If the data has a validity mask as well as a map, the mapped data is also
masked.  When mapping a value datum that has a mask value of zero (invalid), 
the mapping process will first look for an entry in the map that corresponds
to the "invalid" value, if one does not exist (for example, the value is NaN), 
the mapped data is set to the pad value.  In either case, the data remains 
masked.  The mask is resized to correspond to the size of the mapped value 
data to properly mask it.  For example, the value data may have the 
dimensions of 
.nf
	width = 100, 
	height = 50, 
	elements = depth = time = 1.  
.fi
.sp
If the map dimensions are 
.nf
	map width = 3, 
	map height = whatever, 
	map element = map time = map spatial = 1, 
.fi
.sp
the resulting value (and mask) size will be 
.nf
	width = 100, 
	height = 50, 
	elements = 3, 
	depth = time = 1.
.fi
