.. _filters.reprojection:

filters.reprojection
===========================

The **reprojection filter** converts the X, Y and/or Z dimensions to a
new spatial
reference system. The old coordinates are replaced by the new ones.
If you want to preserve the old coordinates for future processing, use a
:ref:`filters.ferry` to create copies of the original dimensions before
reprojecting.

.. note::

    When coordinates are reprojected, it may significantly change the precision
    necessary to represent the values in some output formats.  Make sure
    that you're familiar with any scaling necessary for your output format
    based on the projection you've used.

.. embed::

.. streamable::

Example 1
--------------------------------------------------------------------------------

This pipeline reprojects terrain points with Z-values between 0 and 100 by first
applying a range filter and then specifing both the input and output spatial
reference as EPSG-codes. The X and Y dimensions are scaled to allow enough
precision in the output coordinates.

.. code-block:: json

  [
      {
          "filename":"input.las",
          "type":"readers.las",
          "spatialreference":"EPSG:26916"
      },
      {
          "type":"filters.range",
          "limits":"Z[0:100],Classification[2:2]"
      },
      {
          "type":"filters.reprojection",
          "in_srs":"EPSG:26916",
          "out_srs":"EPSG:4326"
      },
      {
          "type":"writers.las",
          "scale_x":"0.0000001",
          "scale_y":"0.0000001",
          "scale_z":"0.01",
          "offset_x":"auto",
          "offset_y":"auto",
          "offset_z":"auto",
          "filename":"example-geog.las"
      }
  ]

Example 2
--------------------------------------------------------------------------------

In some cases it is not possible to use a EPSG-code as a spatial reference.
Instead `Proj.4 <http:/proj4.org>`_ parameters can be used to define a spatial
reference.  In this example the vertical component of points in a laz file is
converted from geometric (ellipsoidal) heights to orthometric heights by using
the ``geoidgrids`` parameter from Proj.4.  Here we change the vertical datum
from the GRS80 ellipsoid to DVR90, the vertical datum in Denmark. In the
writing stage of the pipeline the spatial reference of the file is set to
EPSG:7416. The last step is needed since PDAL will otherwise reference the
vertical datum as "Unnamed Vertical Datum" in the spatial reference VLR.


.. code-block:: json

  [
      "./1km_6135_632.laz",
      {
          "type":"filters.reprojection",
          "in_srs":"EPSG:25832",
          "out_srs":"+init=epsg:25832 +geoidgrids=C:/data/geoids/dvr90.gtx"
      },
      {
          "type":"writers.las",
          "a_srs":"EPSG:7416",
          "filename":"1km_6135_632_DVR90.laz"
      }
  ]

Options
-------

in_srs
  Spatial reference system of the input data. Express as an EPSG string (eg
  "EPSG:4326" for WGS84 geographic), Proj.4 string or a well-known text
  string. [Required if not part of the input data set]

out_srs
  Spatial reference system of the output data. Express as an EPSG string (eg
  "EPSG:4326" for WGS84 geographic), Proj.4 string or a well-known text
  string. [Required]

in_axis_ordering
  An array of numbers that override the axis order for the in_srs (or if 
  not specified, the inferred SRS from the previous Stage). "2, 1" for 
  example would swap X and Y, which may be commonly needed for 
  something like "EPSG:4326". 
  
out_axis_ordering
  An array of numbers that override the axis order for the out_srs. 
  "2, 1" for example would swap X and Y, which may be commonly needed for 
  something like "EPSG:4326". 
