[R-sig-Geo] Still a bug when converting from tess to spatial polygon (sp) format?

[Roger Bivand]

On Wed, 14 May 2014, Fabricio Vasselai wrote:

> Dear all,
>
>
> After playing around with the tessellation functions of the spatstat
> package to reply to a previous question on the list, I realized that there
> seems to be something odd when converting from tess format to spatial
> polygon format.
>
> Usually, spatial polygonal shapefiles read into R with functions like
> readShapePoly or readOGR, are objects which have a slot called "data", that
> can be assessed with @data and has the format of a data.frame. In there,
> each polygonal feature of the map is a row, while the columns are the
> variables that are eventually present in the shapefiles's data. Therefore,
> if you try to check names() of a spatial polygon object read from a
> shapefile, you get these columns, while row() gives you the polygon
> features present in the map.

Why is this a surprise? There are *two* classes for each Spatial* object, 
the geometries (SpatialPoints, SpatialLines, SpatialPolygons, SpatialGrid, 
SpatialPixels), and each of these can be extended have attributes/fields 
in a data.frame object in the data slot of Spatial*DataFrame objects. This 
has always been like this, since:

Edzer J. Pebesma and Roger S. Bivand. Classes and methods for spatial data 
in R. R News, 5(2):9-13, November 2005

http://cran.r-project.org/doc/Rnews/Rnews_2005-2.pdf

There is also a vignette:

http://cran.r-project.org/web/packages/sp/vignettes/intro_sp.pdf

The real question is why anyone would expect a tesselation geometry to 
have attributes. If a tesselation has attributes/marks, then it would be 
worth writing a method to coerce to SpatialPolygonsDataFrame, for example 
for E in the examples in ?tess. But here:

> Esp <- as(E, "SpatialPolygons")
> length(Esp)
[1] 11
> row.names(Esp)
  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k"

already preserves the levels, so I don't see a use case for such a method. 
If you can suggest one and contribute a coercion method, it could be added 
to maptools.

Hope this clarifies,

Roger


>
> However, when converting a tess object generated by spatstat to shape
> polygon, by using the as( , "SpatialPolygons"), things look quite
> different. First, no slot called data is created. Second, if you try
> names() on the converted object, you will get each polygon features and
> with row(), you get nothing. This looks odd, since the features should be
> the rows of a data.frame-like slot, shouldn't them?
>
> Here it goes a reproducible example:
>
> ###Code begins here
>
> #Let's first read a shp map for comparison
>
> vectors.path <- system.file("vectors", package = "rgdal")[1]
> loaded.map <- readShapePoly(paste(vectors.path, "/scot_BNG.shp", sep=""))
> plot(loaded.map)      #looking at the loaded map
> class(loaded.map)      #checking if it is of the class we want
>
> #Now let's generate a shp map from tessellation
>
> randompoints <- rpoint(100)      #generate random spatial points
> random.map.tess <- dirichlet(randompoints)      #tessellate the points
> random.map <- as(random.map.tess, "SpatialPolygons")      #convert to shape
> polygon
> plot(random.map)      #looking at the random generated map
> class(random.map)      #checking if it is of the class we want
>
>
> #Finally, let's compare both polygon shape objects:
>
> names(loaded.map)
> names(random.map)
>
> row(loaded.map)
> row(random.map)
>
> nrow(loaded.map)
> nrow(random.map)
>
> ncol(loaded.map)
> ncol(random.map)
>
> View(loaded.map at data)
> View(random.map at data)
>
> ###End of code
>
>
> Notice how things look different for the same sp objects when loading them
> or creating from a tess converted object.  I mean, I am not sure whether
> this is really a bug or it is normal and if yes, why does it happen (that's
> why the subject of this email is a question, not an affirmative sentence),
> but I though it was worth bringing to you all since it can be confusing.
> Anyway, is there any way we can convert tess objects to a sp object in the
> pattern a shapefile map is read, i.e. with the "data" slot and with the
> polygon features as rows, no as columns?
>
> As always, thank you all for your time.
>
> Best,
>
> Fabricio Vasselai
> CPS-University of Michigan
> University of S??o Paulo
>
> 	[[alternative HTML version deleted]]
>
>

-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no