Blank certain areas of a contour plot

Hi,

I'm interpolating a list of syncronous accumulated precipitation 
observations collected over a number of raingauge stations sited over 
land, over a regular lat/lon grid using akima's interp().

Then, I plot and locate geographycally the resulting field with a 
filled.contour() and a call to map(). Everything is fine but I need to 
"blank" (in Golden Surfer nomenclature) the areas of my domain that lay 
over the sea, as I do not have observations there and structures are 
completely artificial.

Is there any function out there that can use the mapsdata database to 
"blank" the sea areas of my plot?
Otherwise, can anybody think of a shortcut to get a similar effect on 
the final map (not showing the interpolated precipitation areas over the 
sea?

Thanks a lot in advance,

V?ctor.

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V?ctor Homar Santaner
Grup de Meteorologia

Edif. Mateu Orfila                      Tel: +34 971 17 1376
Universitat de les Illes Balears        Fax: +34 971 17 3426
07122 Palma de Mallorca (SPAIN)         Email: Victor.Homar at uib.es

              Knowledge is contagious. Infect truth.