Database Reference
In-Depth Information
Given all this variety in representation formats, described resources, ap-
proaches for metadata capture, storage, use, and so forth, there is not a com-
monly agreed taxonomy of types of metadata or types of described resources,
but there are different points of view about how metadata can be generated
and used. We will now go through some of these points of view, illustrating
them with examples.
One of the properties of metadata is that it can be organized in layers; that
is, metadata can refer to raw data (e.g., coming from an instrument or being
available in a database), to information about the process of obtaining the raw
data, or to derived data products. This allows distinguishing different layers
(or chains) of metadata: primary, secondary, tertiary, and so forth. As an ex-
ample, let us consider an application in the satellite imaging domain, such as
the one described in Sanchez-Gestido et al.
2
Raw data coming from satellites
(e.g., images taken by instruments in the satellite) are sent to the ground sta-
tions so that they can be stored and processed. A wide range of metadata can
be associated with these data, such as the times when they were obtained and
transferred, the instrument used for capturing them, the time period when the
image was taken, the position to which it refers, and the like. This is consid-
ered as the primary metadata of the images received. Later on, this metadata
can be used to check whether all the images that were supposed to be obtained
from an instrument in a period of time have actually been obtained or whether
there are any gaps, and new metadata can be generated regarding the group-
ing of pieces of metadata for an instrument, the quality of the results obtained
for that time period, statistical summaries, and so forth. This is considered as
secondary metadata, because it does not refer to the raw data being described
but to the metadata that refer to the analysis, summaries, and observations
about the raw data, so that it forms a set of layers or a chain of metadata
descriptions. Another common example of this organization of metadata into
layers is that of provenance, which is described in the next section.
In all these cases, it is important to determine which type (layer) of meta-
data we use for searching, querying, and so forth, and which type of metadata
we show to users, so that metadata coming from different layers is not merged
together and is shown with the appropriate level of detail, as discussed in
Hunter.
3
The organization of metadata into layers also reflects an interesting charac-
teristic of how metadata is used. To some extent, what is considered metadata
for one application may be considered data for another. In the previous ex-
ample in the satellite domain, metadata about the positions of images on
the Earth is considered as part of the primary metadata that is captured and
stored for the satellite mission application when the information arrives to the
ground station. However, the same spatial information would be considered
as a data source for other applications, such as a map visualization service
(e.g., Google Earth) that positions those resources in a map. In contrast, the
dates when the images were taken or the instruments with which they were
produced may still be considered as metadata in both cases.
