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Default method for extracting 'z values' looks for $Zm, $Z, $z in that order.
# S3 method for default zvals(obj, ...)
an object
additional arguments
the x values
xvals, yvals, zvals, xvals.default, yvals.default, xvals.fromXYZ, yvals.fromXYZ, zvals.fromXYZ, xvals.SpatialGridDataFrame, yvals.SpatialGridDataFrame, zvals.SpatialGridDataFrame