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.. _dev_quickstart:
================================================================================
Quick start
================================================================================
This is a short introduction to the PROJ API. In the following section we
create a simple program that transforms a geodetic coordinate to UTM and back
again. The program is explained a few lines at a time. The complete program can
be seen at the end of the section.
See the following sections for more in-depth descriptions of different parts of
the PROJ API or consult the :doc:`API reference <reference/index>` for specifics.
Before the PROJ API can be used it is necessary to include the ``proj.h`` header
file. Here ``stdio.h`` is also included so we can print some text to the screen:
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 39-40
Let's declare a few variables that'll be used later in the program. Each variable
will be discussed below.
See the :doc:`reference for more info on data types <reference/datatypes>`.
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 43-45
:dedent: 4
For use in multi-threaded programs the ``PJ_CONTEXT`` threading-context is used.
In this particular example it is not needed, but for the sake of completeness
it created here. The section on :doc:`threads <threads>` discusses
this in detail.
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 48
:dedent: 4
Next we create the ``PJ`` transformation object ``P`` with the function
``proj_create``. ``proj_create`` takes the threading context ``C`` created above,
and a proj-string that defines the desired transformation. Here we transform
from geodetic coordinate to UTM zone 32N.
It is recommended to create one threading-context per thread used by the program.
This ensures that all ``PJ`` objects created in the same context will be sharing
resources such as error-numbers and loaded grids.
In case the creation of the ``PJ`` object fails an error message is displayed and
the program returns. See :doc:`errorhandling` for further
details.
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 50-52
:dedent: 4
PROJ uses it's own data structures for handling coordinates. Here we use a
``PJ_COORD`` which is easily assigned with the function ``proj_coord``. Note
that the input values are converted to radians with ``proj_torad``. This is
necessary since PROJ is using radians internally. See :doc:`transformations`
for further details.
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 56
:dedent: 4
The coordinate defined above is transformed with ``proj_trans_coord``. For this
a ``PJ`` object, a transformation direction (either forward or inverse) and the
coordinate is needed. The transformed coordinate is returned in ``b``.
Here the forward (``PJ_FWD``) transformation from geodetic to UTM is made.
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 59-60
:dedent: 4
The inverse transformation (UTM to geodetic) is done similar to above,
this time using ``PJ_INV`` as the direction.
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 61-62
:dedent: 4
Before ending the program the allocated memory needs to be released again:
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:lines: 65-66
:dedent: 4
A complete compilable version of the above can be seen here:
.. literalinclude:: ../../../examples/pj_obs_api_mini_demo.c
:language: c
:linenos:
:lines: 39-
|
