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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
| Interface
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re u INTERNET -.-.-.
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Figure 2. MUIS and WMS Image Server
2. WMS PROTOCOL
The OpenGIS Consortium (OGC) has released the Web Map
Service (WMS) Implementation Specification. This
specification defines a syntax for World Wide Web (WWW)
Uniform Resource Locators (URL's) to invoke queries for maps
and georeferenced data.
The WMS protocol was developed as an open standard for
transferring maps and georeferenced data over the Internet.
Servers compliant with this standard make it possible to
visualise maps using a standard Web browser. Servers can be
specialised, for example the WMS Image Server is specialised
on satellite images. The user can access multiple servers at the
same time and in that way combine many types of information.
Figure shows how a coastline server, a height map server and a
satellite image archive are accessed from a user GIS. The result
is a combined map.
Coastline database WMS Image Server
Global elevation map
"M
MS Pat
“A A
Customer GIS
Figure 3. Multiple WMS servers used by a WMS client
The WMS specification is already supported by several
vendors, and already there exists open source servers as well as
Specific implementations such as the WIS described here.
Thanks to this it is very easy for any owner of data to publish
them on the internet and share them with other GIS information
via this common and simple interface.
The WMS protocol is based upon HTTP requests. The WMS
Image Server responds on three types of requests:
GetCapabilities: A request for a specification on what type of
information can be requested from the server.
GetMap request: A user-selected spatial request for a mosaic
of satellite images. The user can specify what sensor, time
period, type of mosaic and enhancement of the mosaic.
GetFeature request: A metadata request for a user selected
feature (image) in the mosaic.
The WMS servers will also deliver an error message if there is
an error in the request or in the processing of data. Error
messages can be delivered in images or as XML files.
3. MUIS CATALOGUE
The ESA Multi-Mission Catalogue (MUIS) contains over
200.000 browse image products from a large number of sensors
flying on-board ESA and Third Party Mission satellites. The
images represent a wide variety of sensor types, such as SAR,
sweep scanners, push-broom scanners and conical scanners.
Table 1 shows the missions and sensors handled in the present
version of the WMS Image Server.
ERS SAR IRS-P3 MOS
ERS ATSR-1 CHRIS PROBA
ERS ATSR-2 SeaStar SEAWIFS
JERS OPS TERRA MODIS
Landsat TM ENVISAT ASAR
Landsat MSS ENVISAT AATSR
NOAA AVHRR ENVISAT MERIS
Table 1. Imaging missions and sensors in the MUIS catalogue
The MUIS protocol is ESA specific and is used by client
software's for direct access of browse images in the archive.
The WMS Image Server uses the MUIS protocol to access
metadata and browses for processing. The protocol is based on
servlets.
4. SERVER DESIGN
The WMS Image Server was designed to handle a large number
of simultaneous requests from WMS clients. Each request sent
to the WMS Image Server involves heavy image processing and
there was a need to keep response times to a minimum. The
image processing algorithms was optimized and a cache system
was added to the server. The cache system is used to save both
metadata and images for reuse making the server response times
dramatically shorter. A timeout handler was also included to
make it possible to interrupt requests demanding too large
datasets.
The system is divided into three main modules:
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