path: root/docs/source/faq.rst

.. _faq:

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FAQ
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    .. contents::
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Where can I find the list of projections and their arguments?
--------------------------------------------------------------------------------

:doc:`Here<../operations/projections/index>`.

Additinoally, the ``proj`` command itself can report the list of projections
using the ``-lp`` option, the list of ellipsoids with the ``-le`` option,
the list of units with the ``-lu`` option, and the list of built-in datums with
the ``-ld`` option.

How do I do datum shifts between NAD27 and NAD83?
--------------------------------------------------------------------------------

Using the ``cs2cs`` application.  The following example demonstrates using the
default shift parameters for NAD27 to NAD83:

::

    % cs2cs +proj=latlong +datum=NAD27 +to +proj=latlong +datum=NAD83 -117 30

producing:

::

    117d0'2.901"W   30d0'0.407"N 0.000

In order for datum shifting to work properly the various grid shift files must
be available. See below. More details are available in the
section on :doc:`resource files<resource_files>`.

How do I build/configure PROJ to support datum shifting?
--------------------------------------------------------------------------------

After downloading and unpacking the PROJ source, also download and unpack the
set of datum shift files.  See :ref:`download` for instructions how to fetch
and install these files

On Windows the extra nadshift target must be used.  For instance
``nmake /f makefile.vc nadshift`` in the ``proj/src`` directory.

A default build and install on Unix will normally build knowledge of the
directory where the grid shift files are installed into the PROJ library
(usually ``/usr/local/share/proj``).  On Windows the library is normally built
thinking that C:\PROJ\NAD is the installed directory for the grid shift files.
If the built in concept of the PROJ data directory is incorrect, the ``PROJ_LIB``
environment can be defined with the correct directory.

How do I debug problems with NAD27/NAD83 datum shifting?
--------------------------------------------------------------------------------

1. Verify that you have the binary files (eg. ``/usr/local/share/proj/conus``)
   installed on your system.  If not, see the previous question.
2. Try a datum shifting operation in relative isolation, such as with the :program:`cs2cs`
   command listed above.  Do you get reasonable results?  If not it is likely
   the grid shift files aren't being found.  Perhaps you need to define
   :envvar:`PROJ_LIB`?
3. The :program:`cs2cs` command and the underlying :c:func:`pj_transform()` API know how to do a
   grid shift as part of a more complex coordinate transformation; however, it
   is imperative that both the source and destination coordinate system be
   defined with appropriate datum information.  That means that implicitly or
   explicitly there must be a ``+datum=`` clause, a ``+nadgrids=`` clause or a
   ``+towgs84=`` clause.  For instance
   ``cs2cs +proj=latlong +datum=NAD27 +to +proj=latlong +ellps=WGS84`` won't work because defining the output
   coordinate system as using the ellipse WGS84 isn't the same as defining it
   to use the datum WGS84 (use ``+datum=WGS84``).  If either the input or output
   are not identified as having a datum, the datum shifting (and ellipsoid
   change) step is just quietly skipped!
4. The :envvar:`PROJ_DEBUG` environment can be defined (any value) to force extra output
   from the PROJ library to stderr (the text console normally) with
   information on what data files are being opened and in some cases why a
   transformation fails.

   ::

        export PROJ_DEBUG=ON
        cs2cs ...


   .. note::
        ``PROJ_DEBUG`` support is not yet very mature in the PROJ library.

5. The :option:`-v` flag to :program:`cs2cs` can be useful in establishing more detail on what
   parameters being used internally for a coordinate system.  This will include
   expanding the definition of ``+datum`` clause.

How do I use EPSG coordinate system codes with PROJ?
--------------------------------------------------------------------------------

There is somewhat imperfect translation between 2D geographic and projected
coordinate system codes and PROJ descriptions of the coordinate system
available in the ``epsg`` definition file that normally lives in the ``proj/nad``
directory.  If installed (it is installed by default on Unix), it is possible
to use EPSG numbers like this:

::


    % cs2cs -v +init=epsg:26711
    # ---- From Coordinate System ----
    #Universal Transverse Mercator (UTM)
    #       Cyl, Sph
    #       zone= south
    # +init=epsg:26711 +proj=utm +zone=11 +ellps=clrk66 +datum=NAD27 +units=m
    # +no_defs +nadgrids=conus,ntv1_can.dat
    #--- following specified but NOT used
    # +ellps=clrk66
    # ---- To Coordinate System ----
    #Lat/long (Geodetic)
    #
    # +proj=latlong +datum=NAD27 +ellps=clrk66 +nadgrids=conus,ntv1_can.dat

The proj/nad/epsg file can be browsed and searched in a text editor for
coordinate systems.  There are known to be problems with some coordinate
systems, and any coordinate systems with odd axes, a non-greenwich prime
meridian or other quirkiness are unlikely to work properly. Caveat Emptor!

How do I use 3 parameter and 7 parameter datum shifting
--------------------------------------------------------------------------------

Datum shifts can be approximated with 3 and 7 parameter transformations. Their
use with :program:`cs2cs` is more fully described in the
:ref:`geodetic tranformation<cs2cs_specific_options>` section.

More generically, the :ref:`helmert` can be used with :program:`cct`.


Does PROJ work in different international numeric locales?
--------------------------------------------------------------------------------

No. PROJ makes extensive use of the :c:func:`sprintf()` and :c:func:`atof()` C functions
internally to translate numeric values.  If a locale is in effect that modifies
formatting of numbers, altering the role of commas and periods in numbers, then
PROJ will not work. This problem is common in some European locales.

On UNIX-like platforms, this problem can be avoided by forcing the use of the
default numeric locale by setting the :envvar:`LC_NUMERIC` environment variable to C.

::

    $ export LC_NUMERIC=C
    $ proj ...

.. note::

    NOTE: Per ticket #49, in PROJ 4.7.0 and later pj_init() operates with locale
    overridden to "C" to avoid most locale specific processing for applications
    using the API.  Command line tools may still have issues.

Changing Ellipsoid / Why can't I convert from WGS84 to Google Earth / Virtual Globe Mercator?
----------------------------------------------------------------------------------------------

The coordinate system definition for Google Earth, and Virtual Globe Mercator
is as follows, which uses a sphere as the earth model for the Mercator
projection.

::

    +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0
         +x_0=0.0 +y_0=0 +k=1.0 +units=m +no_defs

But, if you do something like:

::

    cs2cs +proj=latlong +datum=WGS84
        +to +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0
                       +x_0=0.0 +y_0=0 +k=1.0 +units=m +no_defs

to convert between WGS84 and mercator on the sphere there will be substantial
shifts in the Y mercator coordinates.  This is because internally :program:`cs2cs` is
having to adjust the lat/long coordinates from being on the sphere to being on
the WGS84 datum which has a quite differently shaped ellipsoid.

In this case, and many other cases using spherical projections, the desired
approach is to actually treat the lat/long locations on the sphere as if they
were on WGS84 without any adjustments when using them for converting to other
coordinate systems.  The solution is to "trick" PROJ into applying no change
to the lat/long values when going to (and through) WGS84.  This can be
accomplished by asking PROJ to use a null grid shift file for switching from
your spherical lat/long coordinates to WGS84.

::

    cs2cs +proj=latlong +datum=WGS84 \
        +to +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 \
        +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs

Note the strategic addition of ``+nadgrids=@null`` to the spherical projection
definition.

Similar issues apply with many other datasets distributed with projections
based on a spherical earth model - such as many NASA datasets.  This coordinate
system is now known by the EPSG code 3857 and has in the past been known as
EPSG:3785 and EPSG:900913.  When using this coordinate system with GDAL/OGR it
is helpful to include the +wktext so the exact PROJ string will be preserved
in the WKT representation (otherwise key parameters like ``+nadgrids=@null`` will
be dropped):

::

    +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0
               +units=m +nadgrids=@null +wktext  +no_defs


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

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


What options does PROJ allow for the shape of the Earth (geodesy)?
--------------------------------------------------------------------------------

See https://github.com/OSGeo/proj.4/blob/master/src/pj_ellps.c
for possible ellipse options. For example, putting ``+ellps=WGS84`` uses
the ``WGS84`` Earth shape.

What if I want a spherical Earth?
--------------------------------------------------------------------------------

Use ``+ellps=sphere``.  See https://github.com/OSGeo/proj.4/blob/master/src/pj_ellps.c
for the radius used in this case.

How do I change the radius of the Earth?  How do I use PROJ for work on Mars?
--------------------------------------------------------------------------------

You can supply explicit values for the semi minor and semi major axes instead
of using the symbolic "sphere" value.  Eg, if the radius were 2000000m:

::

     +proj=laea +lon_0=-40.000000 +lat_0=74.000000 +x_0=1000000 +y_0=1700000 +a=2000000 +b=2000000"

How do I do False Eastings and False Northings?
--------------------------------------------------------------------------------

Use ``+x_0`` and ``+y_0`` in the projection string.