A geoservice is a way to make geographic information available through the Internet.
Today, almost all products allowing the setting and use of a geoservice are based on the OGC standards. The Open Geospatial Consortium, or OGC , is an international non-profit organization founded in 1994 to address issues of interoperability of geographic information systems (GIS). The purpose of this consortium is to bring together all the stakeholders concerned in the development and promotion of the open standards guaranteeing interoperability in the field of geomatics and geographic information and to foster cooperation between developers, suppliers and users.
The OpenGIS standards and specifications developed by the OGC facilitate producing complex systems and information services and promoting open content and services accessible and useful with any application.
In this article we will discuss the two main standards used in geoservices: WMS and WFS.
The OGC WMS standard specifies the interface and settings for dynamically requesting maps from a server. Each WMS is individually configured and can be used for a multitude of different maps, combination of layers and, eventually, can even be stylized with a different symbology.
The Web Feature Service (WFS) service represents a change in the way geographic information is created, modified, and exchanged over the Internet. Rather than sharing geographic information at the file level using the File Transfer Protocol (FTP), for example, WFS provides direct and detailed access to geographic information at e feature and proprietary level. .
This international standard specifies visualization operations, query operations, lock operations, transaction operations, and stored and parameterized query management operations.
To make it short, it can be said that the WMS protocol makes it possible to broadcast formatted maps, while the WFS protocol allows the dissemination of “raw” geographical data. In the first case what will be returned by the server is an image, while in the second it will be vector data as an XML file.
One of the main advantages of a geoservice is to overcome the multitude of GIS formats. The user, just knows the two protocols. It is the geoservice software that takes care of reading and formatting data in different formats. An example of this architecture is the use of Geoserver. The following image shows the different types of data managed by Geoserver.
The WFS protocol
The WFS protocol makes it possible to put data on line. The user accesses the basic information of the entities and their attributes. Of course, it is possible to control and limit access to certain entities or properties, but the result of a request to the server will be a data file.
This international standard specifies discovery operations, quering operations, lock operations, transaction operations, and stored and parameterized query expression management operations.
Discovery operations are used to query the service to determine its capabilities and retrieve the application schema that defines the types of properties that the service provides.
Query operations retrieve entities or entity features values from the underlying data store based on client-defined constraints on entity properties.
Lock operations allow exclusive access to entities for the purpose of editing or deleting entities.
Transactional operations are used to create, modify, replace, and delete features in the underlying data store.
Stored query operations allow clients to create, drop, list, and describe parameterized query expressions that are stored by the server and can be re-called multiple times using different parameter values.
As it stands, the WFS standard defines eleven operations:
- GetCapabilities (discovery operation)
- DescribeFeatureType (discovery operation)
- GetPropertyValue (query operation)
- GetFeature (query operation)
- GetFeatureWithLock (query and lock operation)
- LockFeature (lock operation)
- Transaction (transactional transaction)
- CreateStoredQuery (stored query operation)
- DropStoredQuery (stored query operation)
- ListStoredQueries (stored query operation)
- DescribeStoredQueries (stored query operation)
In the following article, we will discuss the first four since they are the most common. The other operations concern the updating of the data through the geoservice and are, generally, reserved for a limited number of users.
