Coverage Element Working Draft

Table of Contents

• Definition
• Use of Qualifiers
• Use of Schemes
• Examples
  

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Definition

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 or temporally 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 temporal 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 of latitude) or as text. Temporal information may also be given in numeric form or as text. Numbers are preferred. If scheme is not given, none is assumed. No defaults are assumed.

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Use of Qualifiers

The Coverage element can be defined initially as:

• DC.coverage.periodName
• DC.coverage.placeName
• DC.coverage.y
• DC.coverage.z
• DC.coverage.polygon
• DC.coverage.line
• DC.coverage.3d

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 and/or time. 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 set of 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 "3d" modifier is required where a complex three-dimensional extent is required.  The values of the 3d 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 visualized as the shape taken by a balloon enclosing all the points and allowed to deflate.  Definition of a three-dimensional 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:

• DC.coverage.x.min
• DC.coverage.x.max
• DC.coverage.y.min
• DC.coverage.y.max
• DC.coverage.z.min
• DC.coverage.z.max
• DC.coverage.t.min
• DC.coverage.t.max

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 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 which includes, e.g., 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 relationship between associated  x and y values:

• DC.coverage.x.min.1
• DC.coverage.x.max.1
• DC.coverage.y.min.1
• DC.coverage.y.max.1
• DC.coverage.x.min.2
• DC.coverage.x.max.2
• DC.coverage.y.min.2
• DC.coverage.y.max.2

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 inclusion and exclusion regions should be accounted for in the semantics and syntax. The use of the polygon inclusion and exclusion regions, and of exclusion regions in the use of the "3d" option, is given as:

• DC.coverage.polygon.include
• DC.coverage.polygon.exclude
• DC.coverage.3d.exclude

These elements may be repeated. Using a combination of DC.coverage.polygon and DC.coverage.polygon.exculde elements, a complex coverage may be constructed. Points on the boundary of an included region (polygon, 3d, or max-min pair) are included in the defined coverage. Points on the boundary of an excluded polygon or three-dimensional area are excluded from the defined coverage.  For example:

<meta name="DC.coverage.z" content="47">

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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, and specifically that of latitude and longitude in decimal degrees and altitude in metres,  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. Where the grid system is known, users are encouraged to supply that information.

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. Syntax of examples reflects the html-breaking option, of the two options discussed at DC-4 in Canberra; for other option, see: http://www.ukoln.ac.uk/ariadne/issue8/canberra-metadata/

<meta name="DC.coverage.x" scheme="NAD27.DDM" content="-91.89">

<meta name="DC.coverage.z" scheme="WGS84.DDM" content="47">

<meta name="DC.coverage.y" scheme="UTM10" content="5454818">

<meta name="DC.coverage.x" scheme="UTM10" content="483282">
(The abbreviation "DDM" means that the grid information is given in decimal degree and  metres)

A. Spatial schemes should include, but are not 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. To convert degrees-minutes-seconds to decimal degrees, use converter on Webpage currently maintained by Andrew Daviel: http://vancouver-webpages.com/META/DMS.html

OSGB -- Ordnance Survey of Great Britain -- a local planimentric (x,y) system used in Great Britain with units of measure being metres

UTMXX -- Universal Transverse Mercator where the scheme qualifier XX represents the appropriate UTM zone of measure, with units in metres, in either the northern or southern hemispheres.

2. text:

a. authority-list/thesaurus: examples follow

Library of Congress Subject Headings (LCSH):
    Grand Canyon (Ari
    Vancouver (B.C.)

Thesaurus of Geographic Names (TGN)

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]

Vancouver, British Columbia, Canada

B. Temporal schemes include, but are not limited to:

1. text

• a. authority-list/thesaurus: any situation where authorities in a given field have issued a thesaurus or list of terms; examples of sources follow

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)

• b. free text: examples follow

13th century

ca. 1650

before 1725

between 1912 and 1914

2. numeric: the preferred format is ISO; required for min/max

• a. as defined in ISO8601: yyyy-mm-dd for date, hh:mm:22.22M for time, where M is code for meridian used (e.g., Z indicates Zulu, that is, Greenwich time). One records date and time to the level to which one has that information; for example, if one knows only the year, use YYYY; if one knows year and month, use YYYY-MM; and so forth. For a profile of ISO 8601 date and time formats, see: http://www.w3.org/TR/NOTE-datetime

1985-04-12 (April 12, 1985)

23:20:50.52Z (20 minutes and 50.52 seconds after 23^rdhour, Zulu time)

• b. as defined in ANSI X3.30-1985:

A.D. Era to December 31, 9999 A.D.: YYYYMMDD

and in ANSI X3.43-1986: HHMMSSSS

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Examples

1. Geologic data for Mississippi: > `<meta name= "DC.coverage.x.min" scheme = "DD"

content = "-91.89">`

<meta name= "DC.coverage.x.max" scheme = "DD" content = "-87.85">

<meta name= "DC.coverage.y.min" scheme = "DD" content = "29.94">

<meta name= "DC.coverage.y.max" scheme = "DD" content = "35.25">

<meta name= "DC.coverage.placeName scheme="LCSH" content="Mississippi"

2. The Great Wall, seen from the Space Shuttle:

former URL:http://southport.jpl.nasa.gov/imagemaps/html/srl-greatwall.html

<meta name = "DC.coverage.line" scheme = "DD" content = "33 160 32 161 31 160">

<meta name = "DC.coverage.periodName" scheme = "historic" content = "Ming Dynasty">

<meta name = "DC.coverage.periodName" scheme = "historic" content = "Qin Dynasty">