May 04, 2009
GeoWebCache Takes the Ouch out of Tile Caching
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Susan Smith - Managing Editor

by Susan Smith - Managing Editor
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This improves the user experience and also opens up a number of new possibilities. For example, the response time becomes less important, so the WMS server can use more complex rules and render tiles that look better. You can also seed the cache in advance, using the built-in web interface, so that some or all tiles are cached before the instance is used in production.

GeoWebCache has been designed for speed and scalability. Even a laptop can serve tiles at several hundred megabits per second. I have come across blogs and emails where people assume that their instances would be limited to the throughput or seek times of their hard drives, since this is where the tiles are persisted. But this is generally not the case, most modern operating systems have something called disk block caches. This effectively moves the most requested tiles into memory, so they can be accessed at much higher speeds. OpenGeo hopes that this increase in capacity will also allow data providers to make their data available to a wider audience.

GISWeekly: What kind of relationship do you have with Google and Microsoft (Virtual Earth) and how do users benefit from using GeoWebCache with these geographic search engines?

In addition to Summer of Code, Google has contributed to both GeoServer and GeoWebCache by funding the development of three special output formats. Two of them are more closely related to Google Earth, namely raster super-overlays and regionated vectors. The first one works like the regular Google Earth background, improving the resolution of images as you zoom in. GeoWebCache can be used with any WMS server to achieve this effect.

The vector format uses OGC KML, and the key is that we gradually show more features as you zoom in. Developing code that automatically selects what items to show at what zoomlevel was a major undertaking. Both of these types of hierarchies can be cached using GeoWebCache. Google Earth requests a large number of tiles while you are zooming in or spinning the globe, so caching is crucial if you want to serve more than a few simultaneous clients.

The third format is what we call "geosearch" and most of the work is actually done in GeoServer. It is basically an XML sitemap, similar to those used for normal websites, and KML files representing each feature or row in the underlying database. The KML is automatically generated from any backend that provides vector data. Googlebot reads the sitemaps and then fetches all the KML placemarks. It analyzes the description of each feature and its location. After a period of about two weeks your data then becomes visible as a user-contributed placemark om GeoWebCache's role here is to provide fast access in case the person searching wants to view the entire dataset or
download the data as a shapefile.

We have registered that Microsoft is also entering the domain of searching geospatial information, but so far we have not found documentation on how to contribute to the index. So in this sense our relationship is limited to providing an easy to user interface for Virtual Earth JavaScript developers. People who use the SilverLight edition of Virtual Earth have also used GeoWebCache to publish data from WMS servers, it turns out the SilverLight code uses the same tile indexes as Google Maps.

GISWeekly: What are your goals for the product?

The goal is to make tile caching as unobtrusive and easy to use as possible. I hope that the WMTS standard that OGC is working on will make it easier to develop clients and share tiles across applications. On the server side I want to make the cache more dynamic, to automatically expire tiles that are no longer accurate. This is particularly important to OpenGeo, one of our primary goals is to create software that lets end users contribute and edit geospatial information through their web browsers. The term we use for this is "wikiable maps", it is based on WFS with transactions, but maintains multiple versions of the same data.

On the enterprise side of things we are actively looking for clients to fund features that are particularly important for large users. These include clustering, with lateral cache synchronization, for increased scalability and reliability. We would also like to develop tools that make it easier to maintain a cache and gather detailed statistics about usage. To get there I have made a list of
menu items that we hope clients will fund. That said, GeoWebCache is an exciting platform with a lot of possibilities. Things we have listed only represent a small subset of what can be done.

Top News of the Week

PCI Geomatics has joined the
ESRI Business Partner Program. Together with ESRI, they will provide users with optimized desktop and server based workflows for correcting commercial satellite imagery along with tools for processing and analyzing radar imagery.

Safe Software announced that they have received Oracle's Spatial Excellence "Partnership Award" at the 2009 Oracle Spatial User Conference in Tampa, Florida. The Oracle Spatial Excellence Awards are given to those leading organizations that have contributed to the advancement of mainstream enterprise solutions using geospatial technology.

Intergraph and
Stellar Grants Inc. teamed to offer an informative webinar at 1:00 p.m. Eastern on Tuesday, April 28 to assist utilities in applying for Smart Grid grants under the federal economic stimulus and recovery act.

The no-cost, one-hour webinar provided grant application strategy and process guidelines for utilities interested in the approximately $15.5 billion available in grants to modernize their grid infrastructure and operations. The grants are part of the nearly $800 billion included in the American Recovery and Reinvestment Act (ARRA) of 2009.

Acquisitions, Agreements, Alliances

Getmapping has teamed up with
New Zealand Aerial Mapping (NZAM) in an arrangement which enables both companies to share resources and equipment to increase their flying and aerial survey capabilities. Getmapping has ambitious plans for the UK flying season in 2009 including the completion of its aerial survey of Scotland and a new survey of Wales. Thanks to this tie up with NZAM Getmapping has up to four aircraft at its disposal for UK surveys during the 2009 flying season.

ESRI announced that
Interactive Visualization Systems (IVS)
3D has joined its business partner program. IVS 3D is a leading vendor of interactive 3D visualization and analysis software for marine information. This partnership will benefit organizations with workflows that visualize and analyze bathymetric data for use in the maritime community including the defense, petroleum, environment, and hydrographic sectors.

ImageTree Corporation has signed a strategic investment and technology development agreement with independent strategic investment firm In-Q-Tel. Under this agreement, ImageTree will leverage its patented technology and processes to help In-Q-Tel support the interests of the United States Intelligence Community, which includes the Central Intelligence Agency and 15 other organizations.


GeoTec Media, organizers of the 2009 GeoTec Event, June 1-4, 2009, Vancouver, BC, announced that the new online registration system is now live. This innovative new system allows attendees to manage their own registrations online, without the need for paper trails and faxing.

Team discounts saving 30 percent are available for organizations registering three or more people for the Three-Day Conference. All team registrations must be submitted together along with payment to receive the discount. This special offer expires May 8.

here for early registration and to save on conference fees. Early pricing ends 8 May and standard rates apply starting 9 May 2009.

The Open Geospatial Consortium, Inc. (OGC) announces the release of two Discussion Papers:
Uncertainty Markup Language (UncertML) and the OpenGIS Web Coverage Service Standard (WCS) Extension for CF-netCDF Encoding (

UncertML is a conceptual model and XML encoding designed for encapsulating probabilistic uncertainties and may be used to quantify and exchange complex uncertainties in data. Most data contains uncertainty arising from sources such as measurement error, observation operator error, processing/modeling errors, or corruption. Processing uncertain data propagates and often increases uncertainty. Thus there is a need for a standard way of characterizing uncertainty that is readily interpreted by software systems. UncertML is based on a number of ISO and OGC standards, such as ISO 19138 Data Quality Measures, and
addresses the ISO/IEC guide to the expression of uncertainty in measurement (GUM). UncertML utilizes the OGC Geography Markup Language (GML) Standard and the OGC Sensor Web Enablement Common (SWE) Standard.

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-- Susan Smith, Managing Editor.


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