[R-sig-Geo] Question re: Intamap package / Interpolate method: Inputs NOT both Spatial Points Data Frame?

[Rick Reeves]

Hello:

I note a conflict in the documentation for Intamap/interpolate(), need 
to know how to resolve:

I have two SpatalPointsDataFrame objects:

  growingDegreeObservations <- readShapePoints("OhioGDDStationsGP.shp")  
# computed surface phenomena, for individual lat/lon points
  OrgStaLocs <- readShapePoints("Org_Locations.shp")                    
# species observation stations, for DIFFERENT lat/lon points

# Objective: Interpolate the growingDegree points into a SpatialGrid (?) 
raster field, and then
# overlay the observation stations on to the field, and assign field 
values to each observation station points

# according to intamap documentation, the interpolate() method accepts 
two SptatialPointsDataFrame objects, interpolates
# the first ($value field) into a grid, and assigns interpolated 
observations to the predicted locations (monarchStaLocs).

interpImage <- interpolate(growingDegreeObservations 
,OrgStaLocs,list(mean=TRUE, variance=TRUE))

However, when this line is executed, the error occurs:

Error in function (classes, fdef, mtable)  :
   unable to find an inherited method for function "interpolate", for 
signature "SpatialPointsDataFrame"

I guess that this refers to the first argument, and that the argument 
must be a SpatialPixels or SpatialGrid data frame.
Questions:

1) Do I understand the inputs and outputs of interpolate() correctly?

2) How best to transform the incorrect (SpatialPointsDataFrame) argument
      to match the required inputs? E.G., create the interpolated 
SpatialPixelsDataFrame
      of observations before calling interpolate())

3) Is there a better R technique (e.g., direct call to krige()) for 
computing my solution?

Thanks, Rick R