[gradsusr] Problem with grid points on map projection.

[Cypresslin]

Hello,
I got a .ctl file with PDEF settings, using Lambert Conformal projection.
The data file contains 270*300 dots, there are about 550*530 dots
after the Automatic Grid Interpolation by GrADS

Now I want to place a mark on a given grid point, but I only know the
point on the grid BEFORE the automatic grid interpolation taken place
(ie. i = 120, j = 200 on 270*300).

How should I do to place a mark on the interpolated grid? (550*530)

Thanks!