[gdal-dev] Fwd: [MetaCRS] Common SQLite-based dictionaries

[Even Rouault]

Forwarding to a few lists that might be interested in the below proposal as 
metacrs is perhaps not widely followed. But please keep the discussion to 
metacrs to avoid cross postings.

----------  Forwarded Message  ----------

Subject: [MetaCRS] Common SQLite-based dictionaries
Date: Sunday 02 August 2015, 08:15:12
From: Howard Butler <howard at hobu.co>
To: metacrs at lists.osgeo.org

All,

libgeotiff and GDAL all use a system of bespoke CSV files for their coordinate 
system dictionaries. proj.4 uses derived dictionaries made from GDAL's. Each 
is a slightly different subset and/or mix of the EPSG db along with other 
catalogs, customizations, and overrides. The situation is messy, fragile, and 
incomplete, especially for folks like me, who are interested in support of 
ever more complex systems of horizontal and vertical datums, time epochs 
associated with them, and direct transformation.

There have been multiple attempts to build a C tribe API that handles the 
coordinate system description problem, but all have failed for various 
reasons.The one true library to rule them all is probably a pipe dream, but 
maybe it is possible to collaborate in a slightly messier way -- at the 
dictionary level.

One significant technology that was not widely available when GDAL, proj.4, and 
libgeotiff all originated is SQLite. The idea of a single file, sql'able 
database is a standard assumption in today's software, especially in things 
like HTML5 (wars between WebSQL and IndexedDB), just about every significant 
phone application, and your favorite OGC super format [1]. 

I'd like to propose an attempt to standardize the GDAL, proj.4, and libgeotiff 
SRS coordinate system handling dictionaries on a SQLite database that starts 
with EPSG, with each project adding its own auxiliary tables as necessary. I 
am writing this message to MetaCRS to see if there is support for such an 
effort, and to determine if there are other related projects who would like to 
collaborate at this level. 

For the GDAL stack, the benefits of this approach are significant. Multiply-
defined, potentially conflicting definitions no longer need to be resolved. The 
dictionaries could release on their own schedule, rather than with each 
individual project. Powerful new functionality would be much closer to 
software developers instead of hidden behind a rather opaque and fragile CSV 
dictionary generating process. Mundane but important details like 
multithreaded access get handed off to a library and project who do that stuff 
all the time instead of one-off implementations inside of each individual 
project. 

Database views/queries could be standardized for common lookups across. 
Lookups would be faster due to indexed query access. Transformation 
validation, based on EPSG or other databases, could be provided across all 
three projects. More complex topics, like those described above, could be 
developed in a way that have impact across all three projects without tedious 
implementation. 

Consider this email a mix of 

1) is this a good idea? What other benefits do you see this approach providing?
2) Does your project want to collaborate on this?
3) Does this belong in MetaCRS?
4) What are the pitfalls that make this untenable?

windmill-tilting'ly yours,

Howard

[1] http://www.geopackage.org/
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