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 "spatial" or "temporal". qualifiers.
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 a scheme is not given, unqualified spatial information is assumed to be decimal degrees of latitude and longitude (East positive, North positive), with altitude in metres.
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 DC.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. The values of the polygon modifier are stored as a single, closed chain of x and y pairs, which may be separated by commas for readability.
The hull modifier is required where a complex 3-dimensional extent is required. The values of the hull modifier are stored as a single chain of x, y, z triples, which may be separated by commas for readability. The region of coverage is defined as the convex hull of this set of points. This region may be visualised as the shape taken by a balloon enclosing all the points and allowed to deflate. Definition of a 3-D volume in this way permits the specification of simple box shapes using a minimum of data points, and is insensitive to point order.
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 eight 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 optional, 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 may be
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.
Self-documenting data strings may be used, though
the standard form is preferred. Examples
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 regions should be
accounted for in the semantics and syntax. The use of the polygon and hull
exclusion regions is given as:
These elements may be repeated. Using a combination of DC.coverage.polygon and DC.coverage.polygon.exclude elements an arbitrarily complex coverage may be constructed.
Points on the boundary of an included region (polygon, hull, or max-min pair) are included in the defined coverage. Points on the boundary of an excluded polygon or hull are excluded from the defined coverage.
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. By default the spatial coverage is that of the Earth using longitude (x) and latitude (y) in degrees, and altitude (z) in metres. Coordinates are assumed to use the WGS84 system; where this is not the case and significant differences exist, the scheme should be explicitly given. Users are encouraged wherever possible to convert data to this common format.
Other planetary bodies, medical data, etc., may be differentiated by specifying a schema. For imaginary places, no schema or latitude/longitude will be given.
Spatial schemes should include, but not be limited to:
1. numeric: examples follow
OSGB -- Ordnance Survey Grid Base -- a local planimentric (x,y) system
used in Great Britain with units of measure being metres
UTMXXN, UTMXXS -- Universal Transverse Mercator where the scheme qualifier XX represents the appropriate UTM zone of measure, with units in metres, in either the North or South hemisphere.
a. authority-list/thesaurus: examples follow
Library of Congress Subject Headings (LCSH):
b. free text: examples follow
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)
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.