Restructure coordinate operations docs

9 files changed, 34 insertions, 10 deletions

diff --git a/docs/source/usage/operations/transformations/axisswap.rst b/docs/source/usage/operations/conversions/axisswap.rst
index ddaad075..ddaad075 100644
--- a/docs/source/usage/operations/transformations/axisswap.rst
+++ b/docs/source/usage/operations/conversions/axisswap.rst
diff --git a/docs/source/usage/operations/transformations/cart.rst b/docs/source/usage/operations/conversions/cart.rst
index 0c3c7c23..0c3c7c23 100644
--- a/docs/source/usage/operations/transformations/cart.rst
+++ b/docs/source/usage/operations/conversions/cart.rst
diff --git a/docs/source/usage/operations/conversions/index.rst b/docs/source/usage/operations/conversions/index.rst
new file mode 100644
index 00000000..54324ec4
--- /dev/null
+++ b/docs/source/usage/operations/conversions/index.rst
@@ -0,0 +1,18 @@
+.. _conversions_list:
+
+================================================================================
+Conversions
+================================================================================
+
+Conversions are coordinate operations in which both coordinate reference systems
+are based on the same datum. In PROJ.4 projections are differentiated from
+conversions.
+
+.. toctree::
+   :maxdepth: 1
+
+   axisswap
+   cart
+   lonlat
+   latlon
+   unitconvert
diff --git a/docs/source/usage/operations/projections/latlon.rst b/docs/source/usage/operations/conversions/latlon.rst
index 6712fa7b..6712fa7b 100644
--- a/docs/source/usage/operations/projections/latlon.rst
+++ b/docs/source/usage/operations/conversions/latlon.rst
diff --git a/docs/source/usage/operations/projections/lonlat.rst b/docs/source/usage/operations/conversions/lonlat.rst
index 589cc813..589cc813 100644
--- a/docs/source/usage/operations/projections/lonlat.rst
+++ b/docs/source/usage/operations/conversions/lonlat.rst
diff --git a/docs/source/usage/operations/transformations/unitconvert.rst b/docs/source/usage/operations/conversions/unitconvert.rst
index ad848207..ad848207 100644
--- a/docs/source/usage/operations/transformations/unitconvert.rst
+++ b/docs/source/usage/operations/conversions/unitconvert.rst
diff --git a/docs/source/usage/operations/index.rst b/docs/source/usage/operations/index.rst
index 21200c93..baae0e0a 100644
--- a/docs/source/usage/operations/index.rst
+++ b/docs/source/usage/operations/index.rst
@@ -4,15 +4,18 @@
 Coordinate operations
 ================================================================================
 
-Coordinate operations in PROJ.4 are divided into two groups:
-Projections and transformations.
-Projections are purely cartographic mappings of the sphere onto the plane whereas
-transformations are mostly geodetic operations concerned with changes in
-reference frames.
+Coordinate operations in PROJ.4 are divided into three groups:
+Projections, conversions  and transformations.
+Projections are purely cartographic mappings of the sphere onto the plane.
+Technically projections are conversions (according to ISO standards), though in
+PROJ.4 projections are distinguished from conversions. Conversions are coordinate
+operations that do not exert a change in reference frame. Operations that do
+exert a change in reference frame are called transformations.
 
 .. toctree::
    :maxdepth: 1
 
    projections/index
+   conversions/index
    transformations/index
 
diff --git a/docs/source/usage/operations/projections/index.rst b/docs/source/usage/operations/projections/index.rst
index 5ccf0045..f7eb0c67 100644
--- a/docs/source/usage/operations/projections/index.rst
+++ b/docs/source/usage/operations/projections/index.rst
@@ -4,6 +4,11 @@
 Projections
 ================================================================================
 
+Projections are coordinate operations that are technically conversions but since
+projections are so fundamental to PROJ.4 we differentiate them from conversions.
+
+Projections map the spherical 3D space to a flat 2D space.
+
 .. toctree::
    :maxdepth: 1
 
@@ -62,8 +67,6 @@ Projections
    lagrng
    larr
    lask
-   lonlat
-   latlon
    lcc
    lcca
    leac
diff --git a/docs/source/usage/operations/transformations/index.rst b/docs/source/usage/operations/transformations/index.rst
index 2966ed0d..112d9220 100644
--- a/docs/source/usage/operations/transformations/index.rst
+++ b/docs/source/usage/operations/transformations/index.rst
@@ -4,13 +4,13 @@
 Transformations
 ================================================================================
 
+Transformations coordinate operation in which the two coordinate reference
+systems are based on different datums.
+
 .. toctree::
    :maxdepth: 1
 
-   axisswap
-   cart
    helmert
    molodensky
    hgridshift
    vgridshift
-   unitconvert