Data Element Forms for Special Values

The form of the representation of data values is important to applications that will manipulate the data elements. If the values are provided in a standard way, "client" software can help a user to evaluate data. The CSDGM standard specify conventions to describe data elements for the following four types of values:

• Calendar dates
  
• Time of day
  
• Latitude and longitude
  
• Network addresses and file names
  

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 data 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 9999B.C. - - Values for day and month of year, and for years, shall follow the calendar data 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).
  
  
• 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 letters "cd" (general form of cdYYYYYYY....).
  
  
  

Time of Day (Hours, Minutes, and Seconds)

Because of 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.
  
  
• 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 facto 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 facto 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.
  
  
• 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 Zis 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.
  
  
  

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 tot he 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 a minus sigh (-), 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 of +180 (180) degrees.
  
  
• Any spatial address with a latitude of +90 (90) or -90 degrees will specify the position at the North of 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 American National Standards Institute, 1986, Representation of Geographic Point Locations for Information Interchange (ANSI X3.61-1986): New York, American National Standards Institute.
  
  
  

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.
  
  
  

FAQ: What is the Uniform Resource Locator (URL), and what is its form?

A URL is a network extension of the UNIX file naming conventions. A URL can port to any file in any directory of any machine. It also describes the Internet service available for use with the resource. The basic form of the URL is: service://hostname:port/path/filename where service - the name of the service to be used with the resource. Examples include "Field", "ftp", "telnet", "gopher". "wais", "news", "http", "rlogin", and "tn3270". hostname:port - the Internet address of the machine and the port through which the service is provided. The identification of the port is required only if a non-standard implementation is being used. path - the directory path to the file. filename - the name of the file.

FAQ: I'm planning to implement the metadata standard using a data base, but the data base software does not support the forms of the date and time special values. How do I comply with the standard?

Software systems often use special methods internally to encode values such as date and time. The main concern is that metadata provide to others, especially metadata provided through the National Geospatial Data Clearinghouse and for data transfer, be encoded using the forms specified by the metadata standards.

Examples:

Anonymous FTP: ftp://fgdc.er.usgs.gov/gdc/metadata/meta.6984.ps

World Wide Web: http://www.rpi.edu/Internet/Guides/decemj/icmc/top.html

Gopher: gopher://bingsuns.cc.binghamton.edu/11/acad/geogenv

Telnet: telnet://fedworld.doc.gov

News: news:comp.infosystems.gis

Local file on your (DOS) computer: file:///c:/wp51/mad/letter.wpd