Coverages FAQ

Frequently asked questions on coverages and coverage services. -- PeterBaumann - 30 Jun 2014

WCS Extensions refer to WCS Core 2.0 - what about WCS 2.1?

Answer: all WCS 2.0 extensions apply equally to WCS 2.1, see the corresponding clarification in the WCS 2.1 Core specification. In other words: all WCS extension functionality work on CIS 1.1 coverages.

How to represent a 0-dimensional coverage?

Related issues: What happens when all the CRS dimensions get sliced in a WCS request? How to encode the CIS/GMLCOV description?

Answer: As per CIS/GMLCOV, the domain set of a coverage cannot have 0 dimensionality: at least for the GML encoding, the //gml:domainSet/@dimension attribute shall be greater or equal than 1. There is no clear guidance on 0-D in the OGC standards, The recommended solution is to represent such a coverage by the single (atomic or composite) range value in the range set and an empty domain set .

Example: See the rasdaman WCS test oracle for 0-dimensional output rectified grids: browser:systemtest/testcases_services/test_wcs/oracle/

Are time intervals closed or open?

Answer: Discussion on the TC Discussion listhas confirmed that time intervals represent left-closed, right-open intervals, often written as [a:b). This maintains clean interval arithmetics, such as avoiding duplicate values when combining two adjacent intervals.

Related: In its latest release, ISO 8601 seems to drop 24:00 as a valid timestamp as it is equivalent to 00:00 of the next day. This removes an ambiguity where two different notations indicated the same time.

How can I construct the URL of a CRS as needed for a coverage?

Answer: By definition, every coverage has one CRS attached; in case of a grid coverage, there is additionally the internal Cartesian coordinate system of the array representing the range set. Following OGC convention, every CRS is denoted as a URL (so that it can be resolved, ie: its definition can be looked up). There are several branches in the OGC CRS Resolver hosting CRSs, CSs, axes, etc. - see the resolver wiki for details. In short, a CRS looks like this one: . Where no predefined CRS exists - such as for higher-dimensional coverages - this can be composed from existing CRSs and axes, as described in the resolver wiki.
  • Note: CRSs are subject to the rules and constraints given by the CRS standard, ISO 19111 (which is identical to OGC Abstract Topic 2).
Topic revision: r4 - 05 Jun 2019, PeterBaumann
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