DCMI Box Encoding Scheme
DCMI Box Encoding Scheme: specification of the spatial limits of a place, and methods for encoding this in a text string
Is Replaced By:
Status of document:
This is a DCMI Recommendation.
|Description of document:||The DCMI Box encoding scheme is a method for identifying a region of space using its geographic limits. Components of the value correspond to the bounding coordinates in north, south, east and west directions, plus optionally up and down, and also allow the coordinate system and units to be specified, and a name if desired. A method for encoding DCMI Box in a text-string, as a profile of DCSV is described. This notation is intended for recording the value of the DCMES element Coverage, particularly when using HTML meta elements. An alternative encoding for DCMI Box using XML is also shown.|
NOTICE TO IMPLEMENTORS:
The syntax examples included in this document are provisional, and are currently under review as part of the DCMI work on recommending coordinated syntax recommendations for HTML, XML, and RDF. These recommendations and minor editorial changes in this document can be expected to take place in the near future.
Table of Contents
- Identifying a place - the DCMI Box scheme
- Encoding DCMI Box
- 3.1 DCSV encoding
- 3.2 XML encoding
Several methods are available to indicate a place. These include, but are not limited to:
- a name , normally defined in an identifiable enumeration such as a gazetteer or list of jurisdictional localities
- a unique geocode , such as a postal code
- the coordinates of a point , using geographic values or some well-defined projection and units
- a set of arcs or faces describing the polygon or polyhedron comprising the perimeter of the place
- the limits of a regular shaped container which encompasses the place, typically a rectangular box in two or three dimensions, using geographic values or some well-defined projection and units
The Dublin Core™ Metadata Element Set [DCMES] includes an element, Coverage , the value of which may contain an identifier for a place. If a name or geocode is used then the scheme from which that is selected determines valid values. However, there are no simple, commonly used, notations for the identifiers which use coordinates. Here we define DCMI Box, an identifier which specifies the geographic limits of a place, and describe methods for encoding DCMI Box, as a profile of DCSV [DCSV], and using a fragment of XML [XML].
In the simplest usage DCMI Box approximates the extent of a place using a container with a regular shape. For a more precise representation of an irregular shape it is possible to use the approach of "tiling" the place with a set of simple regions defined using DCMI Box. Alternatively, another notation describing a polygon or polyhedron may be used. If an identifier corresponding to a point is required, then DCMI Point [POINT] is available.
We identify a place by considering the minimal rectangular box which fully encloses the place, whose faces are aligned parallel with the axes of an identified cartesian coordinate system [Figure].
We define the following components to describe the box:
|northlimit||The value of the constant coordinate for the northernmost face or edge2||INF3|
|eastlimit||The value of the constant coordinate for the easternmost face or edge2||INF3|
|southlimit||The value of the constant coordinate for the southernmost face or edge2||-INF3|
|westlimit||The value of the constant coordinate for the westernmost face or edge2||-INF3|
|uplimit||The value of the constant coordinate for the uppermost face or edge2||INF3|
|downlimit||The value of the constant coordinate for the lowermost face or edge2||-INF3|
|units||The units applying to unlabelled numeric values of northlimit, eastlimit, southlimit, westlimit||signed decimal degrees|
|zunits||The units applying to unlabelled numeric values of uplimit, downlimit||metres|
|projection||The name of the projection used with any parameters required, such as ellipsoid parameters, datum, standard parallels and meridians, zone, etc||geographic coordinates on Earth for northlimit, eastlimit, southlimit, westlimit; height above mean-sea-level for uplimit, downlimit.|
|name||A name for the place4||-|
1All components are optional. If any *limit component is absent, then this implies an interval unbounded on that side. Thus, a DCMI Box with a single component northlimit="0" would identify the entire southern hemisphere.
2Values are expressed as a text-string representing a number. Units should be included using conventional (SI) notation, unless the relevant units or zunits component is present. However, if units are given as part of any value, then for this component these override those given by units or zunits.
3If this component is absent then the value is undefined. Processors performing numeric comparisons are recommended to set values corresponding to maximally inclusive matching.
4In this context the name is non-normative. In the case of a conflict, the place identified by the coordinate values takes precedence. The name is provided for user convenience only.
The components of a DCMI Box identifier have no meaning when disaggregated, since in any particular instance it is the complete set which acts as the identifier. Thus, use of DCMI Box to identify a place requires that the components are linked together. For systems in which data is encoded using a limited character set, this is conveniently accomplished by packaging the components into a single text-string. Various serialisation syntaxes are available, including DCSV [DCSV] and XML [XML].
In normal usage, the unadorned token "DCMI Box" should be taken to refer to the encoding using DCSV.
Writing DCMI Box using DCSV notation is straightforward, using the component names defined above. A DCMI Box value appears as follows:
northlimit:v1; eastlimit:v2; southlimit:v3; westlimit:v4; uplimit:v5; downlimit:v6; units:v7; zunits:v8; projection:v9; name:v10
where v1 - v10 are values as defined in the table above.
All components are optional but may not be repeated, and the ordering is not significant.
DCMI Box may be written in XML. Given the flexibility of XML many alternative notations are possible. One form looks like this:
<Box projection="v9" name="v10">
defined by the DTD fragment:
<!ELEMENT Box (northlimit?,eastlimit?,southlimit?,westlimit?,uplimit?,downlimit?)>
projection CDATA "geographic, height relative to mean-sea-level"
name CDATA \#IMPLIED >
<!ELEMENT northlimit (#PCDATA)>
<!ATTLIST northlimit units CDATA "signed decimal degrees">
<!ELEMENT eastlimit (#PCDATA)>
<!ATTLIST eastlimit units CDATA "signed decimal degrees">
<!ELEMENT southlimit (#PCDATA)>
<!ATTLIST southlimit units CDATA "signed decimal degrees">
<!ELEMENT westlimit (#PCDATA)>
<!ATTLIST westlimit units CDATA "signed decimal degrees">
<!ELEMENT uplimit (#PCDATA)>
<!ATTLIST uplimit zunits CDATA "m">
<!ELEMENT downlimit (#PCDATA)>
<!ATTLIST downlimit zunits CDATA "m">
The values here are equivalent to the values in the DCSV profile. Note that:
- We have defined an XML element Box. Instances of this would occur within a complete XML document.
- The content model for Box is a (clockwise) sequence of (optional) *limit elements. This is a cleaner representation of the information required to specify the "box" structure than is possible in DCSV. All other components of Box occur as attributes
- units and zunits are recorded in an XML attribute. Since these are associated directly with the local *limit element, it is possible to express different components in different units if desired.
name:Western Australia; northlimit:-13.5; southlimit:-35.5; westlimit:112.5; eastlimit:129 <Box name="Western Australia">
northlimit:5980000; westlimit:644000; eastlimit:647000; southlimit:5966000;
units:m; projection:UTM zone 55 south <Box projection="UTM zone 55 south" name="Lake Jindabyne">
The Western Hemisphere:
A mine, illustrating the use of 3-D coordinates:
northlimit:-21.3; southlimit:-21.4; westlimit:139.8; eastlimit:139.9;
uplimit:400; downlimit:-100; name:Duchess copper mine
<Box name="Duchess copper mine">
1999. Dublin Core™ Metadata Element Set, Version 1.1: Reference Description
Dublin Core™ Metadata Initiative, OCLC, Dublin Ohio.
S. Cox, 1999. DCMI Point - a point location in space and methods for encoding this in a text string
S. Cox, R. Iannella, 2000. A syntax for writing a list of labelled values in a text string
Extensible Markup Language