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Description

Select a grid ensemble member based on a scalar index.
Use case: Produce a relevant flood map based on the current forecast for e.g. water level. Starting point is a number of pre-computed flood maps for different forecast conditions. In an operational setting, you select the relevant flood map, based on the current forecast (water level, rainfall etc.). 

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Since the input is both a SCALAR and a GRID, the headers cannot be matched based on location id (as the scalar and grid will have different location ids).
Instead of the transformation framework automatically matching input and output (based on identical location id) the transformation matches all possible input headers with the output.
Therefore, the transformation can only be run using a single location (and not a location set).

Output

• Grid time series

The output time is determined by the time of the indices input.
The output grid is the grid ensemble member that corresponds to the input index.

Options

Because the time of the output time series is determined by the time of the ensemble member indices, it can happen that the input grids do not have data at the given time. 
Therefore, the transformation has an optional parameter:

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• Missing values in the index time series result in missing values in the output time series.

• Index values for which no matching ensemble member is found result in missing values in the output time series

Use case: select a pre-calculated flood map based on a water level forecast

Preparation steps:

• outside of Delft-FEWS: Create (interpolated) flood maps for the required water level granularity (e.g. every 5cm).
  • Ensure the corresponding water level is in the file name of the flood map (e.g. floodmap_xxxxcm.nc)
• Import the flood map library in Delft-FEWS as an ensemble
  • Create ensembleMemberIds based on the file name with the option fileNameEnsembleMemberIndexPatter (see Import Module configuration options#fileNameEnsembleMemberIndexPattern)
  • You can choose to use importType NETCDF-CF_GRID_REFERENCES (see NETCDF-CF_GRID#Importofmetadataand(OpenDAP)gridreferences(since2019.02))

Operational steps:

• Convert water level forecast to index, using a lookup transformation with a coefficientSetFile (see Simple#Coefficientset)
  • In the example the conversion is a simple as converting a water level forecast of 10.05 m to an index of 1005 (cm)
• Select the corresponding flood map with this gridEnsembleMemberByIndex selection transformation.

XML example

<transformation id="ensembleLookup">
      <selection>
         <gridEnsembleMemberByIndex>
            <inputEnsembleIndices>
               <variableId>InputIndex</variableId>
            </inputEnsembleIndices>
            <inputTimeSeriesGrids>
               <variableId>InputGrids</variableId>
            </inputTimeSeriesGrids>
            <output>
               <variableId>Output</variableId>
            </output>
            <matchInputAndOutputGridTimes>false</matchInputAndOutputGridTimes>
         </gridEnsembleMemberByIndex>
      </selection>
   </transformation>
</transformationModule>

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