Coverages and their status in GeoAPI and GeoTools

Introduction

It is clear that there has been some confusion in the community with regard to the concept and use of Coverages in the open geospatial standards of ISO and OGC. This page collects together information from discussions on various mailing lists, including the GeoTools, GeoAPI and SHAVER lists. Here we define what a Coverage is, give examples of different types of Coverage and discuss how these concepts are implemented in GeoAPI and GeoTools.

What is a Coverage?

(This section is gleaned mostly from an email by Adrian Custer, for which many thanks. I have adapted and decorated Adrian's words.)

The abstract definition of a Coverage is:

Some set of direct positions for all of which we have a set of values.

The domain of a Coverage is, by definition, those points for which we have values. The set of values is known as the range of the Coverage. The values can be of any kind of measure: nominal, ordinal, interval, or ratio and can also be a vector of, possibly mixed, values. The coverage is required to have a value for each position in its domain.

(A "direct position" is simply a point in a given coordinate system. For example a direct position representing a point on the Earth's surface can be specified through its latitude and longitude.)

Coverages can be discrete or continous, according to how values are generated within their domain. Note that the actual values of a Coverage can be anything at all and so Coverages are not classified by their values. This classification is clarified below, with examples.

Discrete Coverages

A Discrete Coverage consists of a finite number of domain elements. Each domain element is associated with a single value, therefore we get the same result whereever we are within the element. This is best explained through some examples:

Coverage	Domain elements	Values (range)	Example domain element	Example value	Notes
Country names	Polygons, one or more for each country	The name of the country	Polygon outline of the island of Great Britain	"United Kingdom"	There are other islands and territories that are part of the UK but are not part of Great Britain.
Geomagnetic readings	Point locations, one for each geomag station	Timeseries of gemagnetic readings	Lerwick Magnetic Observatory

Continuous Coverages

A Continuous Coverage consists of a sin

Outstanding questions

Implementation status in GeoAPI

Implementation status in GeoTools

Implications for Java-CSML

-- JonBlower - 02 Jul 2008