Web Services to Access Spatial Data

How they work, tools use and examples. Recommended Software, relevant standards and exercises

WELCOME!

With the following slides and interactive material you will be able to take part in the journey to discover Spatial Data Infrastructures (SDI) its components and benefits through the observation of multiple examples and exercises.

You can navigate through the course by pressing the navigation arrows at the bottom of each slide or using your arrow keys on your keyboard. You can move horizontally (← →) for viewing each theme and vertically (↑↓) to view extra recommended information

Web services to access spatial data

#	Contents
1	Web Services in Action
2	How the WWW works
3	Relevant Standards
4	Software and Tools
5	Exercises

Web Services in Action

Examples of incompatibility and inconsistency of spatial data

Use case: map of Unesco site in Leuven

Let’s start with a demo…

Use case: map of Unesco site in Leuven

First I’ll go to http://www.geopunt.be/ because I know there is a good chance to find a Flemish map there

I type in the search criteria ‘Unesco’

Use case: map of Unesco site in Leuven

After a click on the first search result

Use case: map of Unesco site in Leuven

Now there are multiple possibilities:

Click on the link of the owner (‘Agentschap Onroerend Erfgoed’)

Click on the ‘Show on map’ (Bekijk op kaart) button

Click on the metadata button

Download the data

Click on the link of the owner

On the website of the data owner I see a link to the own geo-portal.
There I see the same area (brown with yellow), but this map also shows other related layers from the same data producer.

Click on the ‘Show on map’ button

Combine it with other maps (other themes) like historical maps

Click on the metadata button

Abstract, contact information (again with a link to the website / geo-portal of the data producer)…

Use case: map of Unesco site in Leuven

Questions:

So who has the data?
Is the data duplicated for each geo-portal?
Who maintains the data and how is this data updated on all 3 geo-portals?

Answers:

With open standards as Web Viewing Service it is possible to keep the data in one place only, the logical choice would be with the data producer
All the geo-portals link to the same Web Viewing Service
The data itself (shapefile, geo-database…) is kept securely on a private network behind the Web Viewing Service

Why do I need geo web services?

Web mapping is about more than google maps

It's possible to:
- Share your own map data with everyone regardless of the software they use
  - And choose how far you want to go:
    - Only allow viewing = WMS
    - Also allow downloading = WFS
    - Also allow online editing = WFS-T (transactional WFS)
- Combine your map data with map data from others
- Create a web page / web application with your map data

It’s a requirement for any SDI to set up services so your data can be found and accessed

How the WWW works

The World Wide Web: how does it work?

Different protocols work together

Browser
HTTP / HTTPS
TCP/I¨P
IPv4
PPP/…
Cable

The World Wide Web: how does it work?

Definition of web service (3WC):

“A software system designed to support (interoperable) machine-to-machine interaction over a network.”

Web services allow us to create client/server applications

Websites are for humans ↔ Web Services are for software

No GUI (Graphical User Interface) – only usable within applications

Introduction

Web services advantages:

Provide interoperability between various software applications
Accessible via each network supporting open standards
Allow software and services from different organisations and locations to be combined easily to provide an integrated service
Are self-describing

Web Services main disadvantage:

Overhead: transmitting data (usually in XML) is not as efficient as using binary code. What you win in portability, you lose in efficiency.

How to request a service?

Remote procedure calls (RPC)

A Client sends a request message to a known remote server to execute a specified procedure with supplied parameters
The remote server sends a response to the client

Ex.:

Service-oriented architecture (SOA):

The basic unit of communication is a XML message

"message-oriented" services

Relevant Standards

Principles of Open Standards

Availability
- Open Standards are available for all to read and implement.

Maximize End-User Choice
- Open Standards create a fair, competitive market for implementations of the standard. They do not lock the customer in to a particular vendor or group.

No Royalty
- Open Standards are free for all to implement, with no royalty or fee. Certification of compliance by a standards organization may involve a fee.

INSPIRE: Maximum Reuse of existing standards (OGC, ISO, W3C)

World Wide Web Consortium (W3C)

Founded in 1994 (Tim Berners-Lee)

Define standards for the Internet

Initial specifications for URIs (Uniform Resource Identifiers), HTTP (Hypertext Transfer Protocol), and HTML (HyperText Markup Language)

327 members (31/12/14)

449 specifications
- including 131 W3C Recommendations

World Wide Web Consortium (W3C)

XML: eXtensible Markup Language

World Wide Web Consortium (W3C) recommendation

Designed to describe data in textual format

Define your own tags

XML does not DO anything!

XML (describe data), is NOT HTML (present data)

Filename extension: .xml

XML Example

<?xml version="1.0" encoding="UTF-8" ?>

<caption>This is Raphael's Foligno Madonna

</caption>

</painting>

Module “XML and UML”

Open Geospatial Consortium

Founded in 1994

Solve the issue of spatial data sharing and interoperability

Sets the standards that allow geographic information systems to interoperate

470+ members and growing

34 “core” standards
- 15 extensions/profiles

03 | WMS – Web Map Service

As its name implies, it is a service that provides maps

The data leaves the server only as an image

The map is rendered on the server, so styling and presentation are chosen by the data provider

Limited client interactivity with the map

Current version: 1.3 (is the same as ISO 19128)

Note:

Source data from which the image is generated need not to be an image. It can be a Shapefile, PostGIS database, Oracle Spatial,…

Opengeo.org

WMS – Web Map Service

03 | WMS – Web Map Service

WMS Server can do one of 3 things (WMS operations)

Produce a map (as an image)
- getMap

Answer basic queries about the content of the map
- getFeatureInfo (returns an XML with attributes)

Tell other programs what maps it can produce and which of those can be queried further
- getCapabilities (returns an XML file with the metadata)

WMS – GetCapabilities Request

Provides the following information:

Image formats it can serve (jpeg, png, gif…)
List of the map layers
List of layers supporting the optional GetFeatureInfo interface
List of available spatial reference systems
List of exception formats for return of exceptions
(optional) SLD styles
(optional) Vendor specific capabilities (or properties)

Example

http://demo.opengeo.org/geoserver/wms?SERVICE=WMS&REQUEST=GetCapabilities

Response is a XML file

http://demo.opengeo.org/geoserver/wms? The 'root' URL of the service.
SERVICE=WMS Request for a WMS service
REQUEST=GetMap Request for a map image
LAYERS=topp:states Which layer? Multiple layers possible with comma separated list
SRS=EPSG:4326 The projection we are requesting using the EPSG code
BBOX==-124.731,24.956,-66.97,49.372 Bounding box coordinate
- Format: “MinX, MinY, MaxX, MaxY”
WIDTH=600 Width of image we want
HEIGHT=255 Height of image we want
FORMAT=image/pgn Image type (possibilities: png, jpeg, gif & tiff)
STYLES= In case the WMS service offers multiple styling. Blanc → default style
- To be found in the GetCapabilities
TRANSPARENT=FALSE Will image be transparent in areas of no data

WMS – GetMap Request

Image

Error


				<ServiceExceptionReport version="1.1.1"> 
				
				       <ServiceException code=""> 
				
				              WIDTH and HEIGHT incorrectly specified 
				
				       </ServiceException> 
				
				</ServiceExceptionReport>

WMS – GetMap Response

WMS – GetFeatureInfo Request

Example:

http://demo.opengeo.org/geoserver/wms?SERVICE=WMS&REQUEST=getfeatureinfo&LAYERS=topp:states&BBOX=-124.731,24.956,-66.97,49.372&FORMAT=image/png&WIDTH=600&HEIGHT=255&X=100&Y=100&query_layers=topp:states http://www.ruimtemonitor.be/geoserver/wms?service=wms&request=getfeatureinfo&LAYERS=gemeenten_2003_rb&FORMAT=image/png&BBOX=21000,150000,117000,246000&WIDTH=256&HEIGHT=256&X=200&Y=200&query_layers=gemeenten_2003_rb

Response = XML or text file (dependent on your WMS server software)

<?xml version="1.0" encoding="UTF-8"?> <FeatureInfoResponse> <FIELDS FEMALES="226581" MALES="227007" POP1999="482025" STATE_NAME="Wyoming" _ID_="6" _SHAPE_="[Geometry]" _LAYERID_="0"/> </FeatureInfoResponse>

→ response can be formatted by the (web) application

WFS – Web Feature Service

Provides map data (GML) to a (web) client
The client chooses style & presentational details
Geospatial features
Optional: Transactional Web Feature Service (WFS-T) enables the creation, deletion, and updating of features
Current version 2.0
= ISO 19142

Opengeo.org

CSW – Catalogue Service FT Web

A catalogue server publishes collections of descriptive information (metadata) about geospatial data
Defines interface to search for metadata (so you could ‘discover’ geospatial data)
- Transaction: insert, update & delete of metadata
- Harvest (optional) - create/update metadata by asking the server to 'pull' metadata from a specified target

Module “Metadata and catalogue services”

CSW – Some Operations

getCapabilities

Metadata about the service itself (indicating abilities, contact information…)
http://metadata.geopunt.be/zoekdienst/srv/dut/csw?service=CSW&request=GetCapabilities

getRecords

Does a search
http://metadata.geopunt.be/zoekdienst/srv/dut/csw?service=CSW&version=2.0.2&request=GetRecords&CONSTRAINTLANGUAGE=CQL_TEXT&resultType=results

getRecordById

Returns the metadata (default representation) of a specific record using their identifier
http://metadata.geopunt.be/zoekdienst/srv/dut/csw?service=CSW&version=2.0.2&request=GetRecordById&id=DE022DEF-20BF-4016-82A1-57BCC917AF4E

All return a XML document

CSW – GetRecords Response

CSW – GetRecordsByID Request

Geographic Markup Language

Standard way for data to be passed from one geographic application to another

Is the XML grammar defined by the Open Geospatial Consortium (OGC) to express geographical features.
Response of WFS requests is GML
Filename extension: .gml

<gml:MultiLineString srsName="http://www.opengis.net/gml/srs/epsg.xml#27354">
				
					  <gml:lineStringMember> 
				
						    <gml:LineString> 
				
						         <gml:coordinates decimal="." cs="," ts=" "> 494475.71056415,5433016.8189323
				
				                	494982.70115662,5 435041.95096618 </gml:coordinates> 
				
					    	</gml:LineString> 
				
					  </gml:lineStringMember>
				
				</gml:MultiLineString>

Module “Basic concepts of XML and GML”

GML example: Geometries

04 | Software and Tools

Desktop clients:

QGIS
ESRI ArcGIS for Desktop
uDIG
Kosmo
MapWindow

Web mapping clients comparison:
http://geotux.tuxfamily.org/index.php/en/geo-blogs/item/291-comparacion-clientes-web-

WMS/WFS Clients and Servers

Servers:

GeoServer
MapServer
ESRI ArcGIS for Server
Geomajas

DEEGREE
MapGuide OpenSource
OpenGeo Suite

Software to use for the following exercise

Visit the following tools and software sites:

OpenGeo Suite

GeoServer INSPIRE extension

Quantum GIS (QGIS)

Google Earth

Software to use for the following exercise

05 | Exercises

Exercise: objective

05 | Geoserver

Geoserver: installation

05 | Geoserver: installation

Geoserver: installation

Geoserver: configuration

Exercise: Publishing a Shape File

05 | Exercise: Publishing a Shape File

Exercise: Publishing a Shape File

05 | Exercise: Publishing a Shape File

Reference list

Kralidis, A. T. (2007). Geospatial web services: The evolution of geospatial data infrastructure. In The Geospatial Web (pp. 223-228). Springer, London.

van den Brink, L., Barnaghi, P., Tandy, J., Atemezing, G., Atkinson, R., Cochrane, B., ... & Janowicz, K. (2019). Best practices for publishing, retrieving, and using spatial data on the web. Semantic Web, 10(1), 95-114.

Rautenbach, V., Coetzee, S., & Iwaniak, A. (2013). Orchestrating OGC web services to produce thematic maps in a spatial information infrastructure. Computers, Environment and Urban Systems, 37, 107-120.
Clabby, J. (2003). Web services explained: Solutions and applications for the real world. Prentice Hall Professional.