CDM – OPeNDAP Interface

When the CDM reads an OPeNDAP dataset, it makes the following transformations:

OPeNDAP primitive              CDM primitive

* the _Unsigned attribute is set

Constructor Type                     CDM DataType

OPeNDAP Attributes

An OPeNDAP Attribute may be a name/value pair, or may contain other attributes, in which case it is called an attribute container. Containers may be nested. The CDM first matches OPeNDAP attribute containers against Variable names. On a match, the attributes become the Variable's attributes. Anything that doesn't match is made into a global Variable. The CDM does not support nested attributes, so these are flattened by using a "." to separate the container(s) from the name.

Certain OPeNDAP attribute containers are handled separately. An attribute named NC_GLOBAL or HDF_GLOBAL is assumed to contain global attributes. Attribute containers named DODS_EXTRA and EXTRA_DIMENSION are handled as described in the next section.

OPeNDAP Dimensions

OPeNDAP (version 2) does not have explicit shared Dimensions in its object model, except for Map arrays inside of Grids. NetCDF has only shared Dimensions, while the CDM data model allows for both shared and private (non-shared) Dimensions. A shared CDM dimension always has a name, while a private CDM Dimension may or may not be named.

Unnamed OPeNDAP Dimensions are mapped to anonymous (private, unnamed) CDM Dimensions. Named OPeNDAP Dimensions are made into shared CDM Dimensions, in the order they are found in the DDS. If a shared Dimension with the same name already exists, and it has a different length, a named OPeNDAP Dimension is made into a private, named CDM Dimension. Because DAP 2 doesn't have groups, all shared Dimensions will be global. This effectively means that we assume that OPeNDAP Dimensions of the same name and length are the same object.

In order to get the full semantics of NetCDF files transported across the OPeNDAP protocol, we use 2 special global attributes. If there is a global attribute called DODS_EXTRA/Unlimited_Dimension then the value of that attribute is assumed to be a dimension name, which is then set to be an Unlimited dimension. If there is a global attribute called EXTRA_DIMENSION, then it contains one or more (name, length) attributes which are made into shared CDM Dimensions. This allows a server to pass dimensions which are not used in a Variable, but conceivably might be needed by a client. Here are examples of these two cases:

    DODS_EXTRA { String Unlimited_Dimension record; }  

DODS_EXTRA and EXTRA_DIMENSION are ad-hoc Conventions used only by the TDS as far as I know.

OPeNDAP Grids

An OPeNDAP Grid object is decomposed into different Variables for the data array and for each map vector in the Grid. The map vectors are placed at the global level, in the order they are found in the DDS. If a map vector already exists, then it is assumed to be identical with the existing one. Dimensions are currently not checked. For both maps and arrays, the containing grid name is discarded. These assumptions preserve the semantics of NetCDF datasets, but are a violation of the OPeNDAP specification in which Grids constitute a separate namespace. Trying to preserve the namespace makes Netcdf datasets incorrect. This is one of the most important incompatibilities of the NetCDF and OPeNDAP data models, since there is not workaround for DAP 2.

In order to preserve the semantics of the map vectors, the following CDM attribute is added to the data variable:

 _Coordinate.Axes = "map1 map2 map3";

The CDM Coordinate System layer uses this to associate the array and map Variables, as intended by the Grid specification

OPeNDAP Strings

The default case is to map OPeNDAP Strings to CDM Strings. If the Variable has an attribute named DODS/strlen , then the CDM instead uses DataType.CHAR. The value of the DODS/strlen attribute is the length of the innermost dimension. For example, an OPeNDAP String of dimension (5, 7) becomes a char Variable of dimension (5,7,strlen). If the Variable has an attribute named DODS/dimName, then dimName becomes the dimension name of the inner Dimension, otherwise the Dimension is anonymous.

Some older OPeNDAP servers that read NetCDF files map multidimensional char arrays to multidimensional String arrays, with each String being length 1. As a workaround, the CDM reads the data for all String variables from an OPeNDAP dataset when the dataset is opened. If it discovers that the data consists of Strings of length 1, it changes the Variable dataType to DataType.CHAR. In any case, we cache the String data on the client. This workaround is reasonable as long as there isn't huge amounts of String data in the dataset.

OPeNDAP Sequences

An OPeNDAP Sequence is mapped to a one-dimensional CDM Structure of variable length (uses Dimension.VLEN). The CDM now has a Sequence class, but we have not yet converted the CDM opendap code to use it. The semantics of both are the same, the user is only allowed to read the entire sequence (no subsetting). In the general API (ie through the NetcdfFile interface), constraint expressions are not supported . However, a user could break encapsulation and call DODSNetcdfFile.readWithCE() to read data with a relational constraint expression on a Sequence.