Version 4 to 5 API Migration

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


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 latitude and longitude 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_latlong;
    double x, y;

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

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

    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.lp.lam = proj_todeg(c.lp.lam);
        c.lp.phi = proj_todeg(c.lp.phi);
        c = proj_trans(P, PJ_FWD, c);
        printf("%.2f\t%.2f\n", c.xy.x, c.xy.y);


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 superflous. 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










proj_trans_array or proj_trans_generic














No equivalent


No equivalent


No equivalent


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