DCMI Point Encoding Scheme
DCMI Point Encoding Scheme: a point location in space, and methods for encoding this in a text string
Is Replaced By:
Status of document:
This is a DCMI Recommendation.
|Description of document:||We introduce DCMI Point for identifying a point in space using its geographic coordinates. Components of the value correspond to the location coordinates in north and east directions, plus optionally elevation, and also allow the coordinate system and units to be specified, and a name if desired. We describe a method for encoding DCMI Point in a text-string, as a profile of DCSV. This notation is intended for recording the value of the DCES element Coverage, particularly when using HTML meta elements. We also show an alternative encoding for DCMI Point using XML.|
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 Point scheme
- Encoding DCMI Point
- 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 Point, an identifier which specifies the coordinates of the point location of a place, and describe methods for encoding DCMI Point, as a profile of DCSV [DCSV], and using a fragment of XML [XML]. If an identifier corresponding to an extensive region is required, then DCMI Box [BOX] is available for rectangular regions.
We identify a place using a point location, described using coordinates in an identified cartesian system. The point may correspond to some place within an extensive region, such as the areal or volumetric centroid, but we do not specify the nature of the relationship in this document.
We define the following components to describe the point:
|east||The value of the coordinate of the location measured in the east direction2||+/- INF3|
|north||The value of the coordinate of the location measured in the north direction2||+/- INF3|
|elevation||The value of the coordinate of the location measured in the vertical direction2||+/- INF3|
|units||The units applying to unlabelled numeric values of north, east||
signed decimal degrees
|zunits||The units applying to unlabelled numeric values of elevation||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 north, east; height above mean-sea-level for elevation.|
|name||A name for the place4||-|
1 All components are optional.
2 Values 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.
3 If this component is absent then the value is undefined. Processors performing numeric comparisons are recommended to set values corresponding to maximally inclusive matching, i.e. the location is a line if one coordinate is missing, and a plane if two are missing.
4 In 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 Point 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 Point 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 Point" should be taken to refer to the encoding using DCSV.
Writing DCMI Point using DCSV notation is straightforward, using the component names defined above. A DCMI Point value appears as follows:
east:v1; north:v2; elevation:v3; units:v4; zunits:v5; projection:v6; name:v7where v1 - v7 are values as defined in the table above.
All components are optional but may not be repeated, and the ordering is not significant.
DCMI Point may be written in XML. Given the flexibility of XML many alternative notations are possible. One form looks like this:
<Point projection="v6" name="v7">
</Point> defined by the DTD fragment:
<!ELEMENT Point (east?,north?,elevation?)>
projection CDATA "geographic, height relative to mean-sea-level"
name CDATA #IMPLIED >
<!ELEMENT east (#PCDATA)>
<!ATTLIST east units CDATA "signed decimal degrees">
<!ELEMENT north (#PCDATA)>
<!ATTLIST north units CDATA "signed decimal degrees">
<!ELEMENT elevation (#PCDATA)>
<!ATTLIST elevation zunits CDATA "m">
The values here are equivalent to the values in the DCSV profile. Note that:
- We have defined an XML element Point. Instances of this would occur within a complete XML document.
- The content model for Point is a conventionally ordered (x,y,z) sequence of (optional) coordinate elements. This is a cleaner representation of the information required to specify the "point" structure than is possible in DCSV. All other components of DCMI Box occur as attributes
- units and zunits are recorded in an XML attribute. Since these are associated directly with the local coordinate element, it is possible to express different components in different units if desired.
Perth, Western Australia:
name:Perth, W.A.; east:115.85717; north:-31.95301 <Point name="Perth, W.A.">
Bridgnorth, Shropshire, U.K.:
east:372000; north:293000; units:m; projection:U.K. National Grid
<Point projection="U.K. National Grid" name="Bridgnorth">
The Greenwich Meridian:
The highest point in Australia, illustrating the use of 3-D coordinates (and how flat Australia is):
east:148.26218; north:-36.45746; elevation:2228; name:Mt. Kosciusko
<Point name="Mt. Kosciusko">
S. Cox, 2000. DCMI Box Encoding Scheme- specification of the spatial limits of a place, and methods for encoding this in a text string http://dublincore.org/specifications/dublin-core/dcmi-box/
1999. Dublin Core™ Metadata Element Set, Version 1.1: Reference Description
Dublin Core™ Metadata Initiative, OCLC, Dublin Ohio.
S. Cox, R. Iannella, 2000. A syntax for writing a list of labelled values in a text string
Extensible Markup Language http://www.w3.org/XML/