Sneak Peek: OGC Web Coverage Service (WCS) and compound elevations

This lesson offers a seak peek at a two new features in Web WorldWind since 0.9.0: OGC Web Coverage Service (WCS) and compound elevations. These complementary features work together to enable applications to replace WorldWind’s terrain with elevation data a Web Coverage Service.

Important notes:

OGC Web Coverage Service (WCS)

The Open Geospatial Consortium’s (OGC) Web Coverage Service (WCS) is unique from the other OGC services. WCS has a two distinct capabilities documents (capabilities and coverage description) and the differences between versions is far more pronounced than WMS. To facilitate use of a WCS the WorldWind team wanted to abstract the version negotiation and configuration process. The goal is to provide an easy to use API for reviewing and using a WCS without having to worry about version specific nuances.

WCS can provide a multitude of data and data types. The WorldWind team identified WCS as the desired standardization to deliver elevation data in the future. With that goal, our initial WCS effort has focused on retrieving single band GeoTiffs representing elevation values for a coverage.

WebWorldWind WMS and WMTS operations have required the client to create the url parameters for a request, issue the asynchronous request and then create the object model from the returned XML document. The new WebCoverageService completes all of these steps and only requires the service address thus fully abstracting the version negotiation and document retrieval.

  1. Use the static function create of the WebCoverageService to initialize the configuration process. create returns a Promise of a fully configured WebCoverageService:

     var serviceAddress = "https://worldwind26.arc.nasa.gov/wcs";
 WorldWind.WebCoverageService.create(serviceAddress);

  2. A Promise provides a then function which passes the asynchronous result of the Promise. For the WebCoverageService.create function the callback value returned is the fully configured WebCoverageService; The WebCoverageService maintains references to the capabilities and coverage description documents of the WCS. It also maintains an array of WcsCoverage objects representing the available coverages from the WCS. The WcsCoverage object is a simple object which contains common properties of a coverage that different components of WebWorldWind might utilize (Sector, resolution, etc.) In this step, lets add functionality which adds a list of the names and resolutions of the coverages available from the WorldWind26 server on the globe canvas:

     var serviceAddress = "https://worldwind26.arc.nasa.gov/wcs";
 var coveragesList = document.getElementById("coverage-list");
 WorldWind.WebCoverageService.create(serviceAddress)
    .then(function (webCoverageService) {
        var coverages = webCoverageService.coverages, len = coverages.length, i;
        for (i = 0; i < len; i++) {
            var idAndResolution = coverages[i].coverageId + " - " + coverages[i].resolution;
            var coverageElement = document.createElement("li");
            coverageElement.appendChild(document.createTextNode(idAndResolution));
            coveragesList.appendChild(coverageElement);
        }
    });

Compound Elevations

With WCS, applications need a way to display all the elevations of interest. To support this, we’ve introduced the ElevationCoverage class, and redesigned ElevationModel as a container of coverages. Like the WorldWindow’s layer list, applications can configure the elevation model with one coverage, many coverages, or no coverages. The model’s coverage list and individual coverage properties are flexible enough to change dynamically at runtime. For example, coverages may be added once a WCS connection is made, or a coverage may be enabled/disabled in reponse to a UI action.

WorldWind is initialized with a set of default coverages that retrieve terrain from the WorldWind servers at NASA Ames Research Center. Applications may remove these coverages, or agument them by adding their own.

In this lesson, we’ll replace WorldWind’s default elevation coverages with a WCS coverage. First, we remove any existing elevation coverages. The result is a smooth WGS84 ellipsoid:

var elevationModel = wwd.globe.elevationModel;
elevationModel.removeAllCoverages();

Next, we get the WCS capabilities and select a coverage for this lesson.

var serviceAddress = "https://worldwind26.arc.nasa.gov/wcs";
WorldWind.WebCoverageService.create(serviceAddress).then(function (webCoverageService) {
    // we happen to know the coverage at index 1 contains USGS NED elevations; your application will likely look for a specific coverage by name, or let the user choose the coverage
    var coverage = webCoverageService.coverages[1];
});

WebCoverageService has a list of WcsCoverage instances retrieved from the service. Each WcsCoverage instance has an elevationConfig property that can be used to construct a WorldWind elevation coverage.

var elevationCoverage = new WorldWind.TiledElevationCoverage(coverage.elevationConfig);
elevationModel.addCoverage(elevationCoverage);

We’re changing the WorldWindow object after initialization, so we need to redraw it to see the change.

wwd.redraw();

Here’s the completed result.

Next Steps