In ?kernelUD, the "grid" argument is described as:
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grid: a number giving the size of the grid on which the UD should
be estimated. Alternatively, this parameter may be an object
inheriting the class ?SpatialPixels?. For the function
?kernelUD?, it may in addition be a list of objects of class
?SpatialPixels?, with named elements corresponding to each
level of the factor id.
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I'm not sure what is meant by the "size of the grid". Is this the
number of cells in the output grid, and then cell dimensions are
calculated accordingly?