Click here to view the embedded video.

However modern browsers have the <canvas> element. This element allows for the manipulation of single pixels within this element. So I figured if I could encode the building-dates in a PNG and use canvas to display only those pixels which represent a building older than the given date it should be possible to time-slide through the buildings. Fortunately the fancy new mapping library Leaflet.js has a canvas tile layer build in. The BAG data is already available through EduGIS, so I only needed it to encode the data differently in the PNGs.

The encoding is very simple: per pixel there are 4 values available: red, green, blue and alpha. Since I only needed to encode 200-odd values I used just the red value. The years before 1850 are encoded as groups, since the data is so sparse, after 1850 each year is individually encoded. This means that from 1850 onwards the red value increases with 1. The client retrieves the encoded PNGs as normal tiles and look like this:

The image appears grey because I kept the green and blue values of the PNG the same as the red. This image is loaded into canvas and the imageData is retrieved using ctx.getImageData(0, 0, 256, 256) and stored as a jQuery data object on the canvas. This is important, since for visual effect we will manipulate the imageData on the canvas and we want to keep track of the original values. Once the imageData is attached to the canvas the colors are being calculated. It will take the original values, compares them to the current year and will decide whether or not to show the pixel and in which color.

Since only the grey tile is needed, the actual sliding through time is really fast because it doesn’t need to retrieve more data. With 4 bands of 255 values each you can encode an insane amount of data into a PNG, readily available through canvas for direct manipulation. Apart from time sliding, is detailed representation of DEM data an obvious use case.

• Geoserver 2.0 (SNAPSHOT downloaded 8 december 10.19UTC)
• Mapserver 5.6.0 (MS4W Base installer v3.0 Beta 7 + MapServer 5.6.0 release Upgrade)
• Deegree 2.3rc1 (Apache Tomcat 5.5.28)

To find out what the problem exactly was I’ve done some testing. I’ve taken a GeoTiff and configured all three the WCS applications to serve it. Gdalinfo gives us the following information, basically it is a GeoTIFF in epsg:3035 with a native resolution of 100m/pixel.

My test sequence was:

1. request the full extent in native resolution
2. request the full extent in a 10th of the resolution
3. request the full extent with the same size as the original data (256×256)
4. request part of the data in native resolution
5. request part of the data in non-native, non-multiple resolution

I’m not going to show all the results here, because that would result in an unhealthy long document

To verify the results I loaded the original GeoTiff in Quantum GIS and highlighted a specific feature to show the possible offset of the different results.

Corine GeoTiff

1  request the full extent in native resolution:

deegree: http://localhost:8080/deegree-wcs1/services?service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&resx=100&resy=100&format=GeoTIFF gives a three band(!) GeoTiff of 262,674 bytes with a shift of around 40m:

Deegree result

MapServer: http://localhost/cgi-bin/mapserver56/mapserv.exe?map=maps/corine.map&service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&resx=100&resy=100&format=GTiff gives a three band(!) GeoTiff of 197,477 bytes without a shift:

Mapserver result

GeoServer: http://localhsot:8081/geoserver/ows?service=WCS&version=1.0.0&request=GetCoverage&coverage=corine_merge_gtiff_proj_121_108&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&resx=100&resy=100&format=GeoTiff gives a single band GeoTiff of 8,848 bytes with a shift of  around 50m:

Geoserver result

Note:

Deegree both shifts the data and returns a multi-band image; Mapserver doesn’t shift the data, but returns a multi-band image; Geoserver shifts the data 1/2 a pixel (50m), but does give a single band image.

2. request the full extent in a 10th of the resolution

deegree: http://localhost:8080/deegree-wcs1/services?service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&resx=10&resy=10&format=GeoTIFF produces a 26,237,726 multi band GeoTiff which shifts the data the other way than the first request:

Deegree result on a tenth of the resolution

Mapserver: http://localhost/cgi-bin/mapserver56/mapserv.exe?map=maps/corine.map&service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&resx=10&resy=10&format=GTiff produces a multi-band GeoTiff of 19,681,941 bytes with no shift.

Geoserver: http://localhost:8081/geoserver/ows?service=WCS&version=1.0.0&request=GetCoverage&coverage=corine_merge_gtiff_proj_121_108&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&resx=10&resy=10&format=GeoTiff gives a single band GeoTiff of 158,802 bytes with a shift of around 50m, though there is a small difference with the previous result.

Note:

Mapserver and Geoserver both produce a bigger result (as expected) with the same location as the previous request. Deegree shifts the data differently than previously and also produces a bigger result.

3. request the full extent with the same size as the original data (256×256)

geoserver: http://localhost:8081/geoserver/ows?service=WCS&version=1.0.0&request=GetCoverage&coverage=corine_merge_gtiff_proj_121_108&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&width=256&height=256&format=GeoTiff produces exactly the same image as in the first request.

mapserver: http://localhost/cgi-bin/mapserver56/mapserv.exe?map=maps/corine.map&service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&width=256&height=256&format=GTiff produces exactly the same image as in the first request.

deegree: http://localhost:8080/deegree-wcs1/services?service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&crs=EPSG:3035&BBOX=3539200,3402400,3564800,3428000&width=256&height=256&format=GeoTIFF produces a slightly smaller image than the first request (262,674 bytes) with no shift!.

Deegree result with width en height

Note:

Since the request is a different way of describing the same data one would expect that you get the same results. Mapserver and Geoserver do so, Deegree does not, somehow using resx and resy in the request results in shifted data.

4. request part of the data in native resolution

deegree: http://localhost:8080/deegree-wcs1/services?service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&resx=100&resy=100&crs=EPSG:3035&BBOX=3550000,3413200,3554000,3417200&format=GeoTIFF has the correct left-top location but the size of the pixels is wrong, producing a shift of the data which increases as you go right/down:

Deegree result for a part request

geoserver: http://localhost:8081/geoserver/ows?service=WCS&request=GetCoverage&format=GeoTiff&version=1.0.0&coverage=geodan:Corine&resx=100&resy=100&crs=EPSG:3035&BBOX=3550000,3413200,3554000,3417200 produces an image with almost exactly the same shift as the first request.
mapserver: http://localhost/cgi-bin/mapserver56/mapserv.exe?map=maps/corine.map&service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&resx=100&resy=100&crs=EPSG:3035&BBOX=3550000,3413200,3554000,3417200&format=GTiff produces an image with almost exactly the same location as the first request.

Note:

Mapserver and Geoserver shift (or not) the data similar to the first request, Deegree has once again a different shift than any of the previous ones.

5. request part of the data in non-native, non-multiple resolution

deegree: http://localhost:8080/deegree-wcs1/services?service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&resx=30.0&resy=30.0&crs=EPSG:3035&BBOX=3550000,3413200,3554000,3417200&format=GeoTIFF shifts data:

geoserver: http://localhost:8081/geoserver/ows?service=WCS&request=GetCoverage&format=GeoTiff&version=1.0.0&coverage=geodan:Corine&resx=30.0&resy=30.0&crs=EPSG:3035&BBOX=3550000,3413200,3554000,3417200 shifts data and adds a black border to the left and top of the result:

Geoserver result for non-native, non-multiple resolution

mapserver: http://localhost/cgi-bin/mapserver56/mapserv.exe?map=maps/corine.map&service=WCS&version=1.0.0&request=GetCoverage&coverage=Corine&resx=30.0&resy=30.0&crs=EPSG:3035&BBOX=3550000,3413200,3554000,3417200&format=GTiff shifts data:

Mapserver result for non-native, non-multiple resolution

Note:

This request was my initial prompt to find out why all WCS applications shift the data in the results. However this is a false conclusion. What’s happening here is that the service has to extract 30m wide pixels from 100m wide source data, since 100 divided by 30 isn’t a whole number it will add the remaining 10m in an adjacent pixel. If you overlay the result and the source data you get a pixel problem, data of source pixel 2 will most likely end up in result pixel b, which is partly over source pixel 1, resulting in a data shift:

Overlapping 100m with 30m

Conclusion:

Geoserver has a bug feature which offsets all the results by half a pixel, this is a known issue with the definition of the location of a pixel. Added to this there’s the no-data border which appears with non-native, non-multiple requests. I presume that will be gone once the pixel issue is resolved. (update: apparently I’ve misunderstood the Geoserver developers; there’s an issue between pixel placement according to standards and real world implementations, where Geoserver adheres to the standard).

Mapserver doesn’t offset the data unless it is physically impossible (non-native, non-multiple resolutions, extents which don’t snap to source data) but produces a multi-band (this was a configuration issue, thanks to FrankW for spotting this – proper config is IMAGEMODE BYTE) geotiff where the source data is single band.

Deegree has a bug which offsets some of the results, but I don’t know what causes it and although it is resolution-related I don’t see a pattern. It also produces a multi-band geotiff instead of a single band.

Configuration files and source data can be found here.

We’re involved in both ESDIN and EGN and we decided to use the latter as a trial for the first. Together with our partners we’ve setup a series of servers to fulfill the needs of the projects. The main standard used is the latest WFS and GML versions, which have the annoying disadvantage that there are few clients available.

To be able to show nicely (as in not an XML file) that everything worked I was asked to build a webclient which would show a map and the data from the EGN service for that area. I figured that this was once again a good reason to look into the latest developments for OpenLayers. I quickly discovered that the released version (2.7) doesn’t support WFS 1.1.0 so I asked on the mailinglist if people already tried to implement it (and if not, pointers how to do so). Luckily people already did the work and created various patches for the support. (thanks tschaub, bartvde and others)

The most important patch is the one which implements protocols for WFS: ticket 1648 and its wfs.patch The main disadvantage of this patch was in my case that it tried to minimise the number of requests to the WFS server (which in general is a good thing). It requests all the features which are within twice the size of the viewport and it doesn’t request new features when you zoom in. However our server is limited to 10 features per request this results in very interesting behavior. For a starter all the 10 feature could be outside your viewport and also very funny is that shown feature might dissappear when you move the page too much (it requests new features in that case and the first 10 might be outside the viewport). A second patch, on ticket 1830, provided a more aggressive feature update: each zoom action triggers a new request and I set the request-boundingbox-ratio to 1, meaning only those feature within the viewport are requested. This means that every action triggers a new request, which is heavy on the server.

However this is just a reference implementation and hopefully for actual implementations they remove the 10 feature limit. For those interested the reference implementation can be found at http://research.geodan.nl/egn