Web Services Types

Producer Field Guide

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Producer Field Guide
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A web service is a method of communication between two electronic devices over a network.

ERDAS APOLLO has six web service interfaces for serving imagery.

  • ERDAS APOLLO Essentials - SDI provides OGC WMS and OGC WCS
  • ERDAS APOLLO Essentials - Image Web Server provides ECWP, OGC WMTS, OGC WMS, ImageX and JPIP
  • ERDAS APOLLO Advantage and Professional - provides OGC WMS and OGC WCS, ECWP, OGC WMTS, OGC WMS, ImageX and JPIP

Web Coverage Service (WCS)

Web Coverage Service (WCS) supports the electronic interchange of geospatial data as "coverages", which is defined as digital geospatial information representing space-varying phenomena.

An OGC WCS supports ‘intact’ delivery of rich and intelligent raster data sources, in formats such as GeoTIFF and DTED, and is usable in client-side rendering as input into scientific models. Unlike WMS, which delivers a ‘portrayal’ of the original data, the WCS returns full, multi-dimensional geospatial data as "coverages," with its original semantics and in its entirety.

A WCS is providing one or more service offerings on coverage data:

  • Spatial query (version 1.0 focuses on grid spatial request)
  • Temporal query (separated from spatial)
  • Logical space query (spatial query in the 'pixel' space)
  • Reprojection offerings
  • Range subsettings
  • Multiple interpolation offerings
  • Multiple output format offerings

ERDAS APOLLO WCS can deliver raw pixel data in any supported imagery format and type to imagery exploitation clients such as ERDAS IMAGINE as interoperable coverage.

Coverage Portrayal Services

Coverage Portrayal Service (CPS) is a mechanism that renders graphical pictures from coverage data. Typically, coverage data are retrieved from a Web Coverage Service (WCS). The CPS facilitates wider use of coverage data by making graphical representations of coverages that can be shown in client web browsers. The CPS works as an intermediary between the client application and the WCS. Once the client has built a GetMap request and a Styled Layer Descriptor (SLD) document, it sends them to the CPS. The CPS will analyze the request and the SLD, and send a GetCoverage request to the underlying WCS. The WCS will return a coverage that the CPS will render by applying portrayal directives (image size, format, reprojection, classification, and so forth) found in the request and in the SLD document.

In the current OGC-CPS specification, the types of portrayal are listed below.

  • Classification
  • Normalization
  • Color mapping
  • Hill shading
  • Cloud cover filtering
  • Time filtering

A CPS is providing one or more service offerings on coverage data:

  • Spatial query (version 1.0 focuses on grid spatial request)
  • Temporal query (separated from spatial)
  • Reprojection offering(s) (optional)
  • Range subsettings
  • Multiple interpolation offerings
  • Multiple output format offerings

Web Map Service (WMS)

WMS is an official specification from the OGC. An OGC WMS delivers a ‘portrayal’, or a picture screenshot, of geographic information presented as a static map, rendered into picture formats such as PNG and GIF and useful for display and view purposes. WMS focuses on rendering customized maps and is an ideal solution for dynamic data or custom-styled maps. A portrayed WMS can also be built on top of other web services sources like WFS and WCS.

ERDAS APOLLO WMS can deliver portrayed GIS-ready and web client consumable ‘styled’ map data to any WMS client application.

For more information see opengeospatial.org.

Thus, WMS is a service interface specification that:

  • Enables the dynamic construction of a map as a picture, as a series of graphical elements, or as a packaged set of geographic feature data
  • Answers basic queries about the content of the map
  • Can inform other programs about the maps it can produce and provides metadata about the served data

WMS Operations

WMS is deployed such that you can query the server and obtain certain information about the data or the capabilities of the data services. The following WMS operations are valid requests to the WMS from a client-side application.

  • GetCapabilities - Obtains the capabilities of the server using service-level metadata. This is a description of the WMS information content.
  • GetMap - Obtains the map (raster data or image file) whose geo-spatial and dimensional parameters should be defined by the data service provider's WMS.
  • GetFeatureInfo - Obtains information about particular features shown on the image or raster file.

When invoking GetMap, a client can specify the following:

  • information to be shown on the map (one or more layers) and possibly the styles of those layers
  • portion of the Earth to be mapped (bounding box)
  • geographic or projected coordinate system to be used (Spatial Reference System, or SRS)
  • desired output format
  • output size (width and height)
  • background transparency
  • color

When more maps are produced with the same bounding box, SRS, and output size, the results can be accurately layered by the client to produce a composite map. The use of image formats that support transparent backgrounds, such as SVG, GIF, or PNG, allows the lower map layers to be visible. Furthermore, individual map layers can be requested from different servers.

WMS Architecture

wms

The above figure illustrates the WMS architecture. Your data is stored and made accessible to others for use by means of the WMS.

ERDAS APOLLO software also provides additional WMS functionality to you above and beyond the OGC specifications. Sophisticated Web Map Servers can enhance the basic OGC WMS specification by using proxy WMS capabilities. A proxy WMS enables a "cascading" process, whereby the proxy WMS acts as a client of other WMSs, and, in turn, performs WMS services to other clients. The OGC WMS specification notes: "This provides a convenient mechanism for aggregating the capabilities of the individual Map Servers into one logical ‘place.’ Additionally, a proxy WMS can perform additional services. Consider a proxy WMS that can convert many different graphics formats, such as GIF, PNG, JPEG, and so forth into GIF format. Then, client viewers that can only display GIF could still benefit from the output of map servers that produce only JPEG or PNG. Similarly, a Proxy WMS might perform coordinate transformations on behalf of other servers.

For more information see the OGC Web Map Server Interface Implementation Specification at opengeospatial.org.

Web Feature Service (WFS)

A Web Feature Service (WFS) is a web interface that allows you to publish and deploy geographic feature data on the internet.

WFS is an official specification from the OGC that "provides an interface for describing data manipulation operations (create, delete, update, and get features) on geographic data stored in databases that are accessible over internet infrastructure, such as World Wide Web, WANs, LANs, and so forth. The WFS interface exposes the data in these repositories as Geography Mark-up Language (GML)."

For more information see opengeospatial.org.

A WFS based on OGC specifications can be implemented in either its Basic version, or a Transactional version. Basic WFSs are READ‑ONLY Web Feature Servers while a transactional Web Feature Server (WFS-T) can support all the interfaces of a basic WFS and also implement a type of file locking (LockFeature) and transactional interfaces. This allows multiple users to do a variety of manual operations, such as create/update/delete data or make informatory requests simultaneously on the same geographic feature.

WFS Operations

  • GetCapabilities -You can send a request to a WFS asking what capabilities the server holds. Specifically, the WFS should indicate which feature types it can service, what operations are supported on each feature type, and what metadata is associated with each data source.
  • DescribeFeatureType - The server should be able, upon request, to describe in an XML schema the structure of any feature type it can service.
  • GetFeature - The server should be able to service a request to retrieve feature instances. In addition, the client should be able to specify which feature properties to fetch and constrain the query spatially and nonspatially.
  • LockFeature - A transactional WFS may be able to process a lock request on one or more instances of a feature type for the duration of a transaction. This ensures that transactions in series are supported.
  • Spatial Operators and Filters - There are several different spatial filters and operators that can be issued to the WFS.
  • Transaction - A transactional WFS may be able to service transaction requests. A transaction request is composed of operations that modify features; that is create, update, and delete operations on geographic features.

WFS Architecture

wfs

The above figure illustrates the WFS architecture. Your feature data are stored and made accessible to others for use by means of the WFS. You are able to choose which users can access your data, and place permissions or restrictions on certain data sets, ensuring the security of your data via the server.

Web Gazetteer Service (WFS-G)

A gazetteer is a geographical dictionary, which is essentially a list of geographic (place or feature) names, with references to their location and containing associations to descriptive information.

A gazetteer service is a network-accessible service that retrieves one or more features from a gazetteer, given a query (filter) request. It exposes both a potentially structured vocabulary and a feature collection, and also may support the capabilities of a hierarchical vocabulary and a Web Feature Server (WFS).

A Web Gazetteer Service is a specialized Web Feature Service that provides additional capabilities specific to a gazetteer-like feature collection.

Additionally, the Web Gazetteer Service exposes the following interfaces (using HTTP) to query, insert, and update location instances in a gazetteer database:

  • Get or Query Features – This request retrieves features based on thesaurus-specific properties;
  • GetCapabilities - This request retrieves properties of the gazetteer database, such as the location type class or the spatial reference system.

Enhanced Compression Wavelet Protocol (ECWP)

ECWP is the fastest protocol for accessing large imagery datasets in the world. This powerful streaming image delivery method enables organizations to serve more than 5,000 clients with a single server by transferring compressed blocks of imagery from the server to the client. ECWP supports local caching of data and provides real-time roam and zoom functionality on image updates. Additionally, ECWP’s quick response significantly reduces network traffic and server loads.

ERDAS APOLLO offers very fast and efficient delivery of ECW and JPEG2000 wavelet compressed imagery through ECWP protocol (ecwp://). ECWP is supported in many GIS and CAD applications, including ERDAS IMAGINE®, ArcGIS®, MapInfo™, AutoCAD®, PCI™, ER Mapper™, Microsoft® Office, Smallworld™, MicroStation® and more.

For more information see ECW Compression.

Web Annotation Service

Imagery analysts and other users of imagery and maps often need to summarize the essential content of an image, point out features of interest, or express similarities or differences between images. Likewise, GIS specialists often need to highlight spatial patterns, label certain features, or otherwise "mark up" a map. These maps and images can communicate valuable information.

Encoding annotations in an interoperable way will make it easier and more flexible for users to:

  • Exchange annotated maps and images.
  • Integrate image interpretation with raw imagery.

Annotations are features describing a given place that can be shared among different users of the system. An annotation feature consists of a geometry, a name, and a description.

The annotation service wraps WFS that is configured to store annotations as features. The ERDAS APOLLO implementation of the annotation server is an authenticated service, so users must provide both an ID and a password. Each authenticated user may create annotations and share or protect them from other users.

ImageX

ImageX is a simple HTTP protocol for image tile delivery. It efficiently generates image thumbnails and extractions from image mosaic files for combination in a web browser or custom application. ImageX is an optimized, proprietary interface used to provide the fastest non-streamed data communication possible between ERDAS APOLLO servers and ERDAS APOLLO client applications.

JPEG2000 Interactive Protocol (JPIP)

JPIP is a standards-based image streaming protocol for JPEG2000 compressed images, and rapidly serves both imagery and associated metadata. The JPIP streaming protocol can also stream data in the National Imagery Transmission Format (NITF) commonly used by United States defense agencies. NITF is a multiple image format paradigm that also contains sensor model information, and JPIP includes this information as it supports on-demand dissemination of the data.

ERDAS APOLLO provides fast and efficient delivery of JPEG2000 wavelet compressed imagery through the JPIP protocol (jpip://) to JPIP-compliant clients (web applications and IAS 3.1.x).

For more information see JPEG2000 in Raster Data from Other Software Vendors.

OGC Web Map Tile Service (WMTS)

WMTS enables high-performance serving of spatially-referenced map tiles. WMTS trades the flexibility achieved by custom map rendering in WMS for the scalability and simplicity made possible by serving static image tiles. A fixed set of tiles can be scaled by tile cache systems and served via a simple web server.

WMTS in ERDAS APOLLO is designed to complement WMS and provide simple integration and overlay with commercial APIs such as Google Maps and Bing Maps, open source projects like OpenStreetMap, feature services, or other web service layers. While leveraging this new OGC standard, ERDAS APOLLO can implement one service tiling method and integrate across many clients.

OGC WMTS 1.0 is supported as a published protocol in ERDAS APOLLO 2011. ERDAS APOLLO-supported formats like Optimized Tile Delivery Format (OTDF), ECW and many others may be published through the WMTS interface for consumption by third-party applications, no plug-in required. Web developers can build a web map with a WMTS layer and easily create mashups with applications like OpenLayers, Bing Maps, and Google Maps.

The native Google Maps API allows developers to create custom overlays that request WMTS tiles. In the example shown above, ERDAS APOLLO delivers PNG tiles from an ECW file via WMTS, which are overlaid on top of Google base layers.

ERDAS APOLLO Essentials – Image Web Server also provides a WMS interface.

Catalogue Service for the Web (CSW)

Catalogue Service for the Web (CSW) is a web interface that supports the storage, retrieval, and management of data related to web services. It is a key component in a service-oriented architecture that manages shared resources and facilitates the discovery of resources within an open, distributed system. Hexagon Geospatial’s answer to this interface is the CSW endpoint of the ERDAS APOLLO Catalog component, which offers a CSW-compliant view on the content of the ERDAS APOLLO Catalog.

The preferred OGC registry information model is based on the ebXML registry information model. This is called the ebRIM Application Profile for CSW.

CSW supports registration, metadata harvesting and descriptor ingestion, push and pull update of descriptors, and discovery of OGC Web Service types and instances.

Facilities are provided for the URL entry of OGC Web Service capabilities into the system for ingestion as well as the automatic harvesting of this data into the system at previously specified intervals so that it is available for service discovery via "viewer" clients. Service providers may also view, modify, and delete their entries from the system, although modification and deletion operations require preconfigured privileges. In addition, service discovery is facilitated through a well defined search interface.

Catalog - Type Management System

  • Provides persistent information for use in service discovery
  • Facilitates dynamic binding to services instances
  • Guarantees classification of the registered services

WTS

A Web Terrain Service (OGC WTS) is a rendering service which allows a 3D terrain view image to be requested, given a map composition, a terrain model on which to drape the map, and a 3D viewpoint from which to render the terrain view.

A Web Terrain Service (WTS) produces views of georeferenced data. The OGC WTS specification standardizes the way in which clients request views and the way that servers describe their data holdings. Two required operations are defined:

WTS Operations

  • GetCapabilities - Obtain service-level metadata, which is a machine-readable (and human-readable) description of the WTS's information content and acceptable request parameters.
  • GetView - Obtain a 3D scene whose geospatial and dimensional parameters are indirectly defined.

A WTS may potentially get the data required to build the view from other OGC Web Services:

  • Web Coverage Servers may provide the elevation data to model the terrain and some coverage information to be rendered in the scene as 3D shapes.
  • Web Map Servers may provide the image(s) to be draped onto the terrain, such as orthophotos or rendered feature maps.
  • Web Feature Servers may provide features with 2D geometry to be extruded as 3D object, for example annotations or building footprints, or features with 3D geometry to be directly rendered into the scene.