Overlaying SpatialGridDataFrames over SpatialGridDataFrames?

Hello list,  

I've run into a baffling problem, and I hope someone can help.

The last few months I have been successfully overlaying a SpatialGridDataFrame object over a SpatialGridDataFrame object. Here's the code I have been using to do this:

        sPDF_grid <- overlay(sGDF_in, sGDF_ref)
gridded(sPDF_grid) <- TRUE
sGDF_grid <- as(sPDF_grid, "SpatialGridDataFrame")


Today, trying to run this code again, I run into an error telling me that no method exists for overlaying an sGDF object over an sGDF object. This seems true, when I look at the documentation.

What I have been doing is converting "sGDF_in" to "sGDF_grid", with grid definitions matched to "sGDF_ref". All of the sGDF objects have the same grid sizes (1?? x 1??), but are different in the longitude range.

I'm not exactly sure what change prompted the "overlay" function to start acting differently, let alone why things worked before. I think the code originally came from some examples at a blog or a vignette, but I can't remember anymore.

I've just tried updating to the latest R version (2.15.0, 64bit) from 2.14.1(I think), and updated to the latest sp packages, but same problems. I'm on Mac (OSX Lion, 10.7.3, 2.26 GHz Intel C2D, 4GB ram).

Any ideas as to what might be going on?

Thanks,

Jooil

--  
Jooil Kim
Postdoc Fellow
Scripps Institution of Oceanography, UC San Diego
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