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From "Martin Desruisseaux (Jira)" <j...@apache.org>
Subject [jira] [Resolved] (SIS-409) Take in account nested datum shift grids for smaller areas
Date Mon, 17 Feb 2020 11:24:00 GMT

     [ https://issues.apache.org/jira/browse/SIS-409?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Martin Desruisseaux resolved SIS-409.
-------------------------------------
    Resolution: Fixed

Done, but there is three remaining issues:

* The tested point is not the most difficult case. Subgrid in NTv2 file can themselves contains
smaller sub-grids. Such nested sub-grids are supported (in theory), but I haven't yet tested
a point located inside such  "sub-grid in sub-grid" case.

* Performance may be sub-optimal with NTv2 files containing a lot of sub-grids. I used a simplistic
R-Tree for managing those sub-grids, but its performance may not be satisfying. A better R-Tree
implementation would be required. Many exist and there is a rich literature on that topic,
but it would nevertheless require more investigations for integration in SIS because it would
be for wider use in SIS than just NTv2.

* We have an edge case if a point is located very close to the limit between two sub-grids.
For direct transformation this is not a problem. But for inverse transformation, the standard
algorithm is to first estimate the location of the point, then apply corrections iteratively
until the corrections become negligibly small. A problem occur if the first estimation is
on one grid, but then the corrections move that point on the other grid. In current implementation
the code will continue to use data from the first grid, which will result in extrapolations.
Since the corrections are small, the extrapolations should be very close to the grid so the
result would hopefully be not too wrong, but it is nevertheless something we need to improve.


> Take in account nested datum shift grids for smaller areas
> ----------------------------------------------------------
>
>                 Key: SIS-409
>                 URL: https://issues.apache.org/jira/browse/SIS-409
>             Project: Spatial Information Systems
>          Issue Type: Improvement
>          Components: Referencing
>    Affects Versions: 0.8, 1.0
>            Reporter: Martin Desruisseaux
>            Assignee: Martin Desruisseaux
>            Priority: Major
>             Fix For: 1.1
>
>
> Some datum shift grid files begin with a grid over a wide area, followed by smaller grids
over sub-area with a finer resolution. Current Apache SIS implementation use only the grid
in the wide area. We should take in account the grids with finer resolution when applicable.



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