[gradsusr] invalid grid coordinates error: help

[Miles Bliss]

Hello all,
             The following is the error message that i am receiving fro
GrADS. As i am attempting to build an average picture, so to say, of upper
air fields for all my cases as can be found in the attached file.

Data Request Warning:  Request is completely outside file limits
Data Request Warning:  Request is completely outside file limits
Data Request Error: Invalid grid coordinates
  World coordinates convert to non-integer  grid coordinates
    Variable = hgtprs.18  Dimension = 3
  Error ocurred at column 466
DISPLAY error:  Invalid expression
  Expression =
((hgtprs+hgtprs.2(time=00z18jan2002)+hgtprs.3(time=00z11jan2003)+hgtprs.4(time=00z07dec2005)+hgtprs.5(time=00z04dec2007)+hgtprs.6(time=00z08feb2007)+hgtprs.7(time=00z29dec2009)+hgtprs.8(time=00z20feb2009)+hgtprs.9(time=00z07dec2010)+hgtprs.10(time=00z01feb2010)+hgtprs.11(time=00z16jan2011)+hgtprs.12(time=00z01jan2013)+hgtprs.13(time=00z12dec2013)+hgtprs.14(time=00z19nov2014)+hgtprs.15(time=00z06jan2014)+hgtprs.16(time=00z29jan2014)+hgtprs.17(time=00z06feb2014)+hgtprs.18(time=00z08feb2014)+hgtprs.19(time=00z30dec2014))/19)

I have checked using the 'q dims' function the time of each file listed in
the script and each is the appropriately valued T index.
Im not sure where to progress from here, so any help would be great.
-- 
-Miles Bliss
 SUNY Brockport '17
 Meteorology
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