MS RFC 30: Support for WMS 1.3.0¶
dmorissette at mapgears.com
yassefa at dmsolutions.ca
- Last Edited:
This RFC documents the changes required in order to upgrade MapServer’s OGC WMS support to version 1.3.0 of the specification.
MapServer already includes mechanisms to support multiple WMS versions (and already supports WMS versions 1.0.0, 1.1.0 and 1.1.1) so in theory this upgrade should be straightforward and shouldn’t require a RFC. Unfortunately, WMS 1.3.0 contains some tricky changes that while they are not exactly backwards incompatible are likely to make the life of users of WMS miserable.
This RFC is mostly to document those changes and the way MapServer deals with them.
Coordinate Systems and Axis Orientation¶
The main issue introduced by WMS 1.3.0 is the change in the way it handles axis order for several SRS. This has an impact on the way the BBOX is specified in WMS requests and in Capabilities documents and in how the CONNECTIONTYPE WMS code interacts with remote servers.
In previous versions of WMS, for any SRS the first axis was the easting (x or lon) and the second axis was the northing (y or lat). Starting with WMS 1.3.0, some SRS such as the very popular EPSG:4326 have their axis reversed and the axis order becomes lon, lat instead of lat, lon. This change in WMS 1.3.0 was done in order to align with the definitions from the EPSG database (a requirement to make WMS an ISO specification).
This change is sure to confuse simple clients that used to treat all SRS the same way. MapServer and PROJ will need to be extended to carry information about the axis order of all EPSG SRS codes and treat them using the correct axis order.
New CRS codes such as CRS:xxxx and AUTO2:xxxx have also been added by WMS 1.3.0 that will need to be supported by MapServer. Note the wo types of Layer CRS identifiers are supported by the specification: “label” and “URL” identifiers. The intention is to support at this point the label types CRS.
List of CRS planned to be supported are:
CRS:84 (WGS 84 longitude-latitude)
CRS:83 (NAD83 longitude-latitude)
CRS:27 (NAD27 longitude-latitude).
AUTO2:42001 AUTO2:42002 AUTO2:42003 AUTO2:42004 AUTO2:42005
CRS:1 (pixel coordinates) would not be supported at this point (there is a ticket discussing this issue https://github.com/MapServer/MapServer/issues/485)
EPSG codes: when advertising (such as BoundingBox for a layer elemnt) or using a CRS element in a request such as GetMap/GetFeatureInfo, elements using epsg code >=4000 and <5000 will be assumed to have a reverse axes.
All the above need to be done in a way that allows continued support for older versions of the WMS specification (1.0.0 to 1.1.1) and will have the least impact on existing WMS services.
Implementation for the reverse axis will follow closely to what was already done for WCS1.1 support (https://mapserver.org/development/rfc/ms-rfc-41.html)
WMS and SLD¶
All references to SLD support has been removed from the WMS 1.3.0 specifications. It has been replaced by two specifications:
Styled Layer Descriptor profile of the Web Map Service
Symbology Encoding Implementation Specification
The Styled Layer Descriptor profile allows:
to extend the WMS to support additional operations (DescribeLayer, GetLegendGraphic)
additional parameters related to the SLD for the GetMap support
advertise SLD support.
This specification will be supported in the current implementation. Note that the GetStyles operation (available in WMS 1.1.1) might not be supported in the first phase
The Symbology Encoding Implementation represent basically the definitions for the different symbolizers. We need to upgrade the current SLD support to support this specification.
HTTP Post support¶
HTTP Post support is optional and currently supported by MapServer WMS 1.1.1. WMS 1.3.0 defines that if POST is supported, the the request message is formulated as an XML document. Although this is highly deirable, the first implementation of the WMS 1.3.0 might not support the XML Post requests.
OCG compliance tests¶
The OGC Compliance and Interoperability Testing Initiative (CITE) http://cite.opengeospatial.org/test_engine/wms/1.3.0/ provides automatic tests to validate the implementation. The short term intention is to use this service as a first validation tool. The longer term goal is to have MapServer WMS 1.3.0 fully compliant.
GetCapabilities advertises only the text/xml format. During a request we do not parse the optional Format parameter. It is always set to text/xml
None. This affects only the WMS server interface and WMS CONNECTION type.
Backwards compatibility issues¶
The change in the way the axis order is handled is likely to cause lots of confusion.
Passed with +1 from: TomK, Woodbridge, Assefa, Morissette
Questions/Comments from the review period¶
Q: Can libxml2 be used to generate XML responses to continue the work started in mapowscommon.c?
A: I’ll keep libxml2 in mind during the implementation, but I do not plan to refactor and risk breaking any code to convert it to libxml2 as part of this upgrade.
Since WMS 1.3.0 doesn’t implement OWS common, it won’t benefit from any of the code that’s already using libxml2. It will actually mostly reuse existing printf-based code that’s already well tested and working. I think the right time to switch to libxml2 for WMS would be when it will support OWS common and then there will be real benefits by reusing functions from mapowscommon.c.