I went back to the help page, but it still didn't clarify it for me.
Yes, the two files are different length, but both have a zip variable that
they can match on and then keep only those records that match. Or do I need
to additionally create an index for the match to occur?
I am understanding it that unionSpatialPolygons functions works very much in
the same way as merge, except with geographical data. Am I wrong in this
assumption?


On Wed, Apr 22, 2009 at 3:09 AM, Roger Bivand <Roger.Bivand@nhh.no> wrote:

> On Tue, 21 Apr 2009, Alina Sheyman wrote:
>
>  Hi Roger,
>>
>
> Alina,
>
> Don't take this offlist, summarise to the list when done so that others can
> learn from your experiences.
>
>  when I follow the example in  maptools
>>
>> coordinates(mass) <- c("lat", "long")
>> reg4 <- unionSpatialPolygons(zipmaps3, mass$center_id)
>>
>
> Right, you have not read the unionSpatialPolygons help page carefully, have
> you?
>
> Do:
>
> length(mass$center_id)
> length(slot(zipmaps3, "polygons")
>
> They differ - your mass are the districts, zipmaps3 are the zips. zipmaps3
> has a variable giving the district IDs, what is it called? If you do
>
> table(zipmaps3$district_IDs)
>
> you see the numbers of zips in each district. This is your second argument
> to unionSpatialPolygons; it tells the functions which zips belong to each
> district. If you don't have such a variable, you'll have to make it. This is
> all explained on the help page, and in the example. It is very much like the
> other tapply(), by() and aggregate() functions, but only returns the
> geometries.
>
> Hope this helps,
>
> Roger
>
>
>
>> I get the following error message
>> Error in unionSpatialPolygons(zipmaps3, mass$center_id) :
>>  input lengths differ
>>
>> Do you know why that is? Do I in fact need to have two files of identical
>> length in order for the
>> function to work?
>>
>> thank you
>>
>> On Tue, Apr 21, 2009 at 3:17 PM, Roger Bivand <Roger.Bivand@nhh.no>
>> wrote:
>>
>>  On Tue, 21 Apr 2009, Alina Sheyman wrote:
>>>
>>>  To rephrase my question and explain my problem further
>>>
>>>> I have already created a map for revenue by zip code, in the state of
>>>> Massachusetts. To accomplish that I used a shapefile for Massachusetts
>>>> and
>>>> a
>>>> dataframe with my revenue/zip codes.
>>>>
>>>> What I am trying to do now is to have the same map done not by zip code,
>>>> but
>>>> rather by districts, as defined by zip codes.
>>>>
>>>> Basically what I've done was to introduce a new variable "district" into
>>>> my
>>>> dataset along with a zip code. At the same time my basic state map (by
>>>> zip)
>>>> still comes from a shape file and I can't introduce a new variable into
>>>> the
>>>> shape file.
>>>>
>>>>  Does anyone  know if there's any way I can accomplish what I'm trying
>>>> to
>>>> do or do I basically need to create a new shapefile with districts
>>>> already
>>>> in the file?
>>>>
>>>>
>>> OK, that wasn't too clear before. Look at unionSpatialPolygons() in
>>> maptools - it will take a vector of districts matching all your zip
>>> codes,
>>> and join together the geometries belonging to each district, dropping
>>> internal boundaries. The district IDs become the Polygons object IDs.
>>> There
>>> is no data frame in the object, because it isn't obvious how you might
>>> want
>>> to aggregate it.
>>>
>>> Hope this helps,
>>>
>>> Roger
>>>
>>>
>>>
>>>  On Mon, Apr 20, 2009 at 5:53 PM, Don MacQueen <macq@llnl.gov> wrote:
>>>>
>>>>  This is a pretty general question, and there are potentially many ways
>>>> to
>>>>
>>>>> do it.
>>>>>
>>>>> One way to get started would be to follow the example(s) in the help
>>>>> page
>>>>> for the overlay() function in the sp package.
>>>>>
>>>>> -Don
>>>>>
>>>>>
>>>>> At 1:26 PM -0400 4/20/09, Alina Sheyman wrote:
>>>>>
>>>>>  I have a list of made-up districts and corresponding zip codes. Based
>>>>>
>>>>>> on
>>>>>> that I want to define my own polygons (for each district) and then use
>>>>>> them
>>>>>> to create a map. Does anyone know how I would go about defining these?
>>>>>>
>>>>>> thank you
>>>>>>
>>>>>>      [[alternative HTML version deleted]]
>>>>>>
>>>>>> _______________________________________________
>>>>>> R-sig-Geo mailing list
>>>>>> R-sig-Geo@stat.math.ethz.ch
>>>>>> https:// stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>>>>
>>>>>>
>>>>>>
>>>>> --
>>>>> --------------------------------------
>>>>> Don MacQueen
>>>>> Environmental Protection Department
>>>>> Lawrence Livermore National Laboratory
>>>>> Livermore, CA, USA
>>>>> 925-423-1062
>>>>> --------------------------------------
>>>>>
>>>>>
>>>>>        [[alternative HTML version deleted]]
>>>>
>>>> _______________________________________________
>>>> R-sig-Geo mailing list
>>>> R-sig-Geo@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@nhh.no
>>>
>>>
>>>
>>
> --
> 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@nhh.no
>
>

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