Hi Hao Bo,
I suspect your problem is caused by "nRows = 1". Your grid
is probably really the product of numbers of rows and columns
both significantly larger than 1. You need to use these values,
rather than setting nRows = 1 and nCols = the number of points
in your grid. Rather, you need to factor the number of grid
points into nRows * nCols where both are > 1.