The three independent policies an IONEX slant-delay evaluation applies.

• :coverage - :strict refuses a query outside the product's epochs, latitudes or longitudes; :hold holds the nearest map or grid edge and names the miss in the result's held field. Default :strict.

• :missing_nodes - :strict refuses a query whose interpolation weights a node the product gives as non-available; :renormalize interpolates from the weighted nodes that hold values, their weights renormalized to sum to one, and names every node it interpolated around in the result's degraded field. Default :strict.

• :mapping - :single_layer applies 1/cos(z') at the shell height whatever the product declares, naming what a product declaring anything but COSZ declares in the result's assumed_mapping field; :declared applies the factor the product's MAPPING FUNCTION defines and refuses the query where that record defines none. Default :single_layer.

The three are independent, and all three status fields can be set on one value.

What is refused

Nothing here falls back to a default for a name it does not recognize:

• {:invalid_policy_value, field, value} - a choice none of the three fields names.
• {:unknown_policy_key, key} - a key that is not one of the three. The key is returned as it was given, whatever its type, and no atom is created for it.
• {:duplicate_policy_key, key} - a keyword list stating one key twice. Collapsing it would silently drop the earlier statement, including an invalid one, so a list that states a key twice is refused whether the two choices agree or not.
• :bad_slant_policy - an options container that is neither a keyword list nor a plain map.

A key left out takes its documented default.