Vishful thinking…

Reversed co-ordinate axis order for EPSG:4326 vs CRS:84 when requesting WMS 1.3.0 images

Posted in ArcGIS, ESRI, GIS by viswaug on March 15, 2009

I spent quite a bit of time last week over this and thought it might help others who might encounter the same thing as I did. I was requesting images from the WMS service version 1.3.0 in ArcGIS Server and everything was fine as long I was requesting images with the “CRS:84” value for the co-ordinate reference system. According to the GetCapabilities document returned from the WMS Server, both the “CRS:84” and the “EPSG:4326” values are suported by the WMS server.

http://sampleserver1.arcgisonline.com/ArcGIS/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/WMSServer?VERSION=1.3.0&REQUEST=GetCapabilities

But when I started requesting images with “EPSG:4326”, I was seeing some really weird behaviour. I spent quite a bit of time trying to figure out what was going on without luck. The OGC team at ESRI helped me out and pointed out that when using WMS 1.3.0, the co-ordinate axis order in reversed for CRS:84 and EPSG:4326. So, when requesting images in CRS:84 use the coordiate axis order of (long, lat) and when using EPSG:4326 use the (lat, long) order.

http://sampleserver1.arcgisonline.com/ArcGIS/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/WMSServer?VERSION=1.3.0&REQUEST=GetMap&CRS=CRS:84&BBOX=-178.217598,18.924782,-66.969271,71.406235&WIDTH=765&HEIGHT=360&LAYERS=0,1,2&STYLES=,,pointSymbolizer&EXCEPTIONS=application/vnd.ogc.se_xml&FORMAT=image/png&BGCOLOR=0xFFFFFF&TRANSPARENT=TRUE&SLD=http://sampleserver1.arcgisonline.com/arcgis/wms/slds/point_pointSymbolizer.xml

http://sampleserver1.arcgisonline.com/ArcGIS/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/WMSServer?VERSION=1.3.0&REQUEST=GetMap&CRS=EPSG:4326&BBOX=18.924782,-178.217598,71.406235,-66.969271&WIDTH=765&HEIGHT=360&LAYERS=0,1,2&STYLES=,,pointSymbolizer&EXCEPTIONS=application/vnd.ogc.se_xml&FORMAT=image/png&BGCOLOR=0xFFFFFF&TRANSPARENT=TRUE&SLD=http://sampleserver1.arcgisonline.com/arcgis/wms/slds/point_pointSymbolizer.xml

But when requesting images from WMS Server version 1.1.1 with EPSG:4326, we should still use the coordinate axis order of (long, lat)

http://sampleserver1.arcgisonline.com/ArcGIS/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/WMSServer?VERSION=1.1.1&REQUEST=GetMap&SRS=EPSG:4326&BBOX=-178.217598,18.924782,-66.969271,71.406235&WIDTH=765&HEIGHT=360&LAYERS=0,1,2&STYLES=,,pointSymbolizer&EXCEPTIONS=application/vnd.ogc.se_xml&FORMAT=image/png&BGCOLOR=0xFFFFFF&TRANSPARENT=TRUE&SLD=http://sampleserver1.arcgisonline.com/arcgis/wms/slds/point_pointSymbolizer.xml

Do these standards really help when the standards are not really STANDARD?

Then comes the issue of versioning of these standards, especially the OGC standards. The OGC standards are so reliant on one another that the different versions that exist in each standard actually adds a lot more complexity to systems that actually use them. For example, the SLD 1.1.0 standard (Styled Layer Descriptor) depends on the FE 1.1.0 standard (Filter Encoding), the SE 1.1.0 standard (Symbology Encoding) and the GML 3.1.1 standard (Geography Markup Language). As and when the standards evolve and higher versions are created, the versions of the standards they in-turn depend on change making a big mess for the systems that actually have to implement these standards.

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