Sunday, November 24, 2013

Fast tip: Drawing raster NO DATA values with MapServer

MapServer is a great way to draw web maps, using it either as a WMS server or creating image files directly.
Is the raster has a No Data value, MapServer reads it from the GDAL library and doesn't draw the pixels with the value. But what happens if you want to draw the pixels with No Data values?

Fast answer 

The solution, that I found here, is setting the LAYER property PROCESSING in this way:

which disables the regular MapServer behaviour, which is, quite logically,  not drawing the pixels with the NODATA value.
Once this is done, just create a CLASS with the No Data Value  ¡in the EXPRESSION tag, so the pixels are coloured as you prefer.

Why did I need this stuff

Maybe this is not a common need, but when drawing weather radar images, is a good idea to indicate the areas not covered by the radar in some colour, so it's easy to distinguish between zones where there is no rain from the zones where no data is available. Like in these examples:

Catalan Weather Service radar with the range visible in gray
 The Catalan Weather Service radar in La Panadella with the range visible in gray, using the no data values
The Spanish Agency radar network, with the not covered areas in gray.

Strangely, finding the solution was quite difficult, so I put it in this post. Maybe somebody, someday, will find faster than me.

Some links: