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I had the perfect hillshade raster ready to throw on top of my web map, I had previewed it in QGIS and the colour stretching and values looked fine.

I then loaded it into GeoServer and it looked great in the OpenLayers Layer Preview window as well.

However, once I added it as a WMS layer in a Leaflet map, I got really confused. Even though I had my NODATA=255 parameters seemingly working well there appeared a small strip around the outside of the image that was obnoxious.

When GeoServer skews the image it creates a region around the image that has value=0. This image already had nodata=255, so it can be very confusing.

I figured I could change it with an SLD but hadn’t done it before so it took a while to figure out. For a couple hours I tried various permutations of the default “raster” Style, but with no luck. The image, otherwise, looked great so the Style obviously wasn’t a real problem in and of itself.

The trick came when I realised that I need to set my color maps for the gray band explicitly. Up to this point no particular band had been isolated for color mapping. I found the right place to put the elements in the SLD and all worked out well.

My takeaway: there must be a better way to develop SLD for rasters. The options in the GeoServer docs are amazing and exciting, but trying to fit them into a specific context has always been a challenge for me. That’s why I had to write this down, but I’m sure I’ll have to remember it all again a year from now!

Note there is no skewed "collar" around the image as value=0 is ignored. — Note there is no skewed “collar” around the image as value=0 is ignored.

My SLD:

<?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>default_hillshade</Name>
    <UserStyle>
      <Title>Raster Hillshade</Title>
      <Abstract>Grayscale hillshade SLD</Abstract>
      <FeatureTypeStyle>
        <Rule>
          <Name>rule1</Name>
          <Title>Hillshade with transparency</Title>
          <Abstract>Ignore values outside valid range, nodata values transparent</Abstract>
          <RasterSymbolizer>
          	<ChannelSelection>
            	<GrayChannel>
                	<SourceChannelName>1</SourceChannelName>
               	</GrayChannel>                          
            </ChannelSelection>
	        <ColorMap>
            	<ColorMapEntry color="#000000" quantity="1" label="low" opacity="1"/>
                <ColorMapEntry color="#FFFFFF" quantity="254" label="high" opacity="1"/>
                <ColorMapEntry color="#FFFFFF" quantity="255" label="alpha" opacity="0"/>     
			</ColorMap>
          </RasterSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

<?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">

<Name>default_hillshade</Name>

<Title>Raster Hillshade</Title>

<Abstract>Grayscale hillshade SLD</Abstract>

<Rule>

<Title>Hillshade with transparency</Title>

<Abstract>Ignore values outside valid range, nodata values transparent</Abstract>

</GrayChannel>

</ChannelSelection>

</ColorMap>

</RasterSymbolizer>

</Rule>

</FeatureTypeStyle>

</UserStyle>

</NamedLayer>

</StyledLayerDescriptor>

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graychannel

Trick serving a hillshade image from GeoServer

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