[R-sig-Geo] spplot with two rasters

[Roger Bivand]

On Thu, 13 Mar 2008, Edzer Pebesma wrote:

> Agus, I disagree with your claim that R plot functions are not ok for
> real applications. Also, I use real applications for learning and
> teaching -- they are not fundamentally different.
>
> In the trellis panel functions, or in the direct plot/image/lines etc
> functions, you plot in data coordinates. The main thing that the plot
> and spplot methods in package sp control is the aspect ratio. How else
> do data coordinates differ from geographical coordinates when it comes
> to plotting?

Exactly. In fact I think that a grid panel might make it possible to 
insert an image backdrop behind spplot(), which was the original question 
from Dylan - it would make a nice student project in visualisation. 
Something like an sp.image() for sp.layout=, but using its own palette and 
being painted over by the "real" data.

Roger

>
> Are you refering to taking care of geographic (re)projection at that level?
> --
> Edzer
>
> Agustin Lobo wrote:
>> Well, the problem is that R does not have a real geographical
>> display. While things can be done going back and forth from R to GIS,
>> this procedure soon becomes very inconvenient. It's ok
>> for learning and teaching, but not for real applications.
>> Maybe getting an existing GIS to display spatial R objects
>> is actually easier than developing a geographical display for R.
>> Agus
>>
>> Dylan Beaudette escribió:
>>
>>> On Thursday 06 March 2008, Thomas Adams wrote:
>>>
>>>> Dylan,
>>>>
>>>> I think a solution using GRASS can be found on pages 110-111 of "Open
>>>> Source GIS: A GRASS GIS Approach", 3rd Ed. The same material is covered
>>>> in the 2nd Ed. as well, where you use r.mapcalc to combine two rasters
>>>> and judicious use of MASKs; a conditional statement in r.mapcalc is the
>>>> key.
>>>>
>>>> Regards,
>>>> Tom
>>>>
>>> Hi Tom,
>>>
>>> Thanks for the suggestion. This works, but we were hoping to make the plot in
>>> R-- as the PDF output is hard to match with GRASS alone. I suppose I will
>>> just have to try using two rasters with spplot() and see what happens.
>>>
>>> Cheers,
>>>
>>> Dylan
>>>
>>>
>>>> Dylan Beaudette wrote:
>>>>
>>>>> On Tuesday 04 March 2008, Edzer Pebesma wrote:
>>>>>
>>>>>> I find it hard to imagine how you want to plot two raster files on top
>>>>>> of each other. Do you want some form of transparency? If it is just one
>>>>>> overlaying the other, you could use overlay to find out which cells in
>>>>>> raster 1 to replace with those in raster 2 before plotting.
>>>>>> --
>>>>>> Edzer
>>>>>>
>>>>> Hi Edzer,
>>>>>
>>>>> I generally agree that plotting one raster file "over" another raster
>>>>> file would be of little use. In this case, one of the raster files (the
>>>>> interesting one) has been masked with nodata, such that it only really
>>>>> covers about 30% of the region of interest. The other raster is just
>>>>> contextual data, and thus would be useful to plot "behind" the first
>>>>> raster.
>>>>>
>>>>> Ideas?
>>>>>
>>>>> Dylan
>>>>>
>>>>>
>>>>>> Dylan Beaudette wrote:
>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> Is it possible to plot two raster images using spplot() in a manner
>>>>>>> similar to:
>>>>>>>
>>>>>>> pts <- list("sp.points", points_file, pch = 4, col = "black", cex=0.5)
>>>>>>> spplot(raster_file, zcol="elev.pred", sp.layout=list(pts))
>>>>>>>
>>>>>>> Note that one of the raster images is an aerial photo, used only for
>>>>>>> context, while the second one is one with interesting z-values. The
>>>>>>> second raster is masked and thus does not cover the entire region.
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Dylan
>>>>>>>
>>>
>>>
>>
>>
>
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-- 
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