Data types

This section describes the numerous data types in use in PROJ.4. As a rule of thumb PROJ.4 data types are prefixed with PJ_, or in one particular case, is simply called PJ. A few notable exceptions can be traced back to the very early days of PROJ.4 when the PJ_ prefix was not consistently used.

Transformation objects


Object containing everything related to a given projection or transformation. As a user of the PROJ.4 library you are only exposed to pointers to this object and the contents is hidden behind the public API. PJ objects are created with proj_create() and destroyed with proj_destroy().


Enumeration that is used to convey in which direction a given transformation should be performed. Used in transformation function call as described in the section on transformation functions.

Forward transformations are defined with the :c:

typedef enum proj_direction {
    PJ_FWD   =  1,   /* Forward    */
    PJ_IDENT =  0,   /* Do nothing */
    PJ_INV   = -1    /* Inverse    */

Perform transformation in the forward direction.


Identity. Do nothing.


Perform transformation in the inverse direction.


Context objects enable safe multi-threaded usage of PROJ.4. Each PJ object is connected to a context (if not specified, the default context is used). All operations within a context should be performed in the same thread. PJ_CONTEXT objects are created with proj_context_create() and destroyed with proj_context_destroy().


Opaque object describing an area in which a transformation is performed.


This object is not fully implemented yet. It is to be used with proj_create_crs_to_crs() to select the best transformation between the two input coordinate reference systems.

2 dimensional coordinates

Various 2-dimensional coordinate data types.


Geodetic coordinate, latitude and longitude. Usually in radians.

typedef struct { double lam, phi; } PJ_LP;
double PJ_LP.lam

Longitude. Lambda.

double PJ_LP.phi

Latitude. Phi.


2-dimensional cartesian coordinate.

typedef struct { double x, y; } PJ_XY;
double PJ_XY.x


double PJ_XY.y



2-dimensional generic coordinate. Usually used when contents can be either a PJ_XY or PJ_LP.

typedef struct {double u, v; } PJ_UV;
double PJ_UV.u

Longitude or easting, depending on use.

double PJ_UV.v

Latitude or northing, depending on use.

3 dimensional coordinates

The following data types are the 3-dimensional equivalents to the data types above.


3-dimensional version of PJ_LP. Holds longitude, latitude and a vertical component.

typedef struct { double lam, phi, z; } PJ_LPZ;
double PJ_LPZ.lam

Longitude. Lambda.

double PJ_LPZ.phi

Latitude. Phi.

double PJ_LPZ.z

Vertical component.


Cartesian coordinate in 3 dimensions. Extension of PJ_XY.

typedef struct { double x, y, z; } PJ_XYZ;
double PJ_XYZ.x

Easting or the X component of a 3D cartesian system.

double PJ_XYZ.y

Northing or the Y component of a 3D cartesian system.

double PJ_XYZ.z

Vertical component or the Z component of a 3D cartesian system.


3-dimensional extension of PJ_UV.

typedef struct {double u, v, w; } PJ_UVW;
double PJ_UVW.u

Longitude or easting, depending on use.

double PJ_UVW.v

Latitude or northing, depending on use.

double PJ_UVW.w

Vertical component.

Spatiotemporal coordinate types

The following data types are extensions of the triplets above into the time domain.


Spatiotemporal version of PJ_LPZ.

typedef struct {
    double lam;
    double phi;
    double z;
    double t;
double PJ_LPZT.lam


double PJ_LPZT.phi


double PJ_LPZT.z

Vertical component.

double PJ_LPZT.t

Time component.


Generic spatiotemporal coordinate. Useful for e.g. cartesian coordinates with an attached time-stamp.

typedef struct {
    double x;
    double y;
    double z;
    double t;
double PJ_XYZT.x

Easting or the X component of a 3D cartesian system.

double PJ_XYZT.y

Northing or the Y component of a 3D cartesian system.

double PJ_XYZT.z

Vertical or the Z component of a 3D cartesian system.

double PJ_XYZT.t

Time component.


Spatiotemporal version of PJ_UVW.

typedef struct { double u, v, w, t; } PJ_UVWT;
double PJ_UVWT.e

First horizontal component.

double PJ_UVWT.n

Second horizontal component.

double PJ_UVWT.w

Vertical component.

double PJ_UVWT.t

Temporal component.

Ancillary types for geodetic computations


Rotations, for instance three euler angles.

typedef struct { double o, p, k; } PJ_OPK;
double PJ_OPK.o

First rotation angle, omega.

double PJ_OPK.p

Second rotation angle, phi.

double PJ_OPK.k

Third rotation angle, kappa.

Complex coordinate types


General purpose coordinate union type, applicable in two, three and four dimensions. This is the default coordinate datatype used in PROJ.

typedef union {
    double v[4];
    PJ_XYZT xyzt;
    PJ_UVWT uvwt;
    PJ_LPZT lpzt;
    PJ_XYZ  xyz;
    PJ_UVW  uvw;
    PJ_LPZ  lpz;
    PJ_XY   xy;
    PJ_UV   uv;
    PJ_LP   lp;
double v[4]

Generic four-dimensional vector.


Spatiotemporal cartesian coordinate.


Spatiotemporal generic coordinate.


Longitude, latitude, vertical and time components.


3-dimensional cartesian coordinate.


3-dimensional generic coordinate.


Longitude, latitude and vertical component.


2-dimensional cartesian coordinate.


2-dimensional generic coordinate.


Longitude and latitude.

Projection derivatives


Various cartographic properties, such as scale factors, angular distortion and meridian convergence. Calculated with proj_factors().

typedef struct {
    double meridional_scale;
    double parallel_scale;
    double areal_scale;

    double angular_distortion;
    double meridian_parallel_angle;
    double meridian_convergence;

    double tissot_semimajor;
    double tissot_semiminor;

    double dx_dlam;
    double dx_dphi;
    double dy_dlam;
    double dy_dphi;
double PJ_FACTORS.meridional_scale

Meridional scale at coordinate \(\left(\lambda,\phi\right)\).

double PJ_FACTORS.parallel_scale

Parallel scale at coordinate \(\left(\lambda,\phi\right)\).

double PJ_FACTORS.areal_scale

Areal scale factor at coordinate \(\left(\lambda,\phi\right)\).

double PJ_FACTORS.angular_distortion

Angular distortion at coordinate \(\left(\lambda,\phi\right)\).

double PJ_FACTORS.meridian_parallel_angle
Meridian/parallel angle, \(\theta^\prime\), at coordinate \(\left(\lambda,\phi\right)\).
double PJ_FACTORS.meridian_convergence

Meridian convergence at coordinate \(\left(\lambda,\phi\right)\). Sometimes also described as grid declination.

double PJ_FACTORS.tissot_semimajor

Maximum scale factor.

double PJ_FACTORS.tissot_semiminor

Minimum scale factor.

double PJ_FACTORS.dx_dlam

Partial derivative \(\frac{\partial x}{\partial \lambda}\) of coordinate \(\left(\lambda,\phi\right)\).

double PJ_FACTORS.dy_dlam

Partial derivative \(\frac{\partial y}{\partial \lambda}\) of coordinate \(\left(\lambda,\phi\right)\).

double PJ_FACTORS.dx_dphi

Partial derivative \(\frac{\partial x}{\partial \phi}\) of coordinate \(\left(\lambda,\phi\right)\).

double PJ_FACTORS.dy_dphi

Partial derivative \(\frac{\partial y}{\partial \phi}\) of coordinate \(\left(\lambda,\phi\right)\).

List structures


Description a PROJ.4 operation

    char    *id;                 /* operation keyword */
    PJ *(*proj)(PJ *);           /* operation  entry point */
    char    * const *descr;      /* description text */
char *id

Operation keyword.

PJ *(*op)(PJ *)

Operation entry point.

char * const *

Description of operation.


Description of ellipsoids defined in PROJ.4

struct PJ_ELLPS {
    char    *id;
    char    *major;
    char    *ell;
    char    *name;
char *id

Keyword name of the ellipsoid.

char *major

Semi-major axis of the ellipsoid, or radius in case of a sphere.

char *ell

Elliptical parameter, e.g. rf=298.257 or b=6356772.2.

char *name

Name of the ellipsoid


Distance units defined in PROJ.

struct PJ_UNITS {
    char    *id;           /* units keyword */
    char    *to_meter;     /* multiply by value to get meters */
    char    *name;         /* comments */
    double   factor;       /* to_meter factor in actual numbers */
char *id

Keyword for the unit.

char *to_meter

Text representation of the factor that converts a given unit to meters

char *name

Name of the unit.

double factor

Conversion factor that converts the unit to meters.


Prime meridians defined in PROJ.

    char    *id;
    char    *defn;
char *id

Keyword for the prime meridian

char *def

Offset from Greenwich in DMS format.

Info structures


Struct holding information about the current instance of PROJ. Struct is populated by proj_info().

typedef struct {
    int           major;
    int           minor;
    int           patch;
    const char   *release;
    const char   *version;
    const char   *searchpath;
const char *PJ_INFO.release

Release info. Version number and release date, e.g. “Rel. 4.9.3, 15 August 2016”.

const char *PJ_INFO.version

Text representation of the full version number, e.g. “4.9.3”.

int PJ_INFO.major

Major version number.

int PJ_INFO.minor

Minor version number.

int PJ_INFO.patch

Patch level of release.

const char PJ_INFO.searchpath

Search path for PROJ. List of directories separated by semicolons (Windows) or colons (non-Windows), e.g. “C:\Users\doctorwho;C:\OSGeo4W64\share\proj”. Grids and init files are looked for in directories in the search path.


Struct holding information about a PJ object. Populated by proj_pj_info(). The PJ_PROJ_INFO object provides a view into the internals of a PJ, so once the PJ is destroyed or otherwise becomes invalid, so does the PJ_PROJ_INFO

typedef struct {
    const char  *id;
    const char  *description;
    const char  *definition;
    int          has_inverse;
    double       accuracy;
const char *

Short ID of the operation the PJ object is based on, that is, what comes afther the +proj= in a proj-string, e.g. “merc”.

const char *PJ_PROJ_INFO.description

Long describes of the operation the PJ object is based on, e.g. “Mercator Cyl, Sph&Ell lat_ts=”.

const char *PJ_PROJ_INFO.definition

The proj-string that was used to create the PJ object with, e.g. “+proj=merc +lat_0=24 +lon_0=53 +ellps=WGS84”.

int PJ_PROJ_INFO.has_inverse

1 if an inverse mapping of the defined operation exists, otherwise 0.

double PJ_PROJ_INFO.accuracy

Expected accuracy of the transformation. -1 if unknown.


Struct holding information about a specific grid in the search path of PROJ. Populated with the function proj_grid_info().

typedef struct {
    char        gridname[32];
    char        filename[260];
    char        format[8];
    LP          lowerleft;
    LP          upperright;
    int         n_lon, n_lat;
    double      cs_lon, cs_lat;
char PJ_GRID_INFO.gridname[32]

Name of grid, e.g. “BETA2007.gsb”.


Full path of grid file, e.g. “C:\OSGeo4W64\share\proj\BETA2007.gsb”

char PJ_GRID_INFO.format[8]

File format of grid file, e.g. “ntv2

LP PJ_GRID_INFO.lowerleft

Geodetic coordinate of lower left corner of grid.

LP PJ_GRID_INFO.upperright

Geodetic coordinate of upper right corner of grid.

int PJ_GRID_INFO.n_lon

Number of grid cells in the longitudinal direction.

int PJ_GRID_INFO.n_lat

Number of grid cells in the latitudianl direction.

double PJ_GRID_INFO.cs_lon

Cell size in the longitudinal direction. In radians.

double PJ_GRID_INFO.cs_lat

Cell size in the latitudinal direction. In radians.


Struct holding information about a specific init file in the search path of PROJ. Populated with the function proj_init_info().

typedef struct {
    char        name[32];
    char        filename[260];
    char        version[32];
    char        origin[32];
    char        lastupdate[16];

Name of init file, e.g. “epsg”.

char PJ_INIT_INFO.filename[260]

Full path of init file, e.g. “C:\OSGeo4W64\share\proj\epsg

char PJ_INIT_INFO.version[32]

Version number of init-file, e.g. “9.0.0

char PJ_INIT_INFO.origin[32]

Originating entity of the init file, e.g. “EPSG

char PJ_INIT_INFO.lastupdate

Date of last update of the init-file.



Enum of logging levels in PROJ. Used to set the logging level in PROJ. Usually using proj_log_level().


Don’t log anything.


Log only errors.


Log errors and additional debug information.


Highest logging level. Log everything including very detailed debug information.


Special logging level that when used in proj_log_level() will return the current logging level set in PROJ.

New in version 5.1.0.


Function prototype for the logging function used by PROJ. Defined as

typedef void (*PJ_LOG_FUNCTION)(void *, int, const char *);

where the void pointer references a data structure used by the calling application, the int is used to set the logging level and the const char pointer is the string that will be logged by the function.

New in version 5.1.0.