libcoverage.js

Libcoverage.js is an extensible client library for the Open Geospatial Constortium (OGC) Web Coverage Service 2.0+ interface standard using a MIT style license.

It features means to create requests from parameters and parse the results returned from a WCS service.

Dependencies:

• libcoverage.js does not have any further dependency but relies on the function getElementsByTagNameNS, and thus needs a browser that correctly supports it.

Drawbacks:

• Libcoverage.js does not send and receive requests of any kind. This was designed with the intention to maximize compatibility with other DOM/ajax libraries.
• Responses can only be parsed in a general way with the WCS.Core.Parse.parse function.

Libcoverage.js uses namespaces to not clutter the global one.

Request Generation

Lying in the WCS.*.KVP directories the functions ending with ...URL are creating request URLs for the available request methods. For the WCS 2.0 core, these are:

• GetCapabilities (WCS.Core.getCapabilitiesURL): This request gathers general information about the consumed service, e.g: allowed requests, available coverages and various service metadata.
• DescribeCoverage (WCS.Core.describeCoverageURL): This request collects detailed information about one or more coverages.
• GetCoverage (WCS.Core.getCoverageURL): This request downloads a specific coverage or subsets thereof and applies certain pre-processing parameters (e.g: reprojection or band-selection).

The generated requests can be sent to the server via the transmission method of any flavor but typically via ajax.

Response Parsing

To parse the responses returned by the service use the WCS.Core.Parse.parse function. It tries to find the correct parsing functions for the given element name and returns a merged object, containing the result of all registered parsing functions. This approach was taken for the sake of extensibility, please refer to the chapter Extending libcoverage.js for the exact means.

Extending libcoverage.js

Since WCS 2.0 uses a Core/Extension approach it is vital for a client library to be extensible to easily adapt new extensions. This is mostly important for parsing service responses.

Libcoverage.js allows the registration of new parsing functions for the node name of the elements it shall parse. As explained in Response Parsing the results of all registered functions are deep-merged together, so the extending parse functions should only parse information not yet included in the main parsing result.

To extend the core parsing capabilities with some specific functionality, one first has to design the parsing function which always takes the jQuery wrapped XML node as parameter:

var parseExtendedCapabilities = function($node) {
    return {
        // parse data and insert it here
        specialData: $node.find("wcs|SpecialData").text()
    }
}

Then, the function has to be registered for the node name (without the namespace prefix):

WCS.Core.Parse.pushParseFunction("Capabilities", parseExtendedCapabilities);

Example extension: EO-WCS

Libcoverage.js ships with a client extension for Earth Observation (EO-WCS). It provides a new function for generating requests (WCS.EO.KVP.describeEOCoverageSetURL) and new element parsing functions for Capabilities, CoverageDescriptions and EOCoverageSetDescriptions which are registered once the module is loaded.

The extended Capabilities parse function extends the parsed object with additional information about advertised dataset series. The CoverageDescriptions objects, on the other hand, are extended by the time interval and the footprint.

Integrations

Currently there is only one integration, namely for the MVC framework Backbone. The integration provides the models Service and Coverage and the collection CoverageSet. If the EO-WCS extension for libcoverage.js is also available, then the EOCoverageSet is included aswell. The models integrate seamlessly within Backbone and can be used alongside other models and object synchronization.

Unfortunately, as with the current status of Web Coverage Service - Transactional (WCS-T) it is not possible to integrate creation or modification of coverages within backbone and thus all related function calls will fail. This may change once the transactional interface extension for WCS 2.0 is specified.

References

• EOxServer Webclient