Information on Metadata

Metadata Definition:

-- Metadata describe the content, quality, condition, and other characteristics of data. Metadata help a person to locate and understand data. Metadata include information needed to determine the sets of data that exist for a geographic location, information needed to determine if a set of data meets a specific need, information needed to acquire an identified set of data, and information needed to process and use a set of data. The exact order in which data elements are evaluated, and the relative importance of data elements, will not be the same for all users.

Objectives of the Standard:

The objectives of the standard are to provide a common set of terminology and definitions for the documentation of digital geospatial data. The standard establishes the names of data elements and compound elements (groups of data elements) to be used for these purposes, the definitions of these compound elements and data elements, and information about the values that are to be provided for the data elements.

The information included in the standard was selected based on four roles that metadata play:

These roles form a continuum in which a user cascades through a pyramid of choices to determine what data are available, to evaluate the fitness of the data for use, to access the data, and to transfer and process the data. The exact order in which data elements are evaluated, and the relative importance of data elements, will not be the same for all users.

Major uses of metadata include:

Metadata help insure an organization's investment in data. As personnel change or time passes, information about an organization's data will be lost and the data may lose their value. Later workers may have little understanding of the content and uses for a digital data base and may find that they can't trust results generated from these data. Such descriptions also may provide some protection for the producing organization if conflicts arise over the misuse of data.

  • to provide information to data catalogs and clearinghouses.
  • Applications of geographic information systems often require many themes of data. Few organizations can afford to create all data they need. Often data created by an organization also may be useful to others. By making metadata available through data catalogs and clearinghouses, organizations can find data to use, partners to share data collection and maintenance efforts, and customers for their data. The FGDC is sponsoring the development of the National Geospatial Data Clearinghouse through which data producers can provide metadata to others using the Internet.

  • to provide information needed to process and interpret data to be received through a data transfer from an external source.
  • Metadata should accompany the transfer of a data set. The metadata will aid the organization receiving the data to process and interpret data, incorporate data into its holdings, and update internal catalogs describing its data holdings.

    Numbered Sections:

    The standard is organized in a hierarchy of data elements and compound elements that define the information content for metadata to document a set of digital geospatial data. The starting point is "metadata" (section 0). The compound element "metadata" is composed of other compound elements representing different concepts about the data set. Each of these compound elements has a numbered section in the standard. In each numbered section, these compound elements are defined by other compound elements and data elements. The sections "contact information", "time period information", and "citation information" are special sections that specify the data elements for contacting individuals and organizations, time period definitions, and citing data sets or documents. These sections are used by other sections, and are defined once for convenience.

    Section and element numbers are provided for user navigation of the standard. They are neither authoritative nor intended for use in implementation and are subject to change in future revisions of the standard.

    Compound Elements:

    A compound element is a group of data elements and other compound elements. All compound elements are described by data elements, either directly or through intermediate compound elements. Compound elements represent higher-level concepts that cannot be represented by individual data elements. The form for the definition of compound elements is:
    Compound element name -- definition.
    Type: compound
    Short Name:
    The type of "compound" uniquely identifies the compound elements in the lists of terms and definitions. Compound elements are represented by "flat" boxes on the image map graphics.

    Short names consisting of eight alphabetic characters or less are included to assist in implementation of the standard.

    Data Elements:

    A data element is a logically primitive item of data. The entry for a data element includes the name of the data element, the definition of the the data element, a description of the values that can be assigned to the data element, and a short name for the data element. The form for the definition of the data element is:
    Data element name -- definition.
    Type:
    Domain:
    Short Name:
    The information about the values for the data elements include a description of the type of the value, and a description of the domain of the valid values. The type of the data element describes the kind of value to be provided. The choices are "integer" for integer numbers, "real" for real numbers, "text" for ASCII characters, "date" for day of the year, and "time" for time of day.

    The domain describes valid values that can be assigned to the data element. The domain may specify a list of valid values, references to lists of valid values, or restrictions on the range of values that can be assigned to a data element.

    The domain also may note that the domain is free from restrictions, and any values that can be represented by the "type" of the data element can be assigned. These unrestricted domains are represented by the use of the word "free" followed by the type fo the data element (that is, free text, free date, free real, free time, free integer). Some domains can be partly, but not completely, specified. For example, there are several widely used data transfer formats, but there may be many more that are less well known. To allow a producer to describe its data in these circumstances, the convention of providing a list of values followed by the designation of a "free" domain was used. In these cases, assignments of values shall be made from the provided domain when possible. When not possible, providers may create and assign their own value. A created value shall not redefine a value provided by the standard.

    Data elements are represented by "3-dimensional" boxes on the image map graphics.

    Short names consisting of eight alphabetic characters or less are included to assist in user implementation of the standard.

    Another issue is the representation of null values (representing such concepts a "unknown") in the domain. While this is relatively simple for textual entries (one would enter the text "Unknown"), it is not as simple for the integer, real, date, and time types. (For example, which integer value means "unknown"?). Because conventions for providing this information vary among implementations, the standard specifies what concepts shall be represented, but does not mandate a means for representing them.

    In addition to the values to be represented, the form of representation also is important, especially to applications that will manipulate the data elements. The following conventions for forms of values for data elements shall be used:

    Calendar Dates (Years, Months, and Days)

    Time of Day (Hours, Minutes, and Seconds) Latitude and Longitude Network Addresses and File Names

    Optionality

    The standard categorizes elements as being mandatory, mandatory-if-applicable, or optional as follows: The optionality of a section or compound element always takes precedence over the elements that it contains. Once a section or compound element is recognized by the data set producer as applicable, then the optionality of its supordinate elements is to be interpreted.

    Mandatory sections in the standard have some elements that are always required for all types of geospatial data sets. For comparison with other metadata standards, these elements are referred to as "core" elements.

    Extensibility

    Extended elements may be defined by a data set producer or a user community. Extended elements are elements outside the standard, but needed by the data set producer. If extended elements are created, they must follow the guidelines in Content Standard for Digital Geospatial Metadata: Appendix D, Guidelines for creating extended elements to the Content Standard for Digital Geospatial Metadata.

    Return to: Metadata An Image Map of the FGDC's Content Standard for Digital Geospatial Metadata