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:

availability -- data needed to determine the sets of data that exist for a geographic location.
fitness for use -- data needed to determine if a set of data meets a specific need.
access -- data needed to acquire an identified set of data.
transfer -- data needed to process and use a set of data.

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:

to organize and maintain an organization's investment in data.

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)

A.D. Era to December 31, 9999 A.D. -- Values for day and month of year, and for years, shall follow the calendar date convention (general forms of YYYY for years; yyyyMM for month of a year (with month being expressed as an integer), and YYYYMMDD for a day of the year) specified in American National Standards Institute, 1986, Representation for calendar date and ordinal date for information interchange (ANSI X3.30-1985): New York, American National Standards Institute (adopted as Federal Information Processing Standard 4-1).
B.C. Era to 9999 B.C. -- Values for day an dmonth of year, and for years, shall follow the calendar date convention, preceded by the lower case letters "bc" (general forms of bcYYYY for years; bcYYYYMM for month of a year (with month being expressed as an integer), and bcYYYYMMDD for a day of the year).
B.C. Era before 9999 B.C. -- Values for the year shall consist of as many numeric characters as are needed to represent the number of the year B.C., preceded by the lower case letter "cc" (general form of ccYYYYYYY...).
A.D. Era after 9999 A.D. -- Values for the year shall consist of as many numeric characters as are needed to represent the number of the year A.D., preceded by the lower case letter "cd" (general form of cdYYYYYYY...).
Time of Day (Hours, Minutes, and Seconds)

Because some geospatial data and related applications are sensitive to time of day information, three conventions are permitted. Only one convention shall be used for metadata for a data set. The conventions are:

Local Time. For producers who wish to record time in local time, values shall follow the 24-hour timekeeping system for local time of day in the hours, minutes, seconds, and decimal fractions of a second (to the precision desired) without separators convention (general form of HHMMSSSS) specified in American National Standards Institute, 1986, Representations of local time of day for information interchange (ANSI X3.43-1986): New York, American National Standards Institute (adopted as Federal Information Processing Standard 58-1).
Local Time with Time Differential Factor. For producers who wish to record time in local time and the relationship to Universal Time (Greenwich Mean Time), values shall follow the 24-hour timekeeping system for local time of day in hours, minutes, seconds, and decimal fractions of a second (to the resolution desired) without separators convention. This value shall be followed, without separators, by the time differential factor. The time differential factor expresses the difference in hours and minutes between local time and Universal Time. It is represented by a four-digit number preceded by a plus sign (+) or minus sign (-), indicating the hours and minutes the local time is ahead of or behind Universal Time, respectively. The general form is HHMMSSSSshhmm, where HHMMSSSS is the local time using 24-hour timekeeping (expressed to the precision desired), 's' is the plus or minus sign for the time differential factor, and hhmm is the time differential factor. (This option allows producers to record local time and time zone information. For example, Eastern Standard Time has a time differential factor of -0500, Central Standard Time has a time differential factor of -0600, Eastern Daylight Time has a time differential factor of -0400, and Central Daylight Time has a time differential factor of -0500.) This option is specified in American National Standards Institute, 1975, Representations of universal time, local time differentials, and United States time zone reference for information interchange (ANSI X3.51-1975): New York, American National Standards Institute (adopted as Federal Information Processing Standard 59).
Universal Time (Greenwich Mean Time). For producers who wish to record time in Universal Time (Greenwich Mean Time), values shall follow the 24-hour timekeeping system for Universal Time of day in hours, minutes, seconds, and decimal fractions of a second (expressed to the precision desired) without separators convention, with the upper case letter "Z" directly following the low-order (or extreme right hand) time element of the 24-hour clock time expression. The general form is HHMMSSSSZ where HHMMSSSS is Universal Time using 24-hour timekeeping, and Z is the letter "Z". This option is specified in American National Standards Institute, 1975, Representations of universal time, local time differentials, and United States time zone reference for information interchange (ANSI X3.51-1975): New York, American National Standards Institute (adopted as Federal Information Processing Standard 59).

Latitude and Longitude

Values for latitude and longitude shall be expressed as decimal fractions of degrees. Whole degrees of latitude shall be represented by a two-digit decimal number ranging from 0 through 90. Whole degrees of longitude shall be represented by a three-digit decimal number ranging from 0 through 180. When a decimal fraction of a degree is specified, it shall be separated from the whole number of degrees by a decimal point. Decimal fractions of a degree may be expressed to the precision desired.

Latitudes north of the equator shall be specified by a plus sign (+), or by the absence of a minus sign (-), preceding the two digits designating degrees. Latitudes south of the Equator shall be designated by a minus sign (-) preceding the two digits designating degrees. A point on the Equator shall be assigned to the Northern Hemisphere.
Longitudes east of the prime meridian shall be specified by a plus sign (+), or by the absence of a minus sign (-), preceding the three digits designating degrees of longitude. Longitudes west of the meridian shall be designated by minus sign (-) preceding the three digits designating degrees. A point on the prime meridian shall be assigned to the Eastern Hemisphere. A point on the 180th meridian shall be assigned to the Western Hemisphere. One exception to this last convention is permitted. For the special condition of describing a band of latitude around the earth, the East Bounding Coordinate data element shall be assigned the value +180 (180) degrees.
Any spatial address with a latitude of +90(90) or -90 degrees will specify the position at the North or South Pole, respectively. The component for longitude may have any legal value.
With the exception of the special condition described above, this form is specified in Department of Commerce, 1986, Representation of geographic point locations for information interchange (Federal Information Processing Standard 80-1): Washington, Department of Commerce, National Institute of Standards and Technology.
Network Addresses and File Names

Values for file names, network addresses for computer systems, and related services should follow the Uniform Resource Locator convention of the Internet when possible. See http://www.ncsa.uiuc.edu/demoweb/url-primer.html for additional details about the Uniform Resource Locator.

Optionality

The standard categorizes elements as being mandatory, mandatory-if-applicable, or optional as follows:

Mandatory elements must be provided.
Mandatory-if-applicable elements must be provided if the data set exhibits the defined characteristic.
Optional elements are provided at the discretion of the metadata producer.
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.

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