extraction from raster data by mask

Discussion created by eastrobl on Jul 27, 2011
Latest reply on Aug 2, 2011 by sharkawi11

I am extracting from a raster data set by mask with a polygon shape file as the mask - but the strange thing is that the masked raster data seems to have changed the values of raster data.  For example, I know that within this polygon that there are many zero values, but these all disappear when I mask out the area with the polygon. Can anybody tell me what might be happening here?   (by the way, a friend of mine seems to be doing the same thing with the same data, but not does not have the problem).