The Coverage element describes the spatial and temporal characteristics of the object or resource and is the key element for supporting spatial or temporal range searching on document-like objects that are spatially referenced or time-referenced. Coverage may be modified by the "type" qualifiers "spatial" and "temporal".
A resource may have both spatial and temporal coverages, or just one of the two, or none. This element may be used in describing resources from many different fields, e.g., archaeology, art, cartography, geography, geographic information systems, medicine, natural sciences, etc. - any field that deals with georeferenced information, spatial data, or time-referenced data. Thus for example, resources describing the Grand Canyon of the United States include text, maps, music (e.g., Ferde Grofe's Grand Canyon Suite), statistics (e.g., number of visitors per year), works of art (such as the panoramas that appear in the 1882 publication, "Atlas to accompany the monograph on the Tertiary history of the Grand Canon district"), etc.; and each could use Coverage - Spatial and in some cases Coverage - Temporal.
Spatial information may be given in numeric form (e.g., degrees) or in text. Temporal information may also be given in numeric form or in text. Numbers are preferred. If scheme is not given, none is used. No defaults are assumed.
Use of Qualifiers:
The Coverage element can be defined initially as:
where the placeName and periodName are qualifiers for textual representation, and the remainder are for coordinate-based classification. The coordinates used are qualified by the scheme modifier to support different coordinate systems (see below).
The coverage.x, .y, .z, .t options handle the dimensionality of spatial and temporal extent, and - if they are not further qualified - represent a point in space or time or both. The polygon and line modifiers are required where a complex 2-dimensional extent is required -- the polygon in the case of areal extent or "footprint," and the line in the case of a flight path associated with a suite or aerial photographs, for example. Inheritance may be extended such that the x,y,z,t qualifiers reference beginning and ending points in space and time to "bound" a coverage:
With these six properties, a document can be classified as to its rough geographic extent with a beginning and ending time of coverage.
To accommodate both spatial and temporal discontinuities, and as per Dublin-Core general policy that all fields are repeatable and option, all these elements may be repeated. It is conceivable that some data sets may cover multiple, non-contiguous geographic footprints (e.g., the U.S. Exclusive Economic Zone including Hawaii, Puerto Rico, Guam, and American Samoa).
Therefore for x, y, z, and t, a numeric grouping subelement is added
at the end to keep the correct x with the correct y:
where groupings 1 and 2 describe two distinct bounding rectangles of
coverage that may be discontinuous. The time dimension is handled in the
same way. Spatially, footprints can also exhibit what is known as the "swiss
cheese" effect, where there is general overall coverage but there are islands
where there are no data. These exclusion and inclusion regions should be
accounted for in the semantics and syntax. The use of the polygon inclusion
and exclusion regions is given as:
where the values of this qualifier are stored as a single, closed chain of x and y pairs. These inclusion and exclusion regions can be linked with multiple exclusions associated with a given inclusion area, if needed, using the .number syntax referenced above.
Use of Schemes:
This Coverage proposal includes the possibility for the use of multiple classification schemes to further qualify the incoming information. Latitude and longitude must occur in pairs; the schema used must be able to deal with points, lines, and polygons - bounding rectangles and points being the most frequently used. The use of an explicit scheme is encouraged, as there are a large number of "native" coordinate representations that may be easy to encode but difficult for the classifier to translate.
Other planetary bodies, medical data, etc., may be differentiated by specifying a schema. For imaginary places, no schema or latitude/longitude will be given.
Apart from specifying how the data is expressed (e.g., "DMS" for degrees-minutes-seconds;
"DD" for decimal degrees; "text" for text), the geodetic datum (e.g., WGS84,
NAD83, NAD27 CONUS, etc.) should be given when it is known. Examples follow:
Spatial schemes should include, but not be limited to:
1. numeric: examples follow
DMS: Degrees-Minutes-Seconds, in DDD-MM-SSX format, where D=Degrees, M=Minutes, S=Seconds, and X=N or S for North or South latitude, respectively, and E or W for East or West longitude.
DD: Decimal-Degrees, in DD.XXXX format, where D=Degrees and the XXXX represent decimal portions of a degree. This is preceded by a minus sign (-) if the latitude is S or the longitude is W.
Self-documenting data strings should be used whenever possible. Examples
a. authority-list/thesaurus: examples follow
Library of Congress Subject Headings (LCSH): Grand Canyon (Ariz.)
b. free text: examples follow
Grand Canyon, Arizona
13 Lincoln's Inn Fields, London, England [address of Sir John Soane's house]
Grand Canal, Venice [ location shown in Canaletto's view of the Grand Canal]
Temporal schemes include:
Library of Congress Subject Headings
Art & Architecture Thesaurus Styles and Period Hierarchy (bounded periods defined by art historians):
Lexicon of stratigraphic nomenclature (names of geologic formations)
between 1912 and 1914
2. numeric: must include option of providing both beginning and ending dates, e.g.:
1985-04-12 (April 12, 1985)
23:20:50.52Z (20 minutes and 50.52 seconds after 23rd hour,
A.D. Era to December 31, 9999 A.D.: YYYYMMDD
and in ANSI X3.43-1986: HHMMSSSS
1. Geologic data for Mississippi:
Content standard for digital geospatial data.
Date and time on the Internet.
Dublin Core metadata element set: reference description.
Great GIS net sites! Index, GIS WWW resources.
List of formats and standards for spatial data transfer.
RFC 1876 resources.