Download Lecture 3: Geospatial Web Services

Lecture 3: Geospatial Web Services
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From Web sites to Web services
Geospatial Web service functions
Web service types
Interoperability and geospatial service standards
Optimizing Web services
From Web sites to Web services
The need for web services
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Limitations of early web GIS (Fig 3.1):
Isolated systems, difficult to resuse and integrate
Tightly coupled client and server, inflexible to change.
A web service is a program that runs on a web server and exposes programming
interface to other programs on the web.
• The W3C defines a "Web service" as "a software system designed to
support interoperable machine-to-machine interaction over an network". It has an
interface described in a machine-processible format (specifically Web Services
Description Language-WSDL). Other systems interact with the Web service in a
manner prescribed by its description using SOAP messages, typically conveyed
using HTTP with an XML serialization in conjunction with other Web-related
standards.“
• A comparison of web pages and web services:
-Web pages are for users to read and understand. In HTML format.
-Web services are for other computer programs to call. Composed of web-based
programmable components. Results are in XML, JSON, or other structured formats.
Limitations of early web GIS
Three roles in a complete web service
Geospatial industry support of web services:
three main categories of Web GIS products (Fig. 3.3)
1. Server product, e.g., ArcGIS Server: publish
maps, data, and analytic models, enable SaaS
(Software as a Service).
2. Geospatial Web portals (Geoportals) which
serve as brokers b/w server provider and
consumer.
3. Client products: either geo-browsers or
custom applications development
Geospatial Web service functions
Map services: most common geospatial web service
• Allow clients to request maps for a specific
geographic extent; maps returned in an image
format.
• Cached (fulfiling requests with pre-created
tiles from a cache, also called tiled service) or
dynamic (server renders the map each time a
request comes in).
• 2D or 3D (G. Earth, MS Bing Maps 3D, ArcGIS
Online globe).
Data Services
• Allow clients to query, edit, and synchronize data
over the Web. Including:
• Feature editing services
• Search services: index and search GIS
resources(e.g., data layer, table, whole enterprise
database).
• Image services: provide access to raster data
(e.g., RS, DEM)
• Geodata synchronization services: periodically
replicate or synchronize data updates b/w
distributed geodatabases in different locations.
Analytical services
• Perform GIS analysis functions, e.g.:
• Geocoding/reverse geocoding services
• Transportation network analyst services
(routing, calculating service area, finding the
closest facility).
• Geometry services: buffer, merge, split,
area/length, projection, …
• Geoprocessing services: share functions or
models with the Web. E.g., Solar potential
calc.
Metadata Catalog Services
• Meta data: data about data.
• Allow publishing and searching metadata.
• Compare:
- Data search service: indexes data, especially
attribute tables, directly.
- Metadata catalog service indexes metadata.
• E.g. : ArcGIS Server Geoportal Extension.
Web Service Types: SOAP and REST
(Formats of communication b/w server and client)
SOAP-based web services
 SOAP, Simple Object Access Protocol, is
a protocol specification for exchanging
structured information in the implementation
of Web Services.
 relies on Extensible Markup Language (XML)
for its message format
 relies on other Application Layer protocols,
most notably HTTP or Simple Mail Transfer
Protocol (SMTP), for message negotiation and
transmission.
REST-style Web Services
 Representational State Transfer (REST) is a style
of software architecture for distributed systems
such as the WWW.
 REST has emerged as a predominant Web
service design model.
 A RESTful web service (also called a RESTful web
API) is a web service implemented using HTTP
and the principles of REST.
 In the most common RESTful Web services, the
client sends all parameters in the request URL.
SOAP vs. REST
Geospatial Service Standards
Interoperability
Web Service Standards
• Web service standards specify the format of
HTTP requests and HTTP responses. The
standards facilitate interoperability.
• Main standards body:
-Open Geospatial Consortium (OGC)
(http://www.opengeospatial.org/)
-ISO/TC211
-W3C (World Wide Web Consortium)
Geospatial Service Standards
Main Web Service Standards
• WMS (Web Map Service): maps rendered in a pictorial format (PNG,
JPEG, GIF,…).
E.g. NASA Earth Observations.
http://neo.sci.gsfc.nasa.gov/Search.html
• WFS (Web Feature Service): R/W geographic features in vector format.
Two classes: Basic WFS (read-only), and Transactional WFS (WFS-T). E.g.,
National Digital Forecast Database.
• WCS (Web Coverage Service): supports e-retrival of geospatial data as
“coverages”, including RS images, digital air photos, DEM, and other
point measurement data. WCS returns raw data, while WMS returns a
visual representation. WCS for raster, WFS for vector.
• GIS4930/5935/6110 class lab example:
http://students.uwf.edu/trj11/MyWebGIS/lab3/
Other Web Service Standards
• CSW (Catalog Service for the Web), e.g.,
ArcGIS Server Geoportal Extension
• OpenLS (OpenGIS Location Services)
• WPS(Web Processing Service): for geospatial
processing services.
• SWE (sensor web enablement).
Related Standards
• GML (geographical markup Language): is the XML grammar defined by
OGC) to express geographical features. GML serves as a modeling
language for geographic systems as well as an open interchange
format for geographic transactions on the Internet.
• KML and KMZ: Keyhole Markup Language is an XML notation for
expressing geographic annotation and visualization within Internetbased, two-dimensional maps and three-dimensional Earth browsers.
KML was developed for use with Google Earth. KML files are often
distributed in KMZ files, which are zipped files that include KML and its
asscoated images and icons.
• GeoRSS: an emerging standard for encoding location as part of a Web
feed. (Web feeds are used to describe feeds ("channels") of content,
such as news articles, blogs entries. The name "GeoRSS" is derived
from RSS (Really Simple Syndiction), the most known Web feed and
syndication format.
Optimizing Web services by Caching in
Advance
• Map caching is a very effective way to make your
Web map service run faster.
• When you create a map cache, the server draws
the entire map at several different scales, and
stores copies of the map images.
• The server can then distribute these images
whenever someone asks for a map.
• It's much quicker for server to hand out a cached
image than to draw the map each time someone
requests it.
• Also called map tiling. Each image is a tile.