Doc: add preliminary documentation of suffix based specification of u…

…nits (#3346)

Fixes #3345

docs/source/operations/options/lat_0.rst

@@ -3,3 +3,11 @@
     Latitude of projection center.
 
     *Defaults to 0.0.*
+
+    .. note::
+        The default convention is to interpret this value as decimal degrees. To
+        specify radians instead, follow the value with the "r" character.
+
+        Example: `+lat_0=1.5708r`
+
+        See :ref:`Projection Units <projection_units>` for more information.

docs/source/operations/options/lat_1.rst

@@ -3,3 +3,11 @@
     First standard parallel.
 
     *Defaults to 0.0.*
+
+    .. note::
+        The default convention is to interpret this value as decimal degrees. To
+        specify radians instead, follow the value with the "r" character.
+
+        Example: `+lat_1=1.5708r`
+
+        See :ref:`Projection Units <projection_units>` for more information.

docs/source/operations/options/lat_2.rst

@@ -3,3 +3,11 @@
     Second standard parallel.
 
     *Defaults to 0.0.*
+
+    .. note::
+        The default convention is to interpret this value as decimal degrees. To
+        specify radians instead, follow the value with the "r" character.
+
+        Example: `+lat_2=1.5708r`
+
+        See :ref:`Projection Units <projection_units>` for more information.

docs/source/operations/options/lat_ts.rst

@@ -4,3 +4,11 @@
     Takes precedence over ``+k_0`` if both options are used together.
 
     *Defaults to 0.0.*
+
+    .. note::
+        The default convention is to interpret this value as decimal degrees. To
+        specify radians instead, follow the value with the "r" character.
+
+        Example: `+lat_ts=1.5708r`
+
+        See :ref:`Projection Units <projection_units>` for more information.

docs/source/operations/options/lon_0.rst

@@ -3,3 +3,11 @@
     Longitude of projection center.
 
     *Defaults to 0.0.*
+
+    .. note::
+        The default convention is to interpret this value as decimal degrees. To
+        specify radians instead, follow the value with the "r" character.
+
+        Example: `+lon_0=1.5708r`
+
+        See :ref:`Projection Units <projection_units>` for more information.

docs/source/usage/projections.rst

@@ -38,6 +38,8 @@ documenting the individual :doc:`projections<../operations/projections/index>`.
 
 In the sections below most of the parameters are explained in details.
 
+.. _projection_units:
+
 Units
 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
@@ -64,6 +66,32 @@ Scaling of output units can be done by applying the ``+k_0`` argument. The
 returned coordinates are scaled by the value assigned with the ``+k_0``
 parameter.
 
+Input units for parameters that can be understood to be either decimal degrees or
+radians are interpreted to be decimal degrees by convention.
+
+Explict specification of input units can be accomplished by adding the appropriate
+suffix to input values.
+
+
+    +----------------+---------------------+
+    | Suffix         | Unit                |
+    +================+=====================+
+    | d              | Decimal degrees     |
+    +----------------+                     +
+    | D              |                     |
+    +----------------+                     +
+    | °              |                     |
+    +----------------+---------------------+
+    | r              | Radians             |
+    +----------------+                     +
+    | R              |                     |
+    +----------------+---------------------+
+
+Example of use.  The longitude of the central meridian ``+lon_0=90``, can also be expressed more explictly
+with units of decimal degrees as ``+lon_0=90d`` or in radian
+units as ``+lon_0=1.5708r`` (approximately).
+
+
 False Easting/Northing
 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
@@ -159,5 +187,3 @@ They can be combined in +axis in forms like:
 
     The ``+axis`` argument does not work with the :program:`proj` command line
     utility.
-
-