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  1. Spatial Information Systems
  2. SIS-502

Allow users to define CRS for custom codes

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Details

    • New Feature
    • Status: Resolved
    • Major
    • Resolution: Fixed
    • 1.0
    • 1.1
    • Referencing
    • None

    Description

      Provide a mechanism for defining easily (without interacting with a database) a few CRS definitions for codes other than EPSG. For example ESRI also provides their own codes and the International Astronomical Union (IAU) makes some experiments.

      The easiest way is to provide CRS definitions in Well-Know Text (WKT) format, which is an OGC/ISO standard. But the WKT format provides a standard way to define a single CRS, while we need to define many CRS associated to codes. I'm not aware of a standard for such registry. Other projects like GDAL and GeoTools use key-value pairs with one entry per line. Example with ESRI codes:

      9248=GEOGCS["Tapi_Aike",DATUM["Tapi_Aike",SPHEROID["International_1924",6378388.0,297.0]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433],AUTHORITY["Esri",9248]]
      9251=GEOGCS["MMN",DATUM["Ministerio_de_Marina_Norte",SPHEROID["International_1924",6378388.0,297.0]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433],AUTHORITY["Esri",9251]]
      

      (Note: above example uses legacy WKT 1 because ESRI file defines CRS that way, but this proposal does not depend on WKT version)

      However above format does not said which authority to associate the 9248 and 9251 keys (ESRI in above example). In order to know the authority, we have to look at the end of WKT strings for the AUTHORITY["Esri",…] elements. But if we have to look for AUTHORITY[…] (WKT 1) or ID[…] (WKT 2) elements anyway, then we do not need the keys at all because they are redundant with AUTHORITY/ID[…] elements. Dropping the keys simplify the file format and remove one possible source of inconsistencies.

      Another issue is that above format puts each WKT definitions on a single line, which is difficult to read. For making the file more human-readable, I propose to allow WKT definitions to span many lines provided that each additional lines is indented relative to the first line. With those two proposed changes, above example would become:

      GEOGCS["Tapi_Aike",
        DATUM["Tapi_Aike",
          SPHEROID["International_1924",6378388.0,297.0]],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.0174532925199433],
        AUTHORITY["Esri",9248]]
      
      GEOGCS["MMN",
        DATUM["Ministerio_de_Marina_Norte",
          SPHEROID["International_1924",6378388.0,297.0]],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.0174532925199433],
        AUTHORITY["Esri",9251]]
      

      i.e. we just put standard WKT definitions one after the other (without "<key>=" prefix) indented in a human-readable way. An inconvenient is that the AUTHORITY[…] or ID[…] elements, which are optional in WKT standard, become mandatory in this format.

      Aliases for WKT fragments

      Files with more than one WKT definition tend to repeat the same WKT fragments many times, e.g. the same BaseGeogCRS[…] element may be repeated in every ProjectedCRS definitions. I propose to allow redundant fragments to be replaced by aliases for making the file more compact, easier to read, faster to parse and with smaller memory footprint. The syntax would be the same than environment variables in Unix shell: each line starting with "SET <identifier>=<WKT>" defines an alias for a fragment of WKT string. The WKT can span many lines as described above. Aliases can be expanded in other WKT strings by "$<identifier>". So above example could become (if user wishes):

      SET Int1924 = SPHEROID["International_1924",6378388.0,297.0]
      SET Greenwich = PRIMEM["Greenwich",0.0]
      SET Degree = UNIT["Degree",0.0174532925199433]
      
      GEOGCS["Tapi_Aike",
        DATUM["Tapi_Aike", $Int1924],
        $Greenwich, $Degree,
        AUTHORITY["Esri",9248]]
      
      GEOGCS["MMN",
        DATUM["Ministerio_de_Marina_Norte", $Int1924],
        $Greenwich, $Degree,
        AUTHORITY["Esri",9251]]
      

      The possibility to define fragments is already a feature of our WKT parser.

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            desruisseaux Martin Desruisseaux
            desruisseaux Martin Desruisseaux
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              Updated:
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