Added a keyword to allow the user to choose the extrapolation mode of…

… the linear interpolation kernel. The default is not to extrapolate (any points outside the ranges will be masked).

cis/collocation/col_implementations.py

@@ -541,10 +541,11 @@ class li(Kernel):
     Linear Interpolation Kernel
     """
 
-    def __init__(self):
+    def __init__(self, extrapolate=False):
         self.coord_names = []
         self.hybrid_ht = False
         self.interpolator = None
+        self.extrapolation_mode = 'linear' if extrapolate else 'error'
 
     def get_value(self, point, data):
         """
@@ -559,12 +560,12 @@ def get_value(self, point, data):
             if len(data.coords('altitude', dim_coords=False)) > 0:
                 self.hybrid_ht = True
 
-            self.interpolator = iris.analysis.Linear().interpolator(data, self.coord_names)
+            self.interpolator = iris.analysis.Linear(self.extrapolation_mode).interpolator(data, self.coord_names)
 
         # Return the data from the result of interpolating over those coordinates which are on the cube.
         slice = self.interpolator([getattr(point,c) for c in self.coord_names])
         if self.hybrid_ht:
-            val = slice.interpolate([('altitude', point.altitude)], iris.analysis.Linear()).data
+            val = slice.interpolate([('altitude', point.altitude)], iris.analysis.Linear(self.extrapolation_mode)).data
         else:
             val = slice.data
         return val

doc/collocation.rst

@@ -68,6 +68,9 @@ where:
         other separation constraints.
 
       * ``lin`` For use with gridded source data only. A value is calculated by linear interpolation for each sample point.
+        The extrapolation mode can be controlled with the ``extrapolate`` keyword. The default mode is not to extrapolate values
+        to points outside of the gridded data source (masking them in the output instead). Setting ``extrapolate=True``
+        will instruct the kernel to extrapolate these values outside of the data source instead.
 
       * ``nn`` For use with gridded source data only. The data point closest to each sample point is found, and the
         data value is set at the sample point.