15.22.2  CHECK 

CHECK=OFF  turns off data set checking.  When CHECK=ON both data sets must
have identical X (and Y if mesh) values, but data set n2 may contain  more
points  than  n1.   If  (DX/DY) is non zero then the both DX and X must be
identical within 1% of DX.  If you  have  data  sets  with  non  identical
values  of  X  you  may  create  a set with identical values using the BIN
command.  If the X values of 2 data sets are not quite identical.  You may
force  TOPDRAWER  to  divide  them  by  setting  DX for the data 100 times
greater than the difference in X or by setting CHECK=OFF