Per #1673, update Grid-Stat docs to clarify that GBETA is only comput…

…ed on the FULL verification domain and not any masking regions.

met/docs/Users_Guide/grid-stat.rst

@@ -120,7 +120,7 @@ While :numref:`grid-stat_fig1` and :numref:`grid-stat_fig2` are helpful in illus
 
    The absolute difference between the distance maps in the bottom row of :numref:`grid-stat_fig3` (top left), the shortest distances from every grid point in B to the nearest grid point in A (top right), and the shortest distances from every grid point in A to the nearest grid points in B (bottom left). The latter two do not have axes in order to emphasize that the distances are now only considered from within the respective event sets. The top right graphic is the distance map of A conditioned on the presence of an event from B, and that in the bottom left is the distance map of B conditioned on the presence of an event from A.
 
-The statistics derived from these distance maps are described in :numref:`Appendix C, Section %s <App_C-distance_maps>`. For each combination of input field and categorical threshold requested in the configuration file, Grid-Stat applies that threshold to define events in the forecast and observation fields and computes distance maps for those binary fields. Statistics for all requested masking regions are derived from those distance maps. Note that the distance maps are computed only once over the full verification domain, not separately for each masking region. Events occurring outside of a masking region can affect the distance map values inside that masking region and, therefore, can also affect the distance maps statistics for that region.
+The statistics derived from these distance maps are described in :numref:`Appendix C, Section %s <App_C-distance_maps>`. To make fair comparisons, any grid point containing bad data in either the forecast or observation field is set to bad data in both fields. For each combination of input field and categorical threshold requested in the configuration file, Grid-Stat applies that threshold to define events in the forecast and observation fields and computes distance maps for those binary fields. Statistics for all requested masking regions are derived from those distance maps. Note that the distance maps are computed only once over the full verification domain, not separately for each masking region. Events occurring outside of a masking region can affect the distance map values inside that masking region and, therefore, can also affect the distance maps statistics for that region.
 
 .. _grid-stat_gbeta:
 
@@ -143,6 +143,8 @@ Whether or not the forecast from :numref:`grid-stat_fig6` is “good” or not d
 
 In some cases, a user may be interested in a much higher threshold than :math:`2.1 mmh^{-1}` of the above example. :ref:`Gilleland, 2021 (Fig. 4) <Gilleland-2021>`, for example, shows this same forecast using a threshold of :math:`40 mmh^{-1}`. Only a small area in Mississippi has such extreme rain predicted at this valid time; yet none was observed. Small spatial areas of extreme rain in the observed field, however, did occur in a location far away from Mississippi that was not predicted. Generally, for this type of verification, the Hausdorff metric is a good choice of measure. However, a small choice of :math:`\beta` will provide similar results as the Hausdorff distance (:ref:`Gilleland, 2021 <Gilleland-2021>`). The user should think about the average size of storm areas and multiply this value by the displacement distance  they are comfortable with in order to get a good initial choice for :math:`\beta`, and may have to increase or decrease its value by trial-and-error using one or two example cases from their verification set.
 
+Since :math:`G_\beta` is so sensitive to the choice of :math:`\beta`, which is defined relative to the number of points in the verification domain, :math:`G_\beta` is only computed for the full verification domain. :math:`G_\beta` is reported as a bad data value for any masking region subsets of the full verification domain.
+
 Practical information
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@@ -332,7 +334,7 @@ ____________________
      beta_value(n)     = n * n / 2.0;
   }
 
-The **distance_map** entry is a dictionary containing options related to the distance map statistics in the **DMAP** output line type. The **baddeley_p** entry is an integer specifying the exponent used in the Lp-norm when computing the Baddeley :math:`\Delta` metric. The **baddeley_max_dist** entry is a floating point number specifying the maximum allowable distance for each distance map. Any distances larger than this number will be reset to this constant. A value of **NA** indicates that no maximum distance value should be used. The **fom_alpha** entry is a floating point number specifying the scaling constant to be used when computing Pratt's Figure of Merit. The **zhu_weight** specifies a value between 0 and 1 to define the importance of the RMSE of the binary fields (i.e. amount of overlap) versus the mean-error distance (MED). The default value of 0.5 gives equal weighting. This configuration option may be set separately in each **obs.field** entry. The **beta_value** entry is defined as a function of n, where n is the total number of grid points in the domain (i.e. Nx * Ny). The resulting beta_value is used to compute the :math:`G_\beta` statistic. The default function, :math:`N^2 / 2`, is recommended in published literature but can be modified as needed.
+The **distance_map** entry is a dictionary containing options related to the distance map statistics in the **DMAP** output line type. The **baddeley_p** entry is an integer specifying the exponent used in the Lp-norm when computing the Baddeley :math:`\Delta` metric. The **baddeley_max_dist** entry is a floating point number specifying the maximum allowable distance for each distance map. Any distances larger than this number will be reset to this constant. A value of **NA** indicates that no maximum distance value should be used. The **fom_alpha** entry is a floating point number specifying the scaling constant to be used when computing Pratt's Figure of Merit. The **zhu_weight** specifies a value between 0 and 1 to define the importance of the RMSE of the binary fields (i.e. amount of overlap) versus the mean-error distance (MED). The default value of 0.5 gives equal weighting. This configuration option may be set separately in each **obs.field** entry. The **beta_value** entry is defined as a function of n, where n is the total number of grid points in the full verification domain containing valid data in both the forecast and observation fields. The resulting beta_value is used to compute the :math:`G_\beta` statistic. The default function, :math:`N^2 / 2`, is recommended in :ref:`Gilleland, 2021 <Gilleland-2021>` but can be modified as needed.
 
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