0)


Existing SQL functions affected by PROJ.6

SQL FunctionExtended signatureBehavior under PROJ.6
ST_Transform() ST_Transform ( geom Geometry , newSrid Integer , area Geometry ) : Geometry
ST_Transform ( geom Geometry , newSrid Integer , area Geometry , proj_string_from Text ) : Geometry
ST_Transform ( geom Geometry , newSrid Integer , area Geometry , proj_string_from Text , proj_string_to 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 a fine tuned and more accurate transformation.
  • both the optional arguments proj_string_from and proj_string_to may be NULL (default setting).
    Otherwise they are expected to contain a valid string to be used for identifying the corresponding CRS.
    PROJ.6 accepts the following CRS strings:
    • traditional proj-strings used by all previous versions of PROJ, as e.g.
      • +proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0 +k=1 +units=m +nadgrids=@null +wktext +no_defs
      • +proj=utm +zone=32 +south +ellps=WGS72 +towgs84=0,0,4.5,0,0,0.554,0.2263 +units=m +no_defs
      • +proj=utm +zone=32 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m +no_defs
    • any WKT expression defining a CRS.
    • a CRS name as e.g.
      • WGS 84
      • WGS 84 / UTM zone 32N
      • IGM95 / UTM zone 33N
    • any canonical CRS reference as e.g.
      • EPSG:4326
      • urn:ogc:def:crs:EPSG::3003

    • Special case: if proj_string_to is NULL and proj_string_from is explicitly set, then it will be interpreted as an user defined transformation string (possibly, as a transformation pipeline) as e.g.
      • +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

Notes

  • SpatiaLite does not constrain you in any way by making any magical assumptions and allows the users the freedom to contruct any combination supported by PROJ.
    • 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 from PROJ.6 itself that will simply receive a request in the form from EPSG:32632 to EPSG:3003.
    • If, for whatever reason, you are free to use any kind of proj-strings, WKT definitions or CRS names supported by PROJ.6.
      But do not be suprised if the result of your creative construction results in an exception that contains the text: 'PROJ reports' !!!
      Care must be taken to analyse the reported PROJ error in such a way as to resolve the PROJ error.
  • SpatiaLite encourages and promotes the most creative use of PROJ.6, this including the use of your own customized transformation pipelines..
    Correctly using custom transformations isn't necessarily simple and easy, but offers great opportunities for all skilled and talented power users.



A few practical SQL examples:
-- from EPSG:4326 to EPSG:3003 - default settings; ignoring spatialite_ref_sys and relying on PROJ.6 own private database
SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056 , 4326 ) , 3003 ));
-------------------------------------
SRID=3003;POINT(1732852.942716769 4816277.617690674)


-- from EPSG:4326 to EPSG:3003 - exactly the same as above, but this time in an explicit form
SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003, NULL, 'EPSG:4326', 'EPSG:3003'));
-------------------------------------
SRID=3003;POINT(1732852.942716769 4816277.617690674)


-- from EPSG:4326 to EPSG:3003 - same as above, using the alternative notation
SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003, NULL, 'urn:ogc:def:crs:EPSG::4326', 'urn:ogc:def:crs:EPSG::3003'));
-------------------------------------
SRID=3003;POINT(1732852.942716769 4816277.617690674)


-- from EPSG:4326 to EPSG:3003 - explicitly passing CRS names
SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003, NULL, 'WGS 84', 'Monte Mario / Italy zone 1'));
-------------------------------------
SRID=3003;POINT(1732852.942716769 4816277.617690674)


-- from EPSG:4326 to EPSG:3003 - simulating the classic PROJ.4 approach based on proj-strings stored into spatial_ref_sys
SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003 , NULL , 
  (SELECT proj4text FROM spatial_ref_sys WHERE srid = 4326),
  (SELECT proj4text FROM spatial_ref_sys WHERE srid = 3003)));
-------------------------------------
SRID=3003;POINT(1732852.942716769 4816277.617690674)


-- from EPSG:4326 to EPSG:3003 - fancy mode based on WKT expressions stored into spatial_ref_sys
SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003, NULL, 
  (SELECT srtext FROM spatial_ref_sys WHERE srid = 4326),
  (SELECT srtext FROM spatial_ref_sys WHERE srid = 3003)));
-------------------------------------
SRID=3003;POINT(1732852.942716769 4816277.617690674)
This first test is based on a new SpatiaLite database created with full PROJ.6 support. Nothing special here other that all methods return the same results.

-- from EPSG:4326 to EPSG:3003 - using a transformation pipeline
SELECT AsEWKT ( ST_Transform( SwapCoords ( MakePoint( 11.878056 , 43.463056, 4326 ) ) , 3003 , NULL ,
'+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)


-- from EPSG:4326 to EPSG:3003 - using a customized transformation pipeline
SELECT AsEWKT ( ST_Transform( MakePoint( 11.878056 , 43.463056, 4326 ) , 3003 , NULL ,
'+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:

Tech Note: using projinfo for discovering pipelines and 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


-- from EPSG:426 to EPSG:3003 - default settings, NULL area of use
SELECT AsEWKT ( ST_Transform( MakePoint( 9.169464, 39.478275, 4326 ), 3003 ));
-------------------------------------
SRID=3003;POINT(1514600.134321862 4369874.489269957)


-- from EPSG:426 to EPSG:3003 - default settings, but this time explicitly defining the MBR aka BBOX of 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 final 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 and Italy - Sardinia onshore is one between them.

Lesson learned:
  • The best mode for taking full profit from PROJ.6 is by ignoring all CRS definitions contained into spatial_ref_sys, thus leaving PROJ free to choose the best possible approach. (which is the default setting).
    This way you'll be absolutely sure that the most recent and fully detailed definitions will always be retrieved from the PROJ.6 own private database.
    This will happen even when processing processing some legacy DB that was created by a earlier version and thus potentially contains an outdated spatial_ref_sys table.
  • SpatiaLite fully respects users freedom, avoiding any imposed constraint.
    If you think that using another CRS definitions is a good idea, you are free to use traditional proj-strings, WKT definitions, CRS names and whatever else that may be deemed appropriate.
  • SpatiaLite encourages and promotes the most creative use of PROJ.6: using your own custom transformation pipelines isn't necessarily simple and easy, but offers a flexible capability to resolve complex problems or non-standard solutions.

New auxiliary SQL functions specifically supporting PROJ.6

SQL FunctionSupported argumentsDescription
HasProj6( void ) : Boolean None Will return 1 (TRUE) if the library has been built with 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 currently 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_AsProjString ( auth_name String , auth_srid Integer ) : String
  • auth_name and auth_srid identify the intended CRS.
    auth_name can be NULL, and in this case EPSG will be assumed.
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_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: exactly 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 opens 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 previous 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_AsProjString('EPSG', 32632);
------------------------------
+proj=utm +zone=32 +datum=WGS84 +units=m +no_defs +type=crs


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 difficult 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

  1. 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
  2. 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
  3. 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 verify 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 PROJ_LIB

PROJ_LIB is a standard environment variable supported by PROJ itself.
  • the absolute path to the dirtory containing the proj.db should be used

When PROJ_LIB is set, then PROJ.6 will directly use its own private database (and when needed any grids) in the directory referenced by the variable.
Such a capability is supported on both Windows and Linux (and on any other platform).

Linux example:
  • export "PROJ_LIB=/home/sandro/proj"
    echo $PROJ_LIB
    spatialite


Windows example:
  • SET PROJ_LIB=C:\Users\sandro\proj
    echo %PROJ_LIB%
    spatialite_gui




PROJ.6 support on spatialite_gui

The GUI tool now contains a new feature intended to make it as easy as possible to set PROJ_LIB used by PROJ.6

The main menu now contains an External variable PROJ_LIB item.
After activating the corresponding option a standard File Selection Dialog will be shown, thus enabling the user to locate the private proj.db database of PROJ.6 in the file-system.

Once the proj.db has been correctly located, spatialite_gui will automatically set the PROJ_LIB variable.

And that's not all; spatialite_gui will read the current value of PROJ_LIB when started, so that will be a one time affair.
gui proj6
There is a further enhancement available when spatialite_gui is using PROJ.6
It's a very little evident innovation, but it will probably be highly appreciated by many users.

When loading a Shapefile, the appropriate SRID value will be automatically determined by examining the content of the companion .prj file (if it exists).
gui proj6



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