Changes between Initial Version and Version 1 of markhDataModelNotesCRS


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Timestamp:
04/07/14 10:06:32 (5 years ago)
Author:
markh
Comment:

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  • markhDataModelNotesCRS

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     1[http://kitt.llnl.gov/trac/ticket/107#comment:60 davidhassell]:
     2
     3> Another is that a georeference construct is not, according to some definitions (such as ISO 19123, Geographic information -Schema for coverage geometry and functions) a CRS.
     4
     5ISO19123 is a useful reference point for this discussion.  For the CRS definition, ISO19123 standard references ISO19111:
     6
     7{{{
     85.3.10 Coordinate Reference System
     9
     10The association Coordinate Reference System shall link the CV_Coverage to the coordinate reference system to
     11which the objects in its spatiotemporal domain are referenced. The class SC_CRS is specified in ISO 19111.
     12}}}
     13
     14ISO19111 describes the coordinate reference system:
     15
     16{{{
     17
     186.1 Relationship between coordinates and coordinate reference system
     19
     20In this International Standard, a coordinate is one of n scalar values that define the position of a single point.
     21
     22Coordinates are ambiguous until the system to which those coordinates are related has been fully defined.
     23Without the full specification of the system, coordinates are ambiguous at best and meaningless at worst. A
     24coordinate reference system (CRS) defines the coordinate space such that the coordinate values are
     25unambiguous.
     26}}}
     27
     28This is the functionality which CF-NetCDF grid_mapping variables provide. CF-NetCDF coord/auxcoord variables are a set of values referencing a fully defined system.
     29
     30{{{
     318.2 Coordinate reference system
     32
     338.2.1 General
     34
     35In this International Standard a coordinate reference system shall be defined by one coordinate system and
     36one datum. A datum specifies the relationship of a coordinate system to an object, thus ensuring that the
     37abstract mathematical concept “coordinate system” can be applied to the practical problem of describing
     38positions of features on or near the object's surface by means of coordinates.
     39}}}
     40
     41The CF grid_mapping options defined in Appendix F of the CF conventions relate very closely to this ISO definition.  Each one defines a datum and the system of coordinate names which may be validly used.
     42
     43It appears to me that CF-NetCDF has followed the ISO approach in defining coordinate reference systems, calling them ''grid_mapping'' variables.
     44
     45[http://kitt.llnl.gov/trac/ticket/107#comment:60 davidhassell]:
     46
     47> (An ISO 19123 CRS is actually defined by a subset of the information contained in a georeference construct taken with the coordinate constructs to which it relates.)
     48
     49I do not think this is correct.  The ISO19123 describes the definition of grids:
     50
     51{{{
     528.2 Quadrilateral grid geometry
     53
     54...
     55A grid may be defined in terms of an external coordinate reference system. This requires additional information
     56about the location of the grid’s origin within the external coordinate reference system, the orientation of the grid
     57axes, and a measure of the spacing between the grid lines.
     58
     59}}}
     60
     61The grid is defined by a set of values, quantities with respect to an ISO19111 CRS.  In CF-NetCDF terminology, there are two coordinate variables defined with respect to a grid_mapping variable.
     62
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     64
     65The functionality provided by ''formula terms'' in a CF NetCDF file enables a set of coordinate values to be inferred from other information in the file.  This is a hugely flexible tool, which has so far been used to define parameterised vertical coordinates, common in atmosphere models, but has potentially many further uses.  One, well defined transformation is detailed, enabling a set of coordinate values to be derived following the defined process.
     66
     67The result is a coordinate, just like a coordinate variable or auxiliary coordinate variable.  In many cases, it will be useful to be able to define this derived coordinate with respect to some well specified coordinate reference system; there has been significant recent discussion on this topic on the mailing list.
     68
     69The derived nature is related to the coordinate values.  The spatial referencing is an explicitly separate concept.  I think we will store up a large amount of confusion and complexity for the future if we attempt to conflate these two concepts within the CF data model.
     70
     71If we keep them separate, we may be able to follow the ISO terminology and describe Coordinate Reference System instances within our model in ways which are consistent with the ISO definitions, which I would find particularly useful.
     72
     73