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selection of data subsets in spatial classes

3 messages · karl.sommer at dpi.vic.gov.au, Roger Bivand, Edzer Pebesma

Original
karl.sommer at dpi.vic.gov.au
# 
Hello list

I have been trying to figure out if it is possible to select a subset of
data in a SpatialGridDataFrame without prior conversion to a data.frame
using the method as.data.frame().

I have imported an ESRI " *. bil " file containing different spectral bands
using rgdal.  The file imported as a SpatialGridDataFrame.  The data
originates from an aerial photograph of a row crop and I would like to
select pixels from within a row as opposed to those from the inter-row
space for further interpolation.   Inter-row pixels have a different
signature and therefore may be potentially screened out.  However, I have
been unable to access the SpatialGridDataFrame directly with, for example,
a  subset(x, band5 < lowerLimit, select c(a, b) ) call for selecting values
according to given criteria.

Am I using the wrong strategy, should I just demote the spatial class using
method as.data.frame(), do the manipulation and then promote the screened
values back to a spatial class?

Regards

Karl
_________________________________
Karl J Sommer,
Department of Primary Industries,
PO Box 905
Mildura, VIC, Australia 3502

Tel: +61 (0)3 5051 4390
Fax +61 (0)3 5051 4534

Email:     karl.sommer at dpi.vic.gov.au
Roger Bivand
#
On Wed, 7 Jun 2006 karl.sommer at dpi.vic.gov.au wrote:

            

      
      
      
    

        
This is what the SpatialPixelsDataFrame class is for (unless you want to 
change resolution too):

library(rgdal)
SP27GTIF <- readGDAL(system.file("pictures/SP27GTIF.TIF", package = 
  "rgdal")[1])
summary(SP27GTIF)
image(SP27GTIF, col=grey(1:9/10))
SP27GTIF_a <- as(SP27GTIF, "SpatialPixelsDataFrame")
summary(SP27GTIF_a)
SP27GTIF_b <- SP27GTIF_a[SP27GTIF_a$band1 < 100,]
summary(SP27GTIF_b)
image(SP27GTIF_b)

because SpatialPixelsDataFrame objects have their coordinates recorded 
explicitly, and "know where they are" on a full grid. 

fullgrid(SP27GTIF_b) <- TRUE
summary(SP27GTIF_b)

will drop them again, inserting NAs where band1 >= 100.

This class is borrowed from the Terralib "CELL" data structure, part 
raster, part DB record, only recording cells with data.

Roger

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Edzer Pebesma
    

    

      
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Thanks for this. For the record:

sp has a subset function for SpatialPixelsDataFrame,
but it is not exported (nor documented, I believe).
In addition, it seems not to work:

SP27GTIF_c <- sp:::subset.SpatialPixelsDataFrame(SP27GTIF_a, band1 < 100)
Error in sp:::subset.SpatialPixelsDataFrame(SP27GTIF_a, band1 < 100) :
        object "band1" not found
--
Edzer

        
Roger Bivand wrote: