This section contains the information on the thematic attributes associated with the dataset. According to the CEN this section if optional, that is, although data may be available it is not obligatory to document them. The fields in bold are only obligatory when required by the section or subsection to which they belong. The contents reflect the specifications of the CEN (ENV 12657) but mostly adopt the guidelines of the Entity and Attribute Information chapter of the American (CSDGM) metadata standard and also bear in mind MD_ContentInformation section of the ISO metadata standard (optional section).
Firstly this section includes some metadata that describe in general form the properties of the thematic attributes of the database, like for example the version of the relations between the tables, the date of the content or the update frequency of the database content. It also includes information about the properties of the databases associated with the cartographic database (containing the attributes of the graphical objects) as well as the field properties and the links that these databases may have with other databases.
Most of the information in this section has a tree structure and defines in a nested form the properties of the tables and fields that have a relationship with the cartographic database.
- Structured vectors
For the case of structured vectorial databases there is first a principal table (the table that uniquely relates the graphical objects to the attribute database). The properties of this table and all its fields are defined. It is possible that some fields may be linked to one or more tables that must also have defined properties and fields (at least defined in distributed REL files).
- Rasters and unstructured vectors
The thematic attributes are the assigned values of the raster or each vector object. In each case the attributes of the objects are found within the graphics file and not in an external table. The characteristics of these values and attributes can be explained in the metadata (descriptive name, variable processing, errors, etc).
Each image (band) or unstructured vector is assimilated into the field of a database that would contain a value for each object (pixels in a raster, points, lines or polygons in an unstructured vector). For this reason the characteristics of the rasters and unstructured vectors (data format, quality,...) are found in this tag (Thematic information).
This approach permits the use of a single metadata file for a multi-band image (or multi-band layer) which can be thought of as a database with as many fields as there are bands in the image. It is important to note that in order to be able to create a documentation file of a multi-band image all the bands must have the same Spatial Reference System.
In this context, the information on each of the bands will be the field information in the database, whilst the information about the multiband image (a set of bands) will be the database table. The parallel between the two attribute structures is high but not perfect. For example, many of the field properties of the vectorial database should not be defined in the metadata since many of their properties are defined in the header of the database file. For a band, however, it is necessary to define all these properties.
- Considerations about which concepts to include in the general sections and which to include in the specific sections
For the case of multiband images it is often possible to document, for some metadata entries, a different value for each of the bands to that of the general value (for the whole image). For example, all the bands of the multiband image of the Landsat-7 sensor cover the same geographic area except for the panchromatic band which is slightly shifted relative to the rest by the LPGS process. The multiband value corresponds to the modal value of the bands, whilst the value for the panchromatic band will be a particular value for the metadata refering to that extent. This means that in different tags the metadata appear as radial buttons that allow the display, for each metadata entry, of either the modal data (of the multiband image) or the data for the selected band (from the buttons or the pull-down menu of the main GeM+ dialogue box). Antother example that sometimes presents different values to the modal value is the case of numbers of rows and columns.
Given that for the case of multi-band images there will typically be significant coincidence between many of the attributes of the fields or bands (the different bands are usually of the same type, use the same NODATA value, etc), some keys are defined with the aim of serving for all the fields or bands, with the possibility that they are repeated for each band if required in order to specify exceptions (different types of fields, etc). The REL file will always contain the general part (for example, it will always contain the general horizontal extent, or the most frequent number of columns in a multi-band image) and the sections related to each field can contain, if needed, the exception to the general section value.
The metadata entries that allow particular values to be documented for some bands are:
- Disk location, Alternative Location, Alphanumeric database, Alternative alphanumeric database, Columns, Rows and Number of cells (in the Thematic aspects - Presentation section). For example, with Landsat-7 images the majority of the spectral bands (30m resolution) have the same number of columns and rows but band 8 (panchromatic, 15m) has twice as many.
- Resolution (in X and Y), Cell size (in X and Y), Equivalent scale and the parameters correspondng to the quality of the horizontal reference system of the data (in the Spacial Reference System sections)
- The parameters corresponding to the horizontal extension (in the Extension section)
- All the entries refering to the properties of the bands like, for instance: Data type and Compressed file, Treatment of the variable, units, Show units, NODATA value and NODATA description, and others (in the Thematic Information Section). In fact, these entries belong to the properties of each band, so that it makes no sense to consider a single value for the multiband image since this concept does not apply to all the images in the band but rather to the individual bands. The documented image value (the modal value) is only used to save disk space when saving metadata, but for the user these values have no greater meaning than simply as a way of knowing the most common value of the entry (which sometimes may not be a single value and may not even exist!)
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