FAQ

Which file formats does PROJ support?

The command line applications that come with PROJ only support text input and output (apart from proj which accepts a simple binary data stream as well). proj, cs2cs and cct expects text files with one coordinate per line with each coordinate dimension in a separate column.

Note

If your data is stored in a common geodata file format chances are that you can use GDAL as a frontend to PROJ and transform your data with the ogr2ogr application.

Can I transform from abc to xyz?

Probably. PROJ supports transformations between most coordinate reference systems registered in the EPSG registry, as well as a number of other coordinate reference systems. The best way to find out is to test it with the projinfo application. Here’s an example checking if there’s a transformation between ETRS89/UTM32N (EPSG:25832) and ETRS89/DKTM1 (EPSG:4093):

$ ./projinfo -s EPSG:25832 -t EPSG:4093 -o PROJ
Candidate operations found: 1
-------------------------------------
Operation n°1:

unknown id, Inverse of UTM zone 32N + DKTM1, 0 m, World

PROJ string:
+proj=pipeline +step +inv +proj=utm +zone=32 +ellps=GRS80
+step +proj=tmerc +lat_0=0 +lon_0=9 +k=0.99998 +x_0=200000 +y_0=-5000000 +ellps=GRS80

See the projinfo documentation for more info on how to use it.

Coordinate reference system xyz is not in the EPSG registry, what do I do?

Generally PROJ will accept coordinate reference system descriptions in the form of WKT, WKT2 and PROJ strings. If you are able to describe your desired CRS in either of those formats there’s a good chance that PROJ will be able to make sense of it.

If it is important to you that a given CRS is added to the EPSG registry, you should contact your local geodetic authority and ask them to submit the CRS for inclusion in the registry.

I found a bug in PROJ, how do I get it fixed?

Please report bugs that you find to the issue tracker on GitHub. Here’s how.

If you know how to program you can also try to fix it yourself. You are welcome to ask for guidance on one of the communication channels used by the project.

How do I contribute to PROJ?

Any contributions from the PROJ community is welcome. See Contributing for more details.

How do I calculate distances/directions on the surface of the earth?

These are called geodesic calculations. There is a page about it here: Geodesic calculations.

What is the best format for describing coordinate reference systems?

A coordinate reference system (CRS) can in PROJ be described in several ways: As PROJ strings, Well-Known Text (WKT) and as spatial reference ID’s (such as EPSG codes). Generally, WKT or SRID’s are preferred over PROJ strings as they can contain more information about a given CRS. Conversions between WKT and PROJ strings will in most cases cause a loss of information, potentially leading to erroneous transformations.

For compatibility reasons PROJ supports several WKT dialects (see projinfo -o). If possible WKT2 should be used.

Why is the axis ordering in PROJ not consistent?

PROJ respects the axis ordering as it was defined by the authority in charge of a given coordinate reference system. This is in accordance to the ISO19111 standard [ISO19111]. Unfortunately most GIS software on the market doesn’t follow this standard. Before version 6, PROJ did not respect the standard either. This causes some problems while the rest of the industry conforms to the standard. PROJ intends to spearhead this effort, hopefully setting a good example for the rest of the geospatial industry.

Customarily in GIS the first component in a coordinate tuple has been aligned with the east/west direction and the second component with the north/south direction. For many coordinate reference systems this is also what is defined by the authority. There are however exceptions, especially when dealing with coordinate systems that don’t align with the cardinal directions of a compass. For example it is not obvious which coordinate component aligns to which axis in a skewed coordinate system with a 45 degrees angle against the north direction. Similarly, a geocentric cartesain coordinate system usually has the z-component aligned with the rotational axis of the earth and hence the axis points towards north. Both cases are incompatible with the convention of always having the x-component be the east/west axis, the y-component the north/south axis and the z-component the up/down axis.

In most cases coordinate reference systems with geodetic coordinates expect the input ordered as latitude/longitude (typically with the EPSG dataset), however, internally PROJ expects an longitude/latitude ordering for all projections. This is generally hidden for users but in a few cases it is exposed at the surface level of PROJ, most prominently in the proj utility which expects longitude/latitude ordering of input date (unless proj -r is used).

In case of doubt about the axis order of a specific CRS projinfo is able to provide an answer. Simply look up the CRS and examine the axis specification of the Well-Known Text output:

projinfo EPSG:4326
PROJ.4 string:
+proj=longlat +datum=WGS84 +no_defs +type=crs

WKT2_2018 string:
GEOGCRS["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]],
    USAGE[
        SCOPE["unknown"],
        AREA["World"],
        BBOX[-90,-180,90,180]],
    ID["EPSG",4326]]

Why am I getting the error “Cannot find proj.db”?

The file proj.db must be readable for the library to properly function. Like other resource files, it is located using a set of search paths. In most cases, the following paths are checked in order:

  • A path provided by the environment variable PROJ_LIB.

  • A path built into PROJ as its resource installation directory (typically ../share/proj relative to the PROJ library).

  • The current directory.

Note that if you’re using conda, activating an environment sets PROJ_LIB to a resource directory located in that environment.