DAP Design: shared dimensions, groups and types

Back: DAP3/4#NC-DAP

Document Organization

In the DDX all items are declarations which describe things actually defined (assigned or otherwise associated with values) elsewhere. The term Type Definitions refers to the representation of something in a data set which defines a data type; the types are defined in the data set and the DDX merely holds a representation of that definition - a declaration.

The DDX will be modified so that it contains one or more Groups. If only one Group is present (which describes the case for DAP 3.2 and earlier) then the declaration can be left out, but if there are two or more groups, the declarations must be present.

Group characteristics:

• Any configuration of Groups other than one (anonymous) Group which holds all the variables in a data set must be declared.
• If declared, Groups must be named.
• A Group can contain any type, including a Group
• Variables and Attributes are named using / <group name> / ... / <variable name> to reflect their hierarchy.
• Each Group declares a new lexical scope for values.
• A Group cannot be an Array or a Grid (although the distinction between those two might become blurred or non-existent; Group is fundamentally a scalar container-type).
• This definition does not completely subsume the HDF5 Group type but is equivalent to the netCDF 4 version of it.

Examples:

This data set contains one Group - the root group - which has by convention the name '/'

<Dataset ... >
    ...
</Dataset>

This data set contains two Groups, one after the other.

<Dataset ... >
<group name="primary">
    ...
</group>
<group name="secondary">
    ...
</group>
</Dataset>

This data set contains more Groups, and shows they can be nested.

<Dataset ... >
<group name="primary">
    ...
    <group name="in_situ">
        ...
    </group>

</group>
<group name="secondary">
    ...
</group>
</Dataset>

Shared Dimensions

Shared dimensions will be added to DAP so that special variables used to identify the independent parameters in non-scalar variables can be clearly labeled as such. When a dimension is named, each use of that dimension means the same values. Each dimension will have both a name and a size in addition to a type. The moniker Shared Dimension is really redundant. Any Dimension in DAP 3.3 can be shared. There is no requirement that a dimension be used; it can be declared and never used.

Dimension examples

Declaring dimensions in the DDX:

<Dataset name="dimension_ex_1" ...>
    <!-- The 'dimensions' section must come first if present -->
    <dimensions>
        <!-- Dimensions are declared like an array except that the 
             'dimension' element is replaced by an 'extent' element
             and the extent has no name (since the dimension itself
             is named -->
        <Int32 name="latitude">
            <extent size="1024">
        </Int32>
        <String name="color">
            <extent size="3">
        </String>
    </dimensions>

    <!-- remainder of the document -->

</Dataset>

Using dimensions in the DDX. In DAP, dimensions can only be used in a Grid variable;

<Dataset name="dimension_ex_1" ...>
    <dimensions>
        <Int32 name="latitude">
            <dimension size="1024">
        </Int32>
        <Int32 name="longitude">
            <dimension size="1024">
        </Int32>
    </dimensions>

    <!-- The two declarations that follow are effectively Grids,
         but now share the same dimentions -->
    <Byte name="uwnd">
        <!-- note that only the name is used, not the size -->
        <map name="latitude">
        <map name="longitude">
    </Byte>

    <Byte name="vwnd">
        <map name="latitude">
        <map name="longitude">
    </Byte>

    ...

</Dataset>