Trick serving a hillshade image from GeoServer

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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 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.
Hillshade in QGIS

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.
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:

 

 

Geospatial Power Tools Reviews [Book]

Thinking of buying my latest book?  We’ve finally got a few reviews on Amazon that might help you decide.  See my other post for more about the book.  Buy the PDF on Locate Press.com.

Reader Reviews

Geospatial Power Tools book cover
From Amazon.com

5.0 out of 5 stars This book makes a great reference manual for using GDAL/OGR suite of command line …,
January 24, 2015 By Leo Hsu
“The GDAL Toolkit is chuckful of ETL commandline tools for working with 100s of spatial (and not so spatial data sources). Sadly the GDAL website only provides the basic API command switches with very few examples to get a user going with. I was really excited when this book was announced and purchased as soon as it came out. This book makes a great reference manual for using GDAL/OGR suite of command line utilities.
Several chapters are devoted to each commandline tool, explaining what its for, the switches it has, and several examples of how to use each one. You’ll learn how to work with both vector/(basic data no vector) data sources and how to convert from one vector format to another. You’ll also learn how to work with raster data and how to transform from one raster data source to another as well as various operations you can perform on these.”

 

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