OGC、WMS、WFS、WCS

OGC: Open Geospatial Consortium-OGC, Committed to the standardization of software and data and services in the geographic information industry.

OGC's full name was Open GIS Consortium from 1994 to 2004, and it was renamed because of business needs.

OGC is a non-profit, voluntary international standardization organization that leads the development of spatial geographic information standards and the positioning of basic services. In the field of spatial data interoperability, the interoperability method based on the public interface access mode is a basic operation method.Through the International Organization for Standardization (ISO/TC211) or technical alliance (such as OGC) to develop interface specifications for spatial data interoperability, GIS software vendors develop spatial and spatial read and write functions that follow this interface specification, which can realize the mutual operating.

OGC defines three geo-referenced information models: - Web Map Server(WMS) - Web Feature Server(WFS) - Web Coverage Server(WCS)

Free and open source software, open specifications and open data

The current use and maintenance costs of commercial GIS software are getting higher and higher, for example, the ESRI ArcGIS software, which includes a complete set of client and server, is priced at about RMB 700,000. Moreover, the sales strategy is that if the server software is purchased, the client software must be purchased, which the reason is that since the user uses the server software to publish the service, it is necessary to use the client software to process the data. This is far beyond the reach of some of the smaller WebGIS applications. Moreover the data and operations of many commercial software GIS are not fully convertible and shared, resulting in some information islands.

However, open source GIS is active across the commercial GIS software. Founded in 1994, OGC is committed to researching and establishing open geospatial data interaction standards that enable users and developers to interoperate. The Open Source Geospatial Foundation was established in February 2006 with the mission of supporting the development of open source geographic information software and remote sensing software and promoting its wider application. It also provides organizational, legal and financial assistance to the projects it supports, and promotes the development, promotion and popularization of the OSGeo Foundation's development of standard software based on geographic information and its interoperability technologies. The OSGeo China Center was established in September 2006 to help users and developers in China better use the source code, products and services provided by the OSGeo Foundation.   

Web Mapping Service (WMS)

The Web Map Service (WMS) can return the corresponding map (including PNG, GIF, JPEG, etc., or vector forms such as SVG and WEB CGM) according to the user's request.

WMS supports the network protocol HTTP, and the supported operations are defined by URLs.

There are three important operations GetCapabilitiesGetMapGetFeatureinfo

  • GetCapabilities returns service level metadata.
  • GetMap returns a map image.
  • GetFeatureinfo returns information about some of the special features displayed on the map.   There are some other operations like DescribeLayerGetLegendGraphicGetStylesSetSytles 。  In fact, using a traditional point of view,GetMap What you get is the result of drawing on the control in the desktop program, which is the performance of the data.

GetFeatureInfo is easier to understand, which has the same function as the Info button used on almost all desktop programs. It is used to get some information about the screen coordinates. The parameters in GetFeatureInfo are the screen coordinates, the current view range, etc., to a certain extent. It is also convenient for the client to write.

GetFeatureInfo can return feature information in multiple layers at the same time, which is the same as ArcGIS Desktop. WMS also includes requests for returning legend information such as GetLegend, which are also defined in full accordance with the existing standards on the desktop.

Web Feature Services (WFS)

Web Feature Services (WFS) supports the insertion, update, delete, retrieval, and discovery services for geographic features. The service returns GML data based on HTTP client requests.

The basic interfaces are: GetCapabilities, DescribeFeatureType, GetFeature GetCapabilities is the same as above.

DescribeFeatureType returns the feature structure for the client to query and other operations.

GetFeature returns a GML-compliant data document based on query requirements. GetFeature is the most important interface.

Other interfaces such as Transaction not only provide feature reading, but also support feature online editing and transaction processing.

WFS corresponds to the conditional query function in common desktop programs. WFS constructs query conditions through OGC Filter, supports queries based on spatial geometric relations, queries based on attribute domains, and of course, common queries based on spatial relationships and attribute domains.

On the Web, WFS requests are not implemented in SQL, but are implemented through Filter XML, which is more scalable. What WFS returns is the result set of the query. To some extent, it is different from the "data representation" of WMS.

The result set of WFS is a result set defined and constrained by the complete Schema, and is based on GML. This result set is similar to the data table for desktop program query results.

Web Coverage Services (WCS)

Web Coverage Services (WCS): Provides a spatial raster layer that contains geographic location information or attributes, rather than a static map.

Sending data according to HTTP client requirements, including images, multispectral images and other scientific data. There are two important operations GetCapabilities, GetCoverage GetCapabilities returns a collection of data from which the service and XML documents are described.

GetCoverage is executed after GetCapabilities determines the query plan and the data to be acquired, and returns the coverage data.

There is also an optional operation DescribeCoverageType.

WCS corresponds to the function based on raster data and corresponds to the characteristics of WMS based on vector data.

Another: Web Processing Server (WPS) is a newly introduced standard, and its function is actually familiar to us. Processing is GeoProcessing in ArcView, such as Union, Intersect, etc. law. What WPS needs to do is expose the URL-based interface to enable the client to call such methods via WebService and return data.

These specifications are basically supported in major mainstream GIS platforms and open source GIS software.

Intergraph has long introduced the WFS server and interoperability development kit.

ESRI has developed related components in ArcIms that support WMS, WFS and other specifications.

MapInfo 8.5 has also added access to WMS and WFS services, as well as an excuse to read GML data.

GeoServer, MapServer plays the role of providing map services to clients in the network. Such map servers can receive unified WMS and WFS requests and return data in multiple formats. This process has strict rules of the WMS/WFS specification, so it is not a big impact on the client's implementation of its map server. Such a specification creates possibilities for public, joint map services.

OpenLayers/MapBuilder, uDig, QGIS client software is divided into browser and desktop client programs. The B/S system client represented by OpenLayers is now very powerful, it can encapsulate WMS requests. Implement the slice loading feature of the map on the browser. In addition, the functions of dragging and zooming are also very perfect, and cross-browser operation can be realized. The recent OpenLayers version also supports vector editing, which can be submitted via WFS-t. The traditional desktop client program is more powerful, supports a variety of data sources including WMS and WFS, and the editing function and operability are more powerful than in the browser.