DCMI Point Encoding Scheme
|Description:||This document defines DCMI Point, a mechanism for indicating a point in space using its geographic coordinates and representing that information as a value string. Components of the value string 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. A method for encoding DCMI Point in a text string using the DCSV syntax is described. This notation is intended for representing a value of the DCMES element Coverage, particularly when using HTML meta elements.|
Table of Contents
- Identifying a place - the DCMI Point scheme
- Encoding DCMI Point with DCSV Syntax
Several methods are available generally 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 be a place. If a name or geocode is used as the value representation for the property, then the enumeration from which that value is selected determines valid value strings. However, there are no simple, commonly used, notations for indicating a point using coordinates. Here we define DCMI Point, an encoding scheme which specifies the coordinates of the point location of a place, and describe a method for encoding DCMI Point in a text string using the DCSV syntax [DCSV]. If an identifier corresponding to an extensive region is required, then DCMI Box [BOX] is available for rectangular regions.
The place is indicated 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, nor is such specification necessary in a DCMI resource description.
We define the following components to describe the point:
|Component Label||Definition||Default Component Value1|
|east||The coordinate of the location measured in the east direction2||+/- INF3|
|north||The coordinate of the location measured in the north direction2||+/- INF3|
|elevation||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 Representations of component values are text strings representing numbers. 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 component, then for this component these override those given by units or zunits.
3 If this component is absent then the value representation is undefined. Processors performing numeric comparisons are recommended to set value representations 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 specified above have no meaning when disaggregated, since in any particular instance it is the complete set which acts to indicate a specific place. 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 according to the DCSV recommendation [DCSV].
A DCMI Point value string using DCSV syntax, and using the component names defined above, appears as follows:
east=v1; north=v2; elevation=v3; units=v4; zunits=v5; projection=v6; name=v7
where v1 - v7 are component values as defined in the table above.
All components are optional but must not be repeated, and the ordering is not significant.
Perth, Western Australia:
name=Perth, W.A.; east=115.85717; north=-31.95301
Bridgnorth, Shropshire, U.K.:
east=372000; north=293000; units=m; projection=U.K. National Grid
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
S. Cox, 2000. DCMI Box Encoding Scheme- specification of the spatial limits of a place, and methods for encoding this in a text string
1999. Dublin Core™ Metadata Element Set, Version 1.1: Reference Description
S. Cox, R. Iannella, 2000. A syntax for writing a list of labelled values in a text string