[R-sig-Geo] spplot handling of overlapping points?

[Edzer Pebesma]

Yes, look for argument alpha e.g. in ?rgb or ?bpy.colors

On 03/06/2012 04:49 PM, Mauricio Zambrano-Bigiarini wrote:
> On 05/03/12 14:40, Edzer Pebesma wrote:
>> This has now been changed/repared in sp on r-forge (svn), and will
>> appear from sp 0.9-97 on. A test run would be:
>>
>> library(sp)
>> xyz = data.frame(expand.grid(x=1:10,y=1:10),rnorm(100))
>> coordinates(xyz)=~x+y
>> spplot(xyz, cex=10)
>>
>> which used to plot points in value order, and does now in data record
>> order.
>>
>> I put up resulting graphs, for comparison, at
>> http://ifgi.uni-muenster.de/~epebe_01/xyz.html
>>
>> As this change may make some of your spplot's for points different, if
>> there are objections against this change (which I consider an
>> improvement, if not a bug fix), it is time now to let me / us know.
> 
> Would it be possible to set a degree of transparency  for the
> overlapping points in spplot ?
> 
> Thanks in advance,
> 
> Mauricio Zambrano-Bigiarini
> -
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> 
>>
>> On 03/02/2012 09:03 PM, MacQueen, Don wrote:
>>> I have a SpatialPointsDataFrame object in which many points are very
>>> close
>>> together, such that the markers (plotting characters) tend to
>>> overlap. (Of
>>> course, this depends on marker size and the scale at which I plot; if I
>>> "zoom in" there is less overlap.)
>>>
>>> It appears that when markers overlap, spplot() places a marker
>>> associated
>>> with the larger value after, and therefore on top of, a marker
>>> associated
>>> with a smaller value.
>>>
>>> Am I correct? Or more generally, what is the algorithm that
>>> determines the
>>> order in which markers are added?
>>>
>>>
>>> I have searched ?spplot and related help pages and haven't found an
>>> explanation (at least, not yet).
>>>
>>> I can add that I don't believe markers are placed in the order in which
>>> they appear in the SpatialPointsDataFrame. I say this because when I
>>> attempt to reproduce an spplot using base graphics plot() I have to sort
>>> from smallest to largest value to succeed.
>>>
>>> I can probably provide a small reproducible example if necessary, but
>>> I'm
>>> hoping it's not necessary.
>>>
>>> Here are my actual commands.
>>>
>>>    tmps is the SpatialPointsDataFrame.
>>>    tmp is coordinates(tmps)
>>> (they have the same number of rows in the same order)
>>>
>>> Note that I'm using the cuts argument to break a continuous variable
>>> ('cpm2') into bins. I have carefully matched the colors in tmps$col2
>>> with
>>> the cbin.cols object passed to spplot(), and the break points for the
>>> bin
>>> boundaries, so I believe that everything else that could affect the
>>> final
>>> appearance, other than the order in which the markers are placed, is
>>> controlled.
>>>
>>> #1 using spplot
>>> spplot(tmps,c('cpm2'),
>>>     key.space='right',
>>>     legendEntries=cbin.lbls,
>>>     cuts=cbin.brks,
>>>     col.regions=cbin.cols,
>>>     cex=0.4)
>>>
>>> #2 using base graphics
>>> plot(tmp[,1],tmp[,2], asp=1, cex=0.6, pch=16, col=tmps$col2)
>>>
>>>
>>>
>>> Thanks
>>> -Don
>>>
>>
> 
> 
> -

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de