# U_g2l_obs

The page document the user-supplied call-back routine `U_g2l_obs`

.

The routine `U_g2l_obs`

is a call-back routine that has to be provided by the user. In the simplified interface the predefined name of the routine is `g2l_obs_pdaf`

, but in the full interface, the user can choose the name of the routine.
The routine is used with all filter algorithms using domain-localization (LSEIK, LETKF, LESTKF) and is independent of the particular algorithm.
The routine is called during the loop over the local analysis domains in the analysis step. It has to provide a local observation vector `mstate_l`

for the observation domain that corresponds to the local analysis domain with index `domain_p`

. Provided to the routine is the full observation vector `mstate_f`

from which the local part has to be extracted.

The interface is the following:

SUBROUTINE U_g2l_obs(domain_p, step, dim_obs_f, dim_obs_l, mstate_f, mstate_l)

with

`domain_p`

:`integer, intent(in)`

Index of current local analysis domain`step`

:`integer, intent(in)`

Current time step`dim_obs_f`

:`integer, intent(in)`

Size of full observation vector for model sub-domain`dim_obs_l`

:`integer, intent(in)`

Size of observation vector for local analysis domain`mstate_f`

:`integer, intent(in), dimension(dim_p)`

Full observation vector for model sub-domain`mstate_l`

:`integer, intent(out), dimension(dim_l)`

Observation vector for local analysis domain

Hints:

- The vector
`mstate_f`

that is provided to the routine is one of the observed state vectors that are produced by`U_obs_op_f`

. - Some operations performed here are analogous to those required to initialize a local vector of observations in
`U_init_obs_l`

. If that routine reads first a full vector of observations (e.g. in`U_init_dim_obs_f`

), this vector has to be restricted to the relevant observations for the current local analysis domain. For this operation, one can for example initialize an index array when`U_init_dim_obs_l`

is executed. (Which happens before`U_g2l_obs`

)