From 6bea41f4649c318bd7e8e5f6204e1f603f50fa3a Mon Sep 17 00:00:00 2001
From: Christina Kalb <kalb@ucar.edu>
Date: Wed, 13 Nov 2024 17:29:58 -0700
Subject: [PATCH] Update data_io.rst

Updated netCDF reading documentation
---
 docs/Users_Guide/data_io.rst | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/docs/Users_Guide/data_io.rst b/docs/Users_Guide/data_io.rst
index 03d7ab02e6..ad407fdb2c 100644
--- a/docs/Users_Guide/data_io.rst
+++ b/docs/Users_Guide/data_io.rst
@@ -30,7 +30,7 @@ The MET tools use following attributes and variables for input CF Compliant NetC
 
 3. The "`coordinates <https://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#coordinate-types>`_" attribute for the data variables. It contains the coordinate variable names.
 
-4. The "`grid_mapping <https://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#appendix-grid-mappings>`_" attribute for the data variables for projections and the matching grid mapping variable (optional for the latitude_longitude projection).
+4. The "`grid_mapping <https://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#appendix-grid-mappings>`_" attribute for the data variables for projections and the matching grid mapping variable (optional for latitude_longitude and rotated_latitude_longitude projections).
 
 5. The gridded data should be evenly spaced horizontally and vertically.
 
@@ -78,7 +78,7 @@ Here are examples for the grid mapping variable ("edr" is the data variable):
             grid_mapping:GRIB_earth_shape = "spherical" ;
             grid_mapping:GRIB_earth_shape_code = 0 ;
 
-When the grid mapping variable is not available, MET detects the latitude_longitude projection in following order:
+When the grid mapping variable is not available, MET can detect either a latitude_longitude or rotated_latitude_longitude projection.  It detects the latitude_longitude projection in following order:
 
 1. the lat/lon projection from the dimensions
 
@@ -98,6 +98,14 @@ MET gets the valid time from the time variable and the "forecast_reference_time"
 
 2. TRMM_3B42_daily_filename (3B42_daily.<yyyy>.<mm>.<dd>.7.G3.nc)
 
+For rotated_latitude_longitude projections, MET detects the projection using the following logic:
+
+1. Looking for a variable that has the same name as a dimension.
+
+2. If a variable is found, checking to make sure the variable has a standard name attribute and that that attribute is not empty.
+
+3. Checking to see if the standard name attribute is called either grid_latitude for latitude variables or grid_longitude for the longitude variable.
+
 .. list-table:: Valid strings for the "units" attribute.
   :widths: auto
   :header-rows: 1