[R-sig-Geo] merging data with SpatialPolygonsDataFrame

Roger Bivand Roger.Bivand at nhh.no
Sat Jan 26 21:11:09 CET 2008

On Fri, 25 Jan 2008, Dylan Beaudette wrote:

> On Friday 25 January 2008, Roger Bivand wrote:
>> On Thu, 24 Jan 2008, Dylan Beaudette wrote:
>>> On Thursday 24 January 2008, Roger Bivand wrote:
>>>> On Thu, 24 Jan 2008, Dylan Beaudette wrote:
>>>>> Hi Roger. Thanks for contributing some answers to this.
>>>>> I was recently working with a colleague on developing some sample
>>>>> exercises for new students. Since joining new attribute data to a GIS
>>>>> layer's table is a very common operation we included some samples on
>>>>> how to do this within R. You have hinted at some possible ways to do it
>>>>> above, but do you have a 'best practices' approach to doing this using
>>>>> 'sp' methods and objects?
>>>>> For example:
>>>>> # contains an attribute col named 'veg_code'
>>>>> veg <- readOGR(something.shp)
>>>>> # code meanings: indexed by 'veg_code'
>>>>> codes <- read.dbf(table.dbf)
>>>>> # what is the best way to join up the attributes in 'veg' with the rows
>>>>> in 'codes' ?
>>>> Hi Dylan,
>>>> This is a different question, but I won't break it out of this thread
>>>> yet.
>>>> You are doing look-up on codes to give labels to veg$veg_code, right?
>>>> veg$veg_code are integer indices to codes$V1 (say V1, I don't know what
>>>> it is). If length(unique(veg$veg_code)) == length(codes$V1), and
>>>> sort(unique(veg$veg_code)) is 1:length(codes$V1), you should think of
>>>> the factor as your friend:
>>>> veg$veg_code_factor <- factor(veg$veg_code,
>>>> labels=as.character(codes$V1))
>>>> If not, you need another layer using perhaps order() or match() on the
>>>> matching substring of codes$V1 to find out which value of veg$veg_code
>>>> should have which label in as.character(codes$V1). Alternatively use the
>>>> levels= argument to factor().
>>>> Something like:
>>>> set.seed(1)
>>>> veg_code <- rpois(100, 4)
>>>> table(veg_code)
>>>> V1 <- paste("code", 0:10)
>>>> V1
>>>> levs <- 0:10
>>>> veg_code_factor <- factor(veg_code, levels=levs, labels=V1)
>>>> table(veg_code_factor, veg_code)
>>>> No merging or messing with veg itself is needed, apart from adding a
>>>> single extra factor column. The factor abstraction is a great strength
>>>> of the S language.
>>>> Have I misunderstood you?
>>>> Roger
>>> I think so.
>>> I was (trying to) describe the process of joining, either 1:1 or many:1,
>>> the att table associated with an sp object and some other data frame.
>>> merge() seems to work fine, but the order of the rows are different from
>>> the original data frame attached to the sp object.
> Thanks for the detailed response Roger.
>> My experience is that merge() is often a false friend, because of object
>> coercion and sorting, as you suggest. The rules for SpatialLinesDataFrame
>> and SpatialPolygonsDataFrame are simple, and SpatialPointsDataFrame (and
>> by extension SpatialPixelsDataFrame) can be made as simple. The first two
>> constructor functions take a match.ID= argument, default TRUE, which
>> matches the geometry ID - the ID slot in the component Lines or Polygons
>> objects - to the data.frame row.name. For SpatialPointsDataFrame(), the
>> same argument exists, but is only used if the coordinate matrix has row
>> names to be matched to the data.frame row names. If it has been used, the
>> data slot row names are the geometry IDs.
>> From there, it gets harder. merge() takes lots of arguments, and a lot of
>> trial and error is needed to reach a satisfactory result, that is a single
>> data frame with row names matching the geometry IDs (not necessarily in
>> the right order, but the same set of IDs).
>>> My question was on the best way to 'update' the data frame attached to an
>>> sp object, based on the results from a merge() with some other data.
>> For one to one, there isn't really an alternative to extracting the data
>> slot, do the merge, check the geometry IDs against the output data frame
>> row names, and re-construct the object. That is, essentially where this
>> thread began.
>> If there is less data than geometries, merge() should fill out with NAs.
>> If there is more data than geometries, the output data frame will need
>> subsetting. Both match(), order(), and %in% are very handy here.
>> For one geometry list to many - see reshape() first, for example to
>> flatten a "tall" set of space/time observations so that the time values
>> become new attribute columns. This could be met stations stacked by
>> station and date, which need widening to stations by date*attributes
>> If need be, the IDs of the geometries can be changed too, see
>> ?"spChFIDs-methods" in maptools.
>> Any closer?
> I think that you have answered my questions and reaffirmed my thoughts 
> on how to proceed. I am mostly interested in 1:1 and 1:many joins 
> (geom:attr), and I think that the following approach should work in the 
> general case:
> # read vector data:
> v <- readOGR(dsn='...', layer='...')
> # safer approach to 1:1 or 1:many (geom:atts) joins
> # add a sorting id for later use
> v at data$sorting_id <- 1:nrow(v at data)

v$sorting_id <- 1:nrow(as(v, "data.frame"))

is cleaner, there is absolutely no need to access the slot directly with 
the @ operator. Almost all R functions do lazy evaluation, so do not 
create new objects unless needed.

If you are using SpatialLines* or SpatialPolygons*, this is better, 
because it uses the geometry IDs (for v SpatialPolygons*):

v$sorting_id <- sapply(slot(v, "polygons"), function(x) slot(x, "ID"))

> # make a copy of the original table
> orig.table <- v at data

orig.table <- as(v, "data.frame")

> # make the table with 'joined' data
> new.table <- merge(x=orig.table, y=veg_codes, by.x='CODE', by.y='code')

maybe sort=FALSE, you can also try by.x=0, which are the geometry IDs

> # re-order this table based on the original row order
> new.table.ordered <- new.table[order(new.table$sorting_id), ]
> # restore the origina row names
> row.names(new.table.ordered) <- row.names(orig.table)

First check the identity of the two, recalling that character strings may 
be cast to factors when constructing data frames.

> # replace the data table
> v at data <- new.table.ordered

Rather not, make a v1 and leave the input v in peace. Reading history 
files a couple of days later where objects get overwritten is ten times 
harder than creating new objects and if need be deleting old ones. I find 
that copying and pasting out-of-order history files can lead to lots of 
confusion if object names are overwritten.

> This should ensure that the merged rows are in the original order, and have
> the original row.names.

Use of all.equal() and identical() on the IDs, row names, etc., is still 
very comforting. Think locales, and you can see what might creep in when 
swapping data with others - just checking takes little time compared to 
recovering from an overconfident assumption that the data and the 
geometries are correctly attached.

It can be done through databases too, the aRT R/Terralib interface offers 
stronger control, and similar effects can be had with OGR/PostGIS.

Best wishes,


> Does that sound correct?
> Thanks,
> Dylan
>> Roger
>>> does that help?
>>> cheers,
>>> Dylan
>>>>> As of now we are using merge to replace the dataframe slot of the
>>>>> original file. We first re-order the results from merge to match the
>>>>> original row ordering:
>>>>> # an example file:
>>>>> veg <- readOGR(dsn='ArcGISLabData/BrownsPond/', layer='vegevector')
>>>>> # some example codes
>>>>> veg_codes <- data.frame(code=1:4, meaning=c('code 1','code 2','code
>>>>> 3','code 4'))
>>>>> # join the original data table with the veg codes table
>>>>> combined <- merge(x=veg at data, y=veg_codes, by.x='CODE', by.y='code')
>>>>> # overwrite the original data frame with the combined version
>>>>> # note that the original order needs to be restored
>>>>> # since the original data was sorted on 'ID', we can use that to
>>>>> restore # the correct order in the 'combined' dataframe:
>>>>> v at data <- combined[order(combined$ID),]
>>>>> In summary, is there a safer or preferred way to do this?
>>>>> thanks,
>>>>> Dylan

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

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