Archive | November 2012

OGD Vienna Preview

If you ever want to simply preview any of the data released by the city of Vienna by their Open Government Data initiative on top of an OpenStreetMap layer, you now can do so.

I present the “OGD Wien Preview and Matcher”. It can be accessed by http://www.gisforge.com/ogdmatcher .

Title Screen of the OGD Wien Preview and Matcher, version 1

Title Screen of the OGD Wien Preview and Matcher, version 1

With this web application it is possible to display any spatial dataset available on the servers of OGD Vienna. The list which displays the datasets is generated automatically each time the application is accessed. So, it always reflects the current state. Also, only when selecting a specific set of data, it is loaded from the OGD source. The downside of this method is that there is a slight delay while waiting for the chosen dataset to arrive. But on the other hand, it is not necessary to store this data anywhere. Additionally to that, the displayed information is always the most recent one.

At this moment information is displayed by entities filled only with yellow colour and black border. When too many points are displayed at once, they are grouped together. Line and polygon features are always displayed as they are. When clicking on a feature all information associated with it is visualized. It has to be mentioned that because of the experimental nature of the methods used to construct this application, umlauts are not encoded.

A possibility to compare point datasets from OGD Vienna with a copy of OpenStreetMap data is given. Due to the bad performance of the server the procedure of comparison runs very slow and is not of much use until now.

The web application has a problem with Microsoft Internet Explorer that I yet have to track down. While data is loaded with this browser, it takes for ever to process its display on the map.

Basemap.at WMTS Layer in QGis

QuantumGIS as it is now has no specific entry in its menu to add a WMTS layer. But with the gaining availability of web maps that are offered as tiled maps this becomes increasingly important.

The states of Austria work together to provide a commonly designed map of the whole country offered under the CC-BY 3.0 AT licence. This initiative is called basemap.at and is accessible by the WMTS protocol.

Despite of the absence of an easily accessible manner to add such a layer to QuantumGIS, it is still possible to do so by rerouting the WMTS management directly to the GDAL driver used by QGis to manage raster layers. Since version 1.7 GDAL itself supports WMTS layers. Michael Douchin describes in his blog post OpenStreetMap Tiles in QGIS how to create an XML file containing parameters to connect to the OpenStreetMap WMTS server. The specification of the GDAL-WMTS driver can be viewed on its web page. This XML file can be opened in QGis as a raster source and, given the projection is set right, works without any further modification.

The contents of the XML file needed to specify the connection to the basemap.at WMTS server are:

<GDAL_WMS>
 <Service name="TMS">
  <ServerUrl>http://maps.wien.gv.at/basemap/geolandbasemap/normal/google3857/${z}/${y}/${x}.jpeg</ServerUrl>
 </Service>
 <DataWindow>
  <UpperLeftX>-20037508.34</UpperLeftX>
  <UpperLeftY>20037508.34</UpperLeftY>
  <LowerRightX>20037508.34</LowerRightX>
  <LowerRightY>-20037508.34</LowerRightY>
  <TileLevel>18</TileLevel>
  <TileCountX>1</TileCountX>
  <TileCountY>1</TileCountY>
  <YOrigin>top</YOrigin>
 </DataWindow>
 <Projection>EPSG:3857</Projection>
 <BlockSizeX>256</BlockSizeX>
 <BlockSizeY>256</BlockSizeY>
 <BandsCount>3</BandsCount>
 <Cache />
</GDAL_WMS>

The values within “DataWindow” need further optimization. When I find the time I will take a closer look at the driver specifications and post a corrected version of this file, but this one already works even though Vorarlberg is clipped.