# Version 4 to 6 API Migration¶

This is a transition guide for developers wanting to migrate their code to use PROJ version 6.

## Code example¶

The difference between the old and new API is shown here with a few examples. Below we implement the same program with the two different API’s. The program reads input longitude and latitude from the command line and convert them to projected coordinates with the Mercator projection.

We start by writing the program for PROJ 4:

#include <proj_api.h>

main(int argc, char **argv) {
projPJ pj_merc, pj_longlat;
double x, y;
int p;

if (!(pj_longlat = pj_init_plus("+proj=longlat +ellps=clrk66")) )
return 1;
if (!(pj_merc = pj_init_plus("+proj=merc +datum=clrk66 +lat_ts=33")) )
return 1;

while (scanf("%lf %lf", &x, &y) == 2) {
x *= DEG_TO_RAD; /* longitude */
y *= DEG_TO_RAD; /* latitude */
p = pj_transform(pj_longlat, pj_merc, 1, 1, &x, &y, NULL);
printf("%.2f\t%.2f\n", x, y);
}

pj_free(pj_longlat);
pj_free(pj_merc);

return 0;
}


The same program implemented using PROJ 6:

#include <proj.h>

main(int argc, char **argv) {
PJ *P;
PJ_COORD c, c_out;

/* NOTE: the use of PROJ strings to describe CRS is strongly discouraged */
/* in PROJ 6, as PROJ strings are a poor way of describing a CRS, and */
/* more precise its geodetic datum. */
/* Use of codes provided by authorities (such as "EPSG:4326", etc...) */
/* or WKT strings will bring the full power of the "transformation */
/* engine" used by PROJ to determine the best transformation(s) between */
/* two CRS. */
P = proj_create_crs_to_crs(PJ_DEFAULT_CTX,
"+proj=longlat +ellps=clrs66",
"+proj=merc +ellps=clrk66 +lat_ts=33",
NULL);
if (P==0)
return 1;

{
/* For that particular use case, this is not needed. */
/* proj_normalize_for_visualization() ensures that the coordinate */
/* order expected and returned by proj_trans() will be longitude, */
/* latitude for geographic CRS, and easting, northing for projected */
/* CRS. If instead of using PROJ strings as above, "EPSG:XXXX" codes */
PJ* P_for_GIS = proj_normalize_for_visualization(PJ_DEFAULT_CTX, P);
if( 0 == P_for_GIS )  {
proj_destroy(P);
return 1;
}
proj_destroy(P);
P = P_for_GIS;
}

/* For reliable geographic <--> geocentric conversions, z shall not */
/* be some random value. Also t shall be initialized to HUGE_VAL to */
/* allow for proper selection of time-dependent operations if one of */
/* the CRS is dynamic. */
c.lpzt.z = 0.0;
c.lpzt.t = HUGE_VAL;

while (scanf("%lf %lf", &c.lpzt.lam, &c.lpzt.phi) == 2) {
/* No need to convert to radian */
c_out = proj_trans(P, PJ_FWD, c);
printf("%.2f\t%.2f\n", c_out.xy.x, c_out.xy.y);
}

proj_destroy(P);

return 0;
}


## Function mapping from old to new API¶

Old API functions

New API functions

pj_fwd

proj_trans()

pj_inv

proj_trans()

pj_fwd3

proj_trans()

pj_inv3

proj_trans()

pj_transform

pj_init

pj_init

pj_free

proj_destroy()

pj_is_latlong

proj_get_type()

pj_is_geocent

proj_get_type()

pj_get_def

proj_pj_info()

pj_latlong_from_proj

No direct equivalent, but can be accomplished by chaining proj_create(), proj_crs_get_horizontal_datum() and proj_create_geographic_crs_from_datum()

pj_set_finder

proj_context_set_file_finder()

pj_set_searchpath

proj_context_set_search_paths()

pj_deallocate_grids

No equivalent

pj_strerrno

No equivalent

pj_get_errno_ref

proj_errno()

pj_get_release

proj_info()

## Backward incompatibilities¶

Access to the proj_api.h is still possible but requires to define the ACCEPT_USE_OF_DEPRECATED_PROJ_API_H macro.

The emulation of the now deprecated +init=epsg:XXXX syntax in PROJ 6 is not fully compatible with previous versions.

In particular, when used with the pj_transform() function, no datum shift term (towgs84, nadgrids, geoidgrids) will be added during the expansion of the +init=epsg:XXXX string to +proj=YYYY ..... If you still uses pj_transform() and want datum shift to be applied, then you need to provide a fully expanded string with appropriate towgs84, nadgrids or geoidgrids terms to pj_init().

To use the +init=epsg:XXXX syntax with proj_create() and then proj_create_crs_to_crs(), proj_context_use_proj4_init_rules(ctx, TRUE) or the PROJ_USE_PROJ4_INIT_RULES=YES environment variable must have been previously set. In that context, datum shift will be researched. However they might be different than with PROJ 4 or PROJ 5, since a “late-binding” approach will be used (that is trying to find as much as possible the most direct transformation between the source and target datum), whereas PROJ 4 or PROJ 5 used an “early-binding” approach consisting in always going to EPSG:4326 / WGS 84.

# Version 4 to 5 API Migration¶

This is a transition guide for developers wanting to migrate their code to use PROJ version 5.

## Background¶

Before we go on, a bit of background is needed. The new API takes a different view of the world than the old because it is needed in order to obtain high accuracy transformations. The old API is constructed in such a way that any transformation between two coordinate reference systems must pass through the ill-defined WGS84 reference frame, using it as a hub. The new API does away with this limitation to transformations in PROJ. It is still possible to do that type of transformations but in many cases there will be a better alternative.

The world view represented by the old API is always sufficient if all you care about is meter level accuracy - and in many cases it will provide much higher accuracy than that. But the view that “WGS84 is the true foundation of the world, and everything else can be transformed natively to and from WGS84” is inherently flawed.

First and foremost because any time WGS84 is mentioned, you should ask yourself “Which of the six WGS84 realizations are we talking about here?”.

Second, because for many (especially legacy) systems, it may not be straightforward to transform to WGS84 (or actually ITRF-something, ETRS-something or NAD-something which appear to be the practical meaning of the term WGS84 in everyday PROJ related work), while centimeter-level accurate transformations may exist between pairs of older systems.

The concept of a hub reference frame (“datum”) is not inherently bad, but in many cases you need to handle and select that datum with more care than the old API allows. The primary aim of the new API is to allow just that. And to do that, you must realize that the world is inherently 4 dimensional. You may in many cases assume one or more of the coordinates to be constant, but basically, to obtain geodetic accuracy transformations, you need to work in 4 dimensions.

Now, having described the background for introducing the new API, let’s try to show how to use it. First note that in order to go from system A to system B, the old API starts by doing an inverse transformation from system A to WGS84, then does a forward transformation from WGS84 to system B.

With cs2cs being the command line interface to the old API, and cct being the same for the new, this example of doing the same thing in both world views will should give an idea of the differences:

$echo 300000 6100000 | cs2cs +proj=utm +zone=33 +ellps=GRS80 +to +proj=utm +zone=32 +ellps=GRS80 683687.87 6099299.66 0.00$ echo 300000 6100000 0 0 | cct +proj=pipeline +step +inv +proj=utm +zone=33 +ellps=GRS80 +step +proj=utm +zone=32 +ellps=GRS80
683687.8667   6099299.6624    0.0000    0.0000


Lookout for the +inv in the first +step, indicating an inverse transform.

## Code example¶

The difference between the old and new API is shown here with a few examples. Below we implement the same program with the two different API’s. The program reads input longitude and latitude from the command line and convert them to projected coordinates with the Mercator projection.

We start by writing the program for PROJ v. 4:

#include <proj_api.h>

main(int argc, char **argv) {
projPJ pj_merc, pj_longlat;
double x, y;

if (!(pj_longlat = pj_init_plus("+proj=longlat +ellps=clrk66")) )
return 1;
if (!(pj_merc = pj_init_plus("+proj=merc +ellps=clrk66 +lat_ts=33")) )
return 1;

while (scanf("%lf %lf", &x, &y) == 2) {
x *= DEG_TO_RAD; /* longitude */
y *= DEG_TO_RAD; /* latitude */
p = pj_transform(pj_longlat, pj_merc, 1, 1, &x, &y, NULL );
printf("%.2f\t%.2f\n", x, y);
}

pj_free(pj_longlat);
pj_free(pj_merc);

return 0;
}


The same program implemented using PROJ v. 5:

#include <proj.h>

main(int argc, char **argv) {
PJ *P;
PJ_COORD c;

P = proj_create(PJ_DEFAULT_CTX, "+proj=merc +ellps=clrk66 +lat_ts=33");
if (P==0)
return 1;

while (scanf("%lf %lf", &c.lp.lam, &c.lp.phi) == 2) {
c = proj_trans(P, PJ_FWD, c);
printf("%.2f\t%.2f\n", c.xy.x, c.xy.y);
}

proj_destroy(P);
}


Looking at the two different programs, there’s a few immediate differences that catches the eye. First off, the included header file describing the API has changed from proj_api.h to simply proj.h. All functions in proj.h belongs to the proj_ namespace.

With the new API also comes new datatypes. E.g. the transformation object projPJ which has been changed to a pointer of type PJ. This is done to highlight the actual nature of the object, instead of hiding it away behind a typedef. New data types for handling coordinates have also been introduced. In the above example we use the PJ_COORD, which is a union of various types. The benefit of this is that it is possible to use the various structs in the union to communicate what state the data is in at different points in the program. For instance as in the above example where the coordinate is read from STDIN as a geodetic coordinate, communicated to the reader of the code by using the c.lp struct. After it has been projected we print it to STDOUT by accessing the individual elements in c.xy to illustrate that the coordinate is now in projected space. Data types are prefixed with PJ_.

The final, and perhaps biggest, change is that the fundamental concept of transformations in PROJ are now handled in a single transformation object (PJ) and not by stating the source and destination systems as previously. It is of course still possible to do just that, but the transformation object now captures the whole transformation from source to destination in one. In the example with the old API the source system is described as +proj=latlon +ellps=clrk66 and the destination system is described as +proj=merc +ellps=clrk66 +lat_ts=33. Since the Mercator projection accepts geodetic coordinates as its input, the description of the source in this case is superfluous. We use that to our advantage in the new API and simply state the destination. This is simple at a glance, but is actually a big conceptual change. We are now focused on the path between two systems instead of what the source and destination systems are.

## Function mapping from old to new API¶

Old API functions

New API functions

pj_fwd

proj_trans()

pj_inv

proj_trans()

pj_fwd3

proj_trans()

pj_inv3

proj_trans()

pj_transform

proj_trans_array or proj_trans_generic

pj_init

proj_create()

pj_init_plus

proj_create()

pj_free

proj_destroy()

pj_is_latlong

proj_angular_output()

pj_is_geocent

proj_angular_output()

pj_get_def

proj_pj_info()

pj_latlong_from_proj

No equivalent

pj_set_finder

No equivalent

pj_set_searchpath

No equivalent

pj_deallocate_grids

No equivalent

pj_strerrno

No equivalent

pj_get_errno_ref

proj_errno()

pj_get_release

proj_info()