In Sweden most of
the 290 municipalities used to have their own local coordinate
systems. The national geodata is stored and distributed in a
separate national system. This difference has created problems over
time when attempting to share and collaborate at both local and
national levels based on each set of geoinformation. Sweden is now
introducing a new national system – SWEREF 99. Today, some cities
and municipalities have changed to SWEREF 99 and the rest are in the
transformation process – coordinate by coordinate.
Many tools are
available for performing coordinate transformations and several
municipalities are now deciding on which approach to use, based on
their individual needs. Other important issues involving data quality also need to be
addressed when entire database are being transformed (see Fig. 1).
{sidebar id=228 align=right} Introduction of a new reference
system
The National Land Survey
of Sweden took the decision in 2001 to introduce a new reference
system. In 2003 it was decided on new map projections, among others
SWEREF 99 TM, a national projection covering the whole country. The
purpose was to completely introduce SWEREF 99 for the Land Survey in
January 2007 and thus replacing the old system RT90.
SWEREF 99 is a Swedish
realization of the European reference system ETRS 89, performed in
accordance with the present EUREF guidelines.
With the introduction
of SWEREF 99 Sweden will get a uniform reference system that, among
other things, facilitates the sharing of data on different levels.
Today, positions are frequently determined using satellites (with GPS
receivers) and the new reference system will give coordinates with a
higher accuracy based on the SWEREF 99 system.
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Data sharing
Modern IT relies on
the sharing of information and creating an environment that supports
collaboration between partners and the public. The current situation
with many different local coordinate systems causes serious problems
when sharing geographic data. Often, this means research relating to the
appropriate transformation parameters to use, verification issues,
data inconsistencies and other factors that arise when receiving
geographic data from different sources. Current trends are toward
greater sharing across borders, on local, regional, national and
global levels, where geodata can be re-purposed for policy,
environmental, local government and needs and these trends are
magnifying the problems associated with coordinate system
differences.
Preparation of the tools
There has been a lot
of work in defining the mathematical principles to make sure that the
transformations are done with greatest care. Municipalities and
cities have worked together to benefit from each others expertise and
capacity in handling
{sidebar id=230 align=left} transformations. One example is from the region
Skåne in southern Sweden, where municipalities have teamed up
to perform control surveys and create deformation models.
The
National Land Survey’s tool, Gtrans, has been prepared with
configuration files based on very accurate calculations. Gtrans can
be run separately on some geographic dataformats: however the
integration in FME, a tool for Spatial ETL (Extract Transform Load)
made by Safe Software, has leveraged the transformation capabilities
to most existing spatial datasources (see Fig. 2).
Choosing strategy
In Sweden, most urban planning offices have the responsibility for providing the tools and transformation parameters to the entire municipality, thus securing the quality of coordinate transformations. Sometimes the urban planning office performs all the transformations on the actual data. In other cases they only provide the tools to organisations, whereupon the work is performed. In other regions several municipalities use the same local coordinate systems and therefore have the possibility to optimize the use of tools and knowledge while cooperating (see Fig.3).
As Lars Freden ,Manager of Mapping & Surveying Department at Göteborg City, says, “Today it is no problem to share data within the city of Göteborg or among the surrounding municipalities. he problem is when the users of map data work over large areas outside the region with the 13 communities. For example: The National Land Survey of Sweden, The National Road Administration, The Swedish State Railways, Telecom companies, Yellow pages, etc, etc.”
A major task for most municipalities is to locate all the data sources that should be transformed. In many cases there are one or a few critical spatial production databases containing the core information for the organisation. These production databases are often easy to locate but call for great caution when transforming. The actual time for converting these databases has to be carefully planned as production might stand still when work is in progress, such as the production databases for property records and road data.
Locating all other possible file based information (for example CAD data, GIS files, GPS logs, etc) can also be a demanding task – especially for a big administration such as a major city. Depending on the organisation’s needs, different strategies can be used.
The City of Malmö, scanning the servers
The City of Malmö’s road administration, which has to convert more than 50.000 files, has chosen to develop a scanning tool. This tool is instructed to "crawl" the fileservers to find geographic data. Metadata is then extracted to confirm what coordinate system is being used. Other metadata such as number of objects, bounding box, etc is also collected for later quality control (see Fig. 4).
The metadata is then sorted and divided into jobs. Each job may contain thousands of references to files which are piped through a re-projection engine. The original files are backed up and then replaced by the transformed data. Once again metadata is collected so that the quality of the reprojection can be verified. The simplest verification is that the number of geographical objects is the same after reprojection as before.
The City of Göteborg, adopting new web technology
The City of Göteborg sought a way to allow end users, who have the best knowledge of their data and requirements, to perform the transformations themselves in a simple way. There are several benefits with letting the owners of the data perform the transformations themselves, since they understand the results for their locations.
Having used the desktop software of FME to perform data translations and coordinate system transformations for years, the GIS department became very interested in solving their problems using the server software while adopting SWEREF 99.
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This approach also makes it possible for the City’s users of geographic data to access an online interface where they can upload and re-project their spatial data into the new coordinate system when needed. The GIS department will only have to create the most commonly required transformation tasks and upload them to FME Server, and thereby enabling end users to perform the conversions they require without assistance or deeper knowledge about coordinate transformations. With this solution, there will be no need for the GIS team to spend time transforming all data, but instead they can allow working files to be transformed on-the-fly by their authors.
The City of Göteborg will be using this method as a part of their spatial data transformation initiative, in September 2008 with the assistance of SWECO, a certified FME reseller in Sweden. There are two major stages in the project. In the first stage, the GIS department will use the software to transform the spatial data held in its Oracle Spatial database. The second stage means enabling users from all departments to share data by uploading their spatial data files to FME Server online, and transforming them as required.
{sidebar id=232 align=right} The City of Göteborg’s GIS team will develop workspaces for transformation of the spatial data used in the City’s various departments. They will create unique workspaces for the data types used by each department and each workspace will include the GtransReprojector transformer for conversions of features into SWEREF 99. The GtransReprojector makes use of the Gtrans libraries developed by the National Land Survey of Sweden to perform accurate coordinate transformations. When the workspaces are ready, they will be uploaded to the server and made available to data authors across all departments.
FME Server is workspace driven, which means that the GIS department will be able to achieve accurate and efficient transformations without complicated and time-consuming development. Since these workspaces will be shared with data authors over the web, there will be no need for the GIS team to manually handle each transformation; however they are still in control of the coordinate system transformation process.
Challenges for other organisations
Private companies and other regional and national organisations, face a somewhat different challenge. They rely on data from various municipalities, cities and other sources. Some of these sources have already switched to SWEREF 99 but far from all.
Up to now many companies, regional and national organisations have still been using RT 90 or local coordinate systems to store their data but currently they are deciding on a time for switching to SWEREF 99. These organisations have to prepare their production systems so that they can cope with different coordinate systems through quite a long transition time. Fortunately modern databases and ETL tools make these challenges easier to face (see Fig. 5).
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Quality awareness
Independently of the approach, transformed data quality is maintained. The need for accuracy is very different depending upon the type of data that are being processed. The right tools enable very sophisticated validation but other issues must also be
considered. A simple but very relevant check that can be performed is to ask if your data still contain the same number of objects after as before the transformation.
Geometric effects normally occur after a coordinate transformation. Previously straight lines might become bent or curved, areas can change and segment lengths are recalculated. The users have to consider if this is a problem or not (see Fig. 6).
The focus should now be on how to measure when the transformation work is finished. Criteria are needed for reaching the goals, with questions such as: are all the wanted data converted? What quality demands are there? Is the quality known? Are the demands fulfilled?
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Ulf Månsson is a FME Certified Professional at SWECO located in Sweden. Email: [email protected].
Karin Gullstrand is employed at SWECO in Sweden. Email: [email protected]
For more information: www.swecogroup.com
References
The National Land Survey of Sweden (www.lantmateriet.se )
Statistiska Centralbyrån (www.scb.se )
”Transformation Between Malmö Local System and RT90 5,0 gon V 60:-1”, Peter Rothstein and Peter Segerstedt, Sweco, 2006
“City of Göteborg Chooses FME Server to Power Efficient Spatial Data Exchange ” – from Safe Software
