Understanding the WMS and WFS geoservices (Part 1)

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.

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