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.. _peirce_q:
********************************************************************************
Peirce Quincuncial
********************************************************************************
The Peirce Quincuncial projection is a conformal map projection
that transforms the circle of the northern hemisphere into a square,
and the southern hemisphere split into four triangles arranged
around the square to form a quincunx. The resulting projection
is a regular diamond shape or can be rotated to form a square.
The resulting tile can be infinitely tessellated. Though this implementation
defaults to a central meridian of 0, it is more common to use a central
meridian of around 25 to optimise the distortions. Peirce's original
published map from 1879 used a central meridian of approx -70.
The diamond and square versions can be produced using the
``+shape=diamond`` and ``+shape=square`` options respectively.
This implementation includes an alternative lateral projection
which places hemispheres side-by-side (``+shape=horizontal`` or
``+shape=vertical``). Combined with a general oblique transformation,
this can be used to produced a Grieger Triptychial projection
(see example below).
+---------------------+----------------------------------------------------------+
| **Classification** | Miscellaneous |
+---------------------+----------------------------------------------------------+
| **Available forms** | Forward spherical projection |
+---------------------+----------------------------------------------------------+
| **Defined area** | Global |
+---------------------+----------------------------------------------------------+
| **Alias** | peirce_q |
+---------------------+----------------------------------------------------------+
| **Domain** | 2D |
+---------------------+----------------------------------------------------------+
| **Input type** | Geodetic coordinates |
+---------------------+----------------------------------------------------------+
| **Output type** | Projected coordinates |
+---------------------+----------------------------------------------------------+
.. figure:: ./images/peirce_q_square.png
:width: 500 px
:align: center
:alt: Peirce Quincuncial (Square)
proj-string: ``+proj=peirce_q +lon_0=25 +shape=square``
.. figure:: ./images/peirce_q_diamond.png
:width: 500 px
:align: center
:alt: Peirce Quincuncial (Diamond)
proj-string: ``+proj=peirce_q +lon_0=25 +shape=diamond``
.. figure:: ./images/peirce_q_horizontal.png
:width: 500 px
:align: center
:alt: Peirce Quincuncial (Horizontal)
proj-string: ``+proj=peirce_q +lon_0=25 +shape=horizontal``
.. figure:: ./images/grieger_triptychial.png
:width: 500 px
:align: center
:alt: Grieger Triptychial
proj-string: ``+proj=pipeline +step +proj=ob_tran +o_proj=peirce_q +o_lat_p=-45 +o_lon_p=45 +o_type=horizontal +o_scrollx=-0.25 +step +proj=affine +s11=-1 +s12=0 +s21=0 +s22=-1``
Parameters
################################################################################
.. note:: All parameters are optional.
.. include:: ../options/lon_0.rst
.. option:: +shape=square/diamond/horizontal/vertical/nhemisphere/shemisphere
.. versionadded:: 9.0
*Defaults to diamond.*
.. warning:: This option was wrongly introduced introduced in 8.2.1 with the
``type`` name, which was inappropriate as it conflicted with
the ``+type=crs`` general hint.
Indicates the shape of transformation applied to the southern hemisphere:
``square`` and ``diamond`` represent the traditional quincuncial form suggested
by Peirce with the southern hemisphere divided into 4 triangles and reflected
outward from the northern hemisphere. The ``square`` shape is rotated by 45
degrees to produce the conventional square presentation. The origin lies at
the centre of the square or diamond.
By contrast, the ``horizontal`` and ``vertical`` forms reflect the southern
hemisphere laterally across the x or y axis respectively to produce a rectangular
form. The origin lies at the centre of the rectangle.
The other two types, ``nhemisphere`` and ``shemisphere``, discard latitudes of less
than 0 or more than 0, respectively, to allow single hemispheres to be selected.
The origin lies at the centre of the square or diamond.
.. option:: +scrollx=<value>
For ``horizontal`` shape allows a scalar circular scroll of resulting x coordinates
to shift sections of the projection to the other horizontal side of the map.
*Defaults to 0.0. Must be a scale between -1.0 and 1.0.*
.. option:: +scrolly=<value>
For ``vertical`` shape allows a scalar circular scroll of resulting y coordinates
to shift sections of the projection to the other vertical side of the map.
*Defaults to 0.0. Must be a scale between -1.0 and 1.0.*
.. include:: ../options/R.rst
.. include:: ../options/x_0.rst
.. include:: ../options/y_0.rst
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