Defining NoData pixels in a raster PLEASE HELP

Discussion created by Cyndy on Jul 12, 2011
Latest reply on Feb 18, 2012 by medhimintu
Hi folks -

I have the need to process a set of Geotiff rasters outside of the ArcGIS environment, and then port them back in.  I am having extreme difficulties with 'NoData' pixels.  I have input rasters  created in ArcGIS and that contain many NoData values.  These NoData values must remain distinguishable from, say, 0 value pixels. (My input rasters all contain float values).  I import these rasters to IDL for numerous calculations.  When saving newly created output rasters, I attempt to define the NoData values as 'NaN' in IDL; but this does not work when my output rasters return to ArcGIS.  I also have attempted to read in the ArcGIS "NoData' pixel value - the exact bit pattern -  and then rewrite it in my created rasters, to FORCE them to work as NoData in ArcGIS - and THIS does not work!  I feel there must be something that defines the ARcGIS 'NoData' values which I am not aware of and cannot set.  What am I missing?

Alternatively, is there a way to take a raster, and set, say, flagged pixel values (ie, '-9999') to NoData values and NOT change anything else in the raster. 

Many thanks for any advice.