why does extract by mask only work for some polygons?!?!

Discussion created by aliceduffesriuk-esridist Employee on Jun 12, 2012
Latest reply on Jun 13, 2012 by JSwain-esristaff

I have landcover data for the whole of africa, I want to extract by mask using all of the seperate countries.
So.. when I do this.. some countries work fine, but then others, only a few pixels have been extracted!!

Why? I am using the same raster for all of them.. and the polygons representing the country boundary have all been created from the same polygon!

I am getting desperate now, I do not understand why it doesn't work.

I have closed arcmap and opened it again a million time, removed and re-added the polygons, removed and re-added the raster, there are no NoData points within the raster!!
Please help!!!