SLD Polygon Styling: Add Point feature to Northern Edge?

Question

I am trying to create an SLD to style a polygon; the goal is a simple block fill along with a single point image added to the most northern edge of the polygon. Something like this:

This example SLD code will add a symbol to the start point of a polygon:

<PolygonSymbolizer>
<Fill>
    <CssParameter name="fill">#000000</CssParameter>
</Fill>
</PolygonSymbolizer>
<PointSymbolizer>
    <Geometry>
        <ogc:Function name="startPoint">
            <ogc:PropertyName>the_geom</ogc:PropertyName>
        </ogc:Function>
     </Geometry>   
    <Graphic>
        <Mark>
            <WellKnownName>square</WellKnownName>
        <Fill>
            <CssParameter name="fill">#00FFFF</CssParameter>
        </Fill>                    
        </Mark>
        <Size>5</Size>
    </Graphic>
</PointSymbolizer>  

However I can't work out how to show the point symbol to the most northern edge of the polygon?

One more thing I tried: Using QGIS I can create styles and export to SLD, and using the geometry generator as follows: closest_point( $geometry, make_point( x($geometry), y( $geometry)+20000)) This works. However when exporting this to SLD it fails due to 'not implemented yet'.

Answer 1

This is similar (but slightly harder) than a previous question - find the centre of the polygon is easy, and extracting X is OK:

 <Geometry>
  <ogc:Function name="geomFromWKT">
   <ogc:Function name="Concatenate">
    <ogc:Literal>POINT(</ogc:Literal>
    <ogc:Function name="getX"> 
      <ogc:Function name="centroid">
         <ogc:PropertyName>the_geom</ogc:PropertyName>
      </ogc:Function>
    </ogc:Function>
    <Literal><![CDATA[ ]]></Literal>
     <!-- Extract 1/2 height here -->
      <ogc:Function name="envelope">
         <ogc:PropertyName>the_geom</ogc:PropertyName>
      </ogc:Function>
    </ogc:Function>
     <Literal>)</Literal>

    </ogc:Function>
      </ogc:Function>

 </Geometry>   

I can't see a way to extract half the height of the polygon here (and that assumes that all your polygons are rectangular). I think at this point the best plan is to write a custom geotools function to generate the point you require from a polygon.

Answer 2

Thank you very much for your assistance, and for pointing me in the right direction! With the use of the geoserver function reference (here) I think I have cracked it. But it will only be valid for the case of rectangular polygons. It's not pretty, but it does work:

<PolygonSymbolizer>
    <Fill>
        <CssParameter name="fill">#000000</CssParameter>
    </Fill>
</PolygonSymbolizer>
<PointSymbolizer>
    <Geometry>
        <ogc:Function name="offset">
            <ogc:Function name="centroid">
                <ogc:PropertyName>the_geom</ogc:PropertyName>
            </ogc:Function>
            <ogc:Literal>0.0</ogc:Literal>
            <ogc:Sub>
                <!-- distance between far north point and centroid -->
                <ogc:Function name="distance">
                    <ogc:Function name="offset">
                        <ogc:Function name="centroid">
                            <ogc:PropertyName>the_geom</ogc:PropertyName>
                        </ogc:Function>
                        <ogc:Literal>0.0</ogc:Literal>                                            
                        <ogc:Literal>99999.0</ogc:Literal>                                            
                    </ogc:Function>
                    <ogc:Function name="centroid">
                        <ogc:PropertyName>the_geom</ogc:PropertyName>
                    </ogc:Function>  
                </ogc:Function>
                <!-- distance between far north point and polygon -->
                <ogc:Function name="distance">
                    <ogc:Function name="offset">
                        <ogc:Function name="centroid">
                            <ogc:PropertyName>the_geom</ogc:PropertyName>
                        </ogc:Function>
                        <ogc:Literal>0.0</ogc:Literal>                                            
                        <ogc:Literal>99999.0</ogc:Literal>                                            
                    </ogc:Function>                                                
                    <ogc:PropertyName>the_geom</ogc:PropertyName>
                </ogc:Function>
            </ogc:Sub>
        </ogc:Function>                                        
    </Geometry>   
    <Graphic>
        <Mark>
            <WellKnownName>square</WellKnownName>
            <Fill>
                <CssParameter name="fill">#00FFFF</CssParameter>
            </Fill>                    
        </Mark>
        <Size>5</Size>
    </Graphic>
</PointSymbolizer>