Add documentation for LOCALIZATION keyword

Author: SAKavli

docs/ert/reference/configuration/observations.rst

@@ -489,6 +489,62 @@ are correctly associated with reservoir zones.
 If a zone is specified but no ZONEMAP is provided, or if the observation location doesn't match
 the expected zone, the observation will be deactivated with a warning during the simulation.
 
+The LOCALIZATION keyword - Configuring observations with location
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A prerequisite for using :ref:`distance based localization <distance_based_localization>`
+is to provide metadata regarding location for the observations.
+
+This can be done for :ref:`summary observations <summary_observation>` and
+:ref:`breakthrough observations <breakthrough_observation>` by inserting a `LOCALIZATION`
+keyword into their declarations.
+
+The `LOCALIZATION` object has two required fields: `NORTH` and `EAST`. Additionally,
+`RADIUS` can be configured here, but will be defaulted to 3000m if absent.
+
+The `LOCALIZATION` object can be defined as follows:
+
+.. code-block:: none
+
+   LOCALIZATION {
+     EAST=70;
+     NORTH=80;
+     RADIUS=2500;
+   };
+
+And inserted into a summary observation like so:
+
+.. code-block:: none
+
+   SUMMARY_OBSERVATION WOPR_OP1_141 {
+     KEY   = WOPR;
+     VALUE = 1e6;
+     ERROR = 3e4;
+     DATE  = 2012-02-13;
+     LOCALIZATION {
+       EAST   = 70;
+       NORTH  = 80;
+       RADIUS = 2500;
+     };
+   };
+
+And similarily into a :ref:`breakthrough observation <breakthrough_observation>`
+
+The RFT observations already contains the keywords `NORTH` and `EAST` in its´ regular
+configuration. These observations are therefore already correctly configured for distance
+based localization.
+
+The radius  for RFTs will be the default radius value of 3000m, but can be overwritten by
+providing a `LOCALIZATION` object containing just the `RADIUS` key, e.g.
+
+.. code-block::
+
+   LOCALIZATION {
+     RADIUS=2500;
+   };
+
+
+
 .. _bulk_configuration_of_summary_observations:
 
 Bulk configuration of summary observations

docs/ert/theory/gentle_introduction_to_history_matching.rst

@@ -520,6 +520,8 @@ That said, adaptive localization has some practical benefits.
 It is relatively easy to understand, and it can be enabled with minimal configuration.
 In practice, the only tunable input is the correlation threshold.
 
+.. _distance_based_localization:
+
 Distance based localization
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~