[R-sig-Geo] map algebra with ascii grid

[Tim Keitt]

On 7/16/07, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> On Sun, 15 Jul 2007, Tim Keitt wrote:
>
> > On 7/14/07, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> >>  On Sat, 14 Jul 2007, Michal Gallay wrote:
> >>
> >> >  Dear R Users,
> >> >
> >> >  could you please advise me on doing map algebra with spatial grids? It's
> >> >  the first time I am using spatial objects in R. I have imported an ascii
> >> >  grid file and wanted to round the values in it or sum with a value, but
> >> >  it
> >> >  gives an error message:
> >> >
> >> > >  require(maptools)
> >> > >  x <- readAsciiGrid(fname="xxxx.asc")
> >> > >  x+3
> >> >  Error in x + 3 : non-numeric argument to binary operator
> >> > >  round(x,2)
> >> >  Error in round(x, digits) : Non-numeric argument to mathematical
> >> >  function
> >> >
> >>
> >>  Arithmetic operations are not defined for SpatialGridDataFrame operations,
> >>  as the columns of the data frame (here a single column) may contain data
> >>  of different classes. Do the operations on the columns directly:
> >>
> >>  names(x)
> >>
> >>  If there is a column called "band1", then
> >>
> >>  x$band1 + 3
> >>
> >>  will print "band1" + 3, and
> >>
> >>  x$band1 <- x$band1 + 3
> >>
> >>  will add 3 to the "band1" column. Think what would happen if band1 was
> >>  categorical or logical to see why doing arithmetic directly isn't such a
> >>  good idea.
> >
> > Roger,
> >
> > While I agree that arithmetic directly on S?DF objects does not make a
> > lot of sense, I have always taken the view that if the name of a type
> > includes "data frame", then you are implicitly promising that the
> > object is substitutable for any other data.frame class object in a
> > function call. So if S?DF is meant to inherit (formally or informally)
> > from data.frame, one should be able to use a S?DF anywhere you could
> > use an ordinary data.frame object without changing the result. Of
> > course, I realize that S?DF is not mean to inherit directly from
> > data.frame (so it is clear from the programming standpoint, ie there
> > is no "inherits" relationship between data.frame and S?DF). Perhaps it
> > would be more clear to users if a different naming scheme were used?
>
> Tim,
>
> While there is something in your suggestion, my understanding of the White
> Book (1992) introduction of data frames is that they are lists of vectors
> of varying types but of equal lengths, and that they should play well with
> formulae. The current Spatial*DataFrame do indeed do this, and you can use
> them with model.frame() inside for example lm(), gstat(), and generally
> almost everywhere a real data frame is needed as a data= argument. I
> cannot recall ever using an arithmetic operation on a data frame myself,
> although I may be an exception (I coerse to matrix).
>
> Rather than trying to inherit from data.frame, the methods ($,[, [[, and
> assignment versions) provide data.frame operability.

Ah. OK. That satisfies my concern. I didn't pay close attention to
which operators were being discussed or whether they were within the
formal definition of "data frame". Sounds like the implementation is
meeting its naming "obligations."

THK

> Certainly I can see that there are differences of approach here,
> especially if remote sensing data are never going to have factors (unlike
> say GIS integration, where categorical data do occur). The exchange on the
> list earlier today (about why read.table() returns a data.frame and by
> default converts a character vector to a factor) seems to back me up (from
> my point of view, of course!).
>
> Best wishes,
>
> Roger
>
> >
> > THK
> >
> >>
> >>  Hope this helps,
> >>
> >>  Roger
> >>
> >> >  Thank you for any suggestions.
> >> >
> >> >  Best wishes,
> >> >
> >> >  Michal
> >> >
> >> >
> >> >
> >> >
> >>
> >>  --
> >>  Roger Bivand
> >>  Economic Geography Section, Department of Economics, Norwegian School of
> >>  Economics and Business Administration, Helleveien 30, N-5045 Bergen,
> >>  Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
> >>  e-mail: Roger.Bivand at nhh.no
> >>
> >>  _______________________________________________
> >>  R-sig-Geo mailing list
> >>  R-sig-Geo at stat.math.ethz.ch
> >>  https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >>
> >
> >
> >
>
> --
> Roger Bivand
> Economic Geography Section, Department of Economics, Norwegian School of
> Economics and Business Administration, Helleveien 30, N-5045 Bergen,
> Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
> e-mail: Roger.Bivand at nhh.no
>


-- 
Timothy H. Keitt, University of Texas at Austin
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