I am trying to make a map with PostGIS and GeoServer, showing weather data. The data that I have is pointdata from observation stations.
The data is vectordata. With an SLD using the Barnes Surface rendering transformation I get the following WMS output (OpenLayers preview): 
Now i want to add contour lines. Of course that is a Raster-to-Vector rendering transformation. I read that it should be possible to use the Barnes Surface transformation as input for my contour transformation. But I cannot find how to do this. There is very little documentation about this.
I only found this question: Chaining Rendering Transformations: Making isotherms using Barnes Surface + Contour process?, but the site that is referenced is not even online anymore and there is not really a clear answer.
The SLD documentation says that it is possible to use multiple rendering transformations, but I do not know how to code it. Can anybody help with that? What am I doing wrong?
Below the SLD code that I am working with now:
<?xml version="1.0" encoding="ISO-8859-1"?>
<StyledLayerDescriptor version="1.0.0"
xsi:schemaLocation="http://www.opengis.net/sld StyledLayerDescriptor.xsd"
xmlns="http://www.opengis.net/sld"
xmlns:ogc="http://www.opengis.net/ogc"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<NamedLayer>
<Name> BarnesContours </Name>
<UserStyle>
<Title> Barnes Surface Contours </Title>
<Abstract> Extracts from a computed surface </Abstract>
<FeatureTypeStyle>
<Transformation>
<ogc:Function name="vec:BarnesSurface">
<ogc:Function name="parameter">
<ogc:Literal>data</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>valueAttr</ogc:Literal>
<ogc:Literal>ff</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>scale</ogc:Literal>
<ogc:Literal>15.0</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>convergence</ogc:Literal>
<ogc:Literal>0.2</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>passes</ogc:Literal>
<ogc:Literal>3</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>minObservations</ogc:Literal>
<ogc:Literal>1</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>maxObservationDistance</ogc:Literal>
<ogc:Literal>10</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>pixelsPerCell</ogc:Literal>
<ogc:Literal>10</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>queryBuffer</ogc:Literal>
<ogc:Literal>40</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>outputBBOX</ogc:Literal>
<ogc:Function name="env">
<ogc:Literal>wms_bbox</ogc:Literal>
</ogc:Function>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>outputWidth</ogc:Literal>
<ogc:Function name="env">
<ogc:Literal>wms_width</ogc:Literal>
</ogc:Function>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>outputHeight</ogc:Literal>
<ogc:Function name="env">
<ogc:Literal>wms_height</ogc:Literal>
</ogc:Function>
</ogc:Function>
</ogc:Function>
</Transformation>
<Rule>
<RasterSymbolizer>
<!-- specify geometry attribute of input to pass validation -->
<Geometry><ogc:PropertyName>point</ogc:PropertyName></Geometry>
<Opacity>0.8</Opacity>
<ColorMap type="ramp" >
<ColorMapEntry color="#FFFFFF" quantity="-990" label="nodata" opacity="0"/>
<ColorMapEntry color="#2E4AC9" quantity="-9" label="values"/>
<ColorMapEntry color="#41A0FC" quantity="-6" label="values" />
<ColorMapEntry color="#58CCFB" quantity="-3" label="values" />
<ColorMapEntry color="#76F9FC" quantity="0" label="values" />
<ColorMapEntry color="#6AC597" quantity="2" label="values" />
<ColorMapEntry color="#479364" quantity="4" label="values" />
<ColorMapEntry color="#2E6000" quantity="6" label="values" />
<ColorMapEntry color="#579102" quantity="8" label="values" />
<ColorMapEntry color="#9AF20C" quantity="10" label="values" />
<ColorMapEntry color="#B7F318" quantity="12" label="values" />
<ColorMapEntry color="#DBF525" quantity="14" label="values" />
<ColorMapEntry color="#FAF833" quantity="16" label="values" />
<ColorMapEntry color="#F9C933" quantity="18" label="values" />
<ColorMapEntry color="#F19C33" quantity="20" label="values" />
<ColorMapEntry color="#ED7233" quantity="22" label="values" />
<ColorMapEntry color="#EA3F33" quantity="24" label="values" />
<ColorMapEntry color="#BB3026" quantity="40" label="values" />
</ColorMap>
</RasterSymbolizer>
</Rule>
</FeatureTypeStyle>
<FeatureTypeStyle>
<Transformation>
<ogc:Function name="ras:Contour">
<ogc:Function name="parameter">
<ogc:Literal>data</ogc:Literal>
</ogc:Function>
<ogc:Function name="parameter">
<ogc:Literal>ff</ogc:Literal>
<ogc:Literal>-5</ogc:Literal>
<ogc:Literal>0</ogc:Literal>
<ogc:Literal>5</ogc:Literal>
<ogc:Literal>10</ogc:Literal>
<ogc:Literal>15</ogc:Literal>
<ogc:Literal>20</ogc:Literal>
<ogc:Literal>25</ogc:Literal>
<ogc:Literal>30</ogc:Literal>
</ogc:Function>
</ogc:Function>
</Transformation>
<Rule>
<Name>rule1</Name>
<Title>Contour Line</Title>
<LineSymbolizer>
<Stroke>
<CssParameter name="stroke">#000000</CssParameter>
<CssParameter name="stroke-width">1</CssParameter>
</Stroke>
</LineSymbolizer>
<TextSymbolizer>
<Label>
<ogc:PropertyName>value</ogc:PropertyName>
</Label>
<Font>
<CssParameter name="font-family">Arial</CssParameter>
<CssParameter name="font-style">Normal</CssParameter>
<CssParameter name="font-size">10</CssParameter>
</Font>
<LabelPlacement>
<LinePlacement/>
</LabelPlacement>
<Halo>
<Radius>
<ogc:Literal>2</ogc:Literal>
</Radius>
<Fill>
<CssParameter name="fill">#FFFFFF</CssParameter>
<CssParameter name="fill-opacity">0.6</CssParameter>
</Fill>
</Halo>
<Fill>
<CssParameter name="fill">#000000</CssParameter>
</Fill>
<Priority>2000</Priority>
<VendorOption name="followLine">true</VendorOption>
<VendorOption name="repeat">100</VendorOption>
<VendorOption name="maxDisplacement">50</VendorOption>
<VendorOption name="maxAngleDelta">30</VendorOption>
</TextSymbolizer>
</Rule>
</FeatureTypeStyle>
</UserStyle>
</NamedLayer>
</StyledLayerDescriptor>
The first part of the code works! The result of that is the image shown above. But the second part, the contour part, is not working. If I try to preview it on the layer I get the following message: 
I think the way I am doing this is wrong, but there is not really a good example online that shows how it should be done. Or at least I have not found that yet..
