Rephrasing my question

[Matt Georgescu]

I think I need to rephrase my question as I likely didn't describe myself
well the first time.

I have hi-res AVHRR SST data.  The problem with the data is that it has
gaps:
http://cep.rutgers.edu/~matt1/sst_07week1_95_raw.gif

The undefined values (could be any value, but it happens to take on a
value of 255) are the same over land as they are over water.  Indeed the
continental areas are just 1 really huge gap.

My goal is to fill in the gaps, everywhere (including land), so that no
points remain undefined.  I did this using a weighted average in Fortran
and while the results look decent enough over water, they are poor over
land, and the model I am using is not accepting the data:
http://cep.rutgers.edu/~matt1/sst_070195.gif

(the values over land have been masked using basemap.gs in the figure
above).

What I would like to do is to fill in these gaps (as in the sst_070195
image above) but to have more realistic values over land (which aren't
evident in the image directly above, preferably from another dataset).
The issue is how to tell GrADS to plot 1 thing over water and another
over land (keeping in mind that the initial data has the same undefined
value everywhere, whether over land or water).

Or perhaps there is some other way?

On a related issue I've found and installed the user-define function
called lterp.  I'm assuming and hoping this function may also help me. But
I cannot seem to find any documentation on its usage.
Can someone point me to such a document?

thanks for your help,
matt