= Porting an Existing Implementation to OMI =

{{{
#!html
<div class="wiki-toc">
<h4>PDAF-OMI Guide</h4>
<ol><li><a href="PDAF_OMI_Overview">Overview</a></li>
<li><a href="OMI_Callback_obs_pdafomi">callback_obs_pdafomi.F90</a></li>
<li><a href="OMI_observation_modules">Observation Modules</a></li>
<li><a href="OMI_observation_operators">Observation Operators</a></li>
<li><a href="OMI_debugging">Debugging functionality</a></li>
<li><a href="OMI_ImplementationofAnalysisStep">Implementing the analysis step with OMI</a></li>
<ol>
<li><a href="ImplementAnalysisGlobal">Implementation for Global Filters</a></li>
<li><a href="ImplementAnalysisLocal">Implementation for Local Filters</a></li>
<li><a href="ImplementAnalysislenkfOmi">Implementation for LEnKF</a></li>
</ol>
<li>Porting an existing implemention to OMI</li>
<li><a href="OMI_use_global_obs">Using domain-limited observations</a></li>
</ol>
</div>
}}}

If you have an implementation of the data assimilation with PDAF without OMI, you can port it to OMI with the following steps:

1.	Install the current PDAF version which includes the OMI functionality (V1.16 or later)
1.	In mod_assimilation declare an array `coords_l` for the coordinates of the local analysis domain. Then fill `coords_l` in `init_dim_l_pdaf`. These coordinates are later used in `init_dim_obs_l_pdafomi`. 
1.	Copy the template file `callback_obs_pdafomi.F90` to your code directory.
1.	In `assimilate_pdaf` replace the call(s) to PDAF_put_state_X or PDAF_assimilate_X by the calls to PDAFomi_put_state_local/global or PDAFomi_assimilate_local/global (You can copy this from the template files in templates/)
1.      In `assimilate_pdaf` further replace the declaration of the 'EXTERNAL' subroutines following the template (several of the routines are no longer present with PDAF-OMI, while those that remain now have the suffix `_pdafomi` (like init_dim_obs_pdafomi)
5.	For each observation type implement an obs-module as described in the [wiki:OMI_observation_modules documention on the observation modules]
7.	Add a call to `deallocate_obs_pdafomi()` are the end of your routine `prepoststep`.
8.	Adapt the compilation to also compile `callback_obs_pdafomi.F90` and the obs-modules. 
9.	Modify the compilation such that the include-directory of your PDAF installation is defined as an include directory (i.e. specify -IPDAF_ROOT/include where PDAF_ROOT is the root directory of your PDAF installation.)

'''Notes:'''
- In the classical implementation variant one had to code separate routines for observation handling in global and local filters (e.g. `init_dim_obs` and `init_dim_obs_f_pdaf`. With OMI is it longer necessary to distinguish, as OMI takes case of the different operations in case of global or local filters.
- With OMI we recommend to define the observation errors (`rms_obs_TYPE`) within each observation module instead of using the general module `mod_assimilation`. 
- While it is not done in the template and tutorial codes, one can move the localization parameters (locweight, local_range, srange) into each observation module. This allos to easily define separate localization values for each observation type.