Understanding the WMS and WFS geoservices (Part 2)

GetCapabilities

The GetCapabilities operation is a request to a WFS server for a list of
operations and services, or capabilities, supported by that server. When you
establish a connection with QGis, for example, on a WFS server, QGis sends this
request to be able to manage the connection and offer you the uploading of the
different available layers.

The request takes the following form:

localhost: 8080 / geoserver / wfs?
wfs & service =
& version = 1.1.0
request = GetCapabilities

The answer to GetCapabilities is a long XML document.  

A GetCapabilities document has five main elements:

ServiceIdentification contains basic information about
the request header, such as the title and type of service. The ServiceType
indicates which versions of WFS are supported.
ServiceProvider Provides contact information for the company that
publishes the WFS service, including phone, website, and email.
OperationsMetadata Describes the operations supported by the WFS server
and the parameters of each operation. A WFS server can be configured so as not
responding to the operations listed above.
FeatureTypeList layers are listed in the namespace form: featuretype.
The default projection is also shown, with the footprint box for the data in
the projection indicated.
Filter_Capabilities A list of filters, or expressions, that are
available to query, for example, spatial operators (such as Equals, Keys) and
comparison operators (such as lessthan, greaterthan). The filters themselves
are not included in the GetCapabilities document.

DescribeFeatureType

DescribeFeatureType requests information on a particular layer before
requesting the actual data. Specifically, the operation will request a list of
features and attributes for the given layer, or list the types of available features.

The request takes the following form:

l ocalhost: 8080 / geoserver / wfs?
wfs & service =
& version = 2.0.0
request = & DescribeFeatureType
typeNames = topp: tasmania_roads &
outputFormat = text / javascript

The answer will be of the following type:

In this example the answer indicates that the tasmanbialayer_roads has
two attributes: the_geom of polyline type and TYPE of string type.

GetFeature

The GetFeature operation returns a selection of entities from the data
source.

The request takes the following form:

localhost: 8080 / geoserver / wfs?
wfs & service =
& version = 2.0.0
request = & GetFeature
typeNames = topp: tasmania_roads &
FEATUREID = tasmania_roads.1 &
outputFormat = text / javascript

This request asks for an entity of the tasmania_roads layer
having as identifier tasmania_roads.1

The answer is:  

We observe the values ​​of the coordinates of the polyline and the value
of TYPE for this entity: alley .

The WMS protocol

The Web Map Service Interface (WMS) provides a simple HTTP interface for
requesting geo-recorded map images from one or more geospatial databases
distributed. A WMS request defines the geographic layer (s) and the area of
​​interest to be processed. The response to the request is one or more
geo-recorded map images (returned as JPEG, PNG, etc.) that can be displayed in
a browser application. The interface also allows you to specify whether the
returned images should be transparent so that multiple server layers can be
combined or not.

A typical GETMAP query is of the following form:

localhost: 8080 / geoserver / topp / wms?
service = WMS &
& version = 1.1.0
request = GetMap &
layers = topp: tasmania_roads &
bbox = 145.19754% 2C-43.423512% 40.852802 2C148.27298000000002% 2C-&

width = 768 &
height = 641 &
srs = EPSG: 4326 &
format = image / png

The layers parameter indicates which layer you want, the bbox
parameter indicates the desired geographic site and the parameter srs whose
coordinate system is requested.

The width and height parameters indicate the exact pixel size
of the returned image from this query, while the format indicates the
desired image format.

This information is essential because the output
of the request will be an image that will be loaded directly into a map window:

The size of the image will be the size of the map window and, therefore,
this image is superimposed on the other information of the window, the corners
of the image must correspond exactly with the current coordinates of the
window.

In order to make a comparison with the WFS
protocol, here is the output of the query requesting the set of entities of the
same layer (tasmania_roads):  

Both results are based on the same data layer of the server, but while
the image is directly usable in a browser, the data will have to be processed
by computer code to be displayed.

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