Add documentation for grid aggregation method

README.rst

@@ -27,7 +27,7 @@ Script grid3d_aggregate_map
 Make aggregated maps (min/max/mean/sum) from a 3D grid parameter, Eclipse or Roff input
 
 Script grid3d_migration_time
----------------------------
+----------------------------
 
 Make migration time maps aimed at tracking plume migration in CCS applications
 

docs/howitworks.rst

@@ -14,9 +14,40 @@ sum hc thickness map per zone or by all zones that are spesified.
 
 For average maps, a thickness weighted average is computed.
 
-This is not applicable to grid3d_aggregate_map and grid3d_migration_time,
+This is not applicable to `grid3d_aggregate_map` and `grid3d_migration_time`,
 which uses a different approach for aggregating layers.
 
+`grid3d_aggregate_map` and `grid3d_migration_time`
+--------------------------------------------------
+
+`grid3d_aggregate_map` and `grid3d_migration_time` uses a different approach
+when aggregating values. In general, the main difference from
+`grid3d_hc_thickness` and `grid3d_average_map` is that the result will be less
+smooth. Secondly, gaps in the grid (typically caused by faults), will not be
+filled.
+
+.. image:: images/grid_aggregation.jpg
+
+Each map node is first connected to all cells whose (approximate) footprint
+overlaps the map node. An artificial set of connections are illustrated by
+colors in the figure above.
+
+Once the connections are established, the value of each map node is calculated
+from all the connected grid cells via one of the specified aggregation
+methods:
+
+- Maximum
+- Minimum
+- Mean
+- Sum
+
+Layer information and lateral distance from map node to grid cell center is
+currently not used. However, filters can be used to apply this per zone etc.
+
+One benefit of this approach is that the map-node-to-grid-cell connection
+information can be re-used across time steps and properties, as long as the
+grid geometry does not change. This is beneficial for performance.
+
 Further details
 ---------------