Support for topological ordering of curves

[ttho]

Add "sorting algorithm" to SortProperty like below.

Open issues:
How to order disconnected strings (e.g. random?)
Other algorithms? e.g. North to South and then East to West?
Support for other geometry types?

  <Query typeNames="aixm:RouteSegment">
      <fes:Filter>
      ...
      </fes:Filter>
      <fes:SortBy>
         <fes-te:SortProperty>
            <fes:ValueReference>aixm:curveExtent</fes:ValueReference>
            <fes:SortOrder>ASC</fes:SortOrder>
            <fes-te:Algorithm>TOPOLOGICAL</fes-te:Algorithm>
         </fes-te:SortProperty>
      </fes:SortBy>
   </Query>

Add this to WFS-TE for now, but contact SWG when done, because it's not temporality-specific.

[ttho]

Other sort algorithms could be LONGITUDE_LATITUDE and LATITUDE_LONGITUDE, which can be applied to points only, meaning: points are ordered by their numerical value of the longitude first, then latitude (1). However, this approach is not feasible, as its extensibility is poor. Longitude and latitude are not defined for a lot of projected SRSs, and there may be more than two axes, e.g. elevation.

Preferred alternative:
The ordinate extraction is done in the value reference by custom XPath functions like this:

<fes:ValueReference>geo:latitude(geo:start(curveExtent))</fes:ValueReference>

Here, geo:start() extracts the starting point of a curve geometry. Alternatives: geo:end() or geo:middle().

If the order is latitude, then longitude, multiple SortProperty have to be combined which is already supported by the WFS/FES standard.

If spatial objects of different SRSs are to be compared, a coordinate transformation has to be applied first. This could also be modeled by a custom XPath function:

<fes:ValueReference>geo:latitude(geo:transform(geo:start(curveExtent), "urn:ogc:def:crs:EPSG::4326"))</fes:ValueReference>

Complete example:
"Order [the route segments] topologically, then by their WGS84 longitude value and then by their WGS84 latitude value":

<Query typeNames="aixm:RouteSegment">
      <fes:Filter>
      ...
      </fes:Filter>
      <fes:SortBy>
         <fes:SortProperty>
            <fes:ValueReference>aixm:curveExtent</fes:ValueReference>
            <fes:SortOrder>ASC</fes:SortOrder>
            <fes:Algorithm>TOPOLOGICAL</fes:Algorithm>
         </fes:SortProperty>
         <fes:SortProperty>
            <fes:ValueReference>geo:longitude(geo:transform(geo:start(curveExtent), "urn:ogc:def:crs:EPSG::4326"))</fes:ValueReference>
            <fes:SortOrder>ASC</fes:SortOrder>
         </fes:SortProperty>
         <fes:SortProperty>
            <fes:ValueReference>geo:latitude(geo:transform(geo:start(curveExtent), "urn:ogc:def:crs:EPSG::4326"))</fes:ValueReference>
            <fes:SortOrder>ASC</fes:SortOrder>
         </fes:SortProperty>
      </fes:SortBy>
   </Query>

Open questions:
How to extract elevation? geo:elevation() - only defined for 2.5D curves and surfaces? How to define the elevation SRS?

Is there previous work in that area, e.g. in GeoAPI?

[porosnie]

Issue raised to OGC: http://ogc.standardstracker.org/show_request.cgi?id=436