[R-sig-Geo] Error message when dissolving polygons
frankma
md.franklin at gmail.com
Mon Aug 25 12:12:47 CEST 2014
OK you were right I was being stupid, I have changed LL to UTM and that
works. Do you know of a way to dissolve polygons by explicitly defineing
which ones you wish to dissolve?
Thanks,
Matt
frankma wrote
> My apologies Roger, thank you for letting me know the mailing list
> etiquette.
>
> I understand the gist of what you are saying but I'm still unsure which
> projections I am mixing up. I am using the shapefile available here:
> http://www.cso.ie/en/census/census2011boundaryfiles/
>
> My code and error are below. I realise that it may seem obvious to you but
> it isn't to me, I've spent hours trying to "dissolve polygons" and there
> is something I am not understanding here, I am sorry for this. Appreciate
> your response so far and any further help is very much appreciated, thank
> you. In terms of affiliations I am currently trying to complete an MSc
> dissertation at the University of Sheffield (UK) - I also work full time.
>
>
> library(maptools) # for geospatial services; also loads foreign and sp
> library(gpclib) # General Polygon Clipping library
> library(rgdal) # for map projection work; also loads sp
> library(PBSmapping) # for GIS_like geospatial object manipulation /
> anslysis iIRLluding poly
> #
> # Part 1:
> # First example, derived from the maptools unionSpatialPolygon() method
> documentation in the R
> # maptools user manual: We calculate and sum the area of each polygon in
> the Ireland map.
> # Then, use unionSpatialPolygons() to dissolve the county polygon
> boundries, and assign each
> # polygon's area to one of four regions, based on longitude thresholds.
> #
> print("start of demo. Hit key...")
> browser()
> print("reading and transforming Ireland Shapefile...")
> #
> IRLBase <- readShapePoly("Shape/counties.shp",proj4string=CRS("+proj=aea
> +ellps=GRS80 +datum=WGS84"))
>
> #
> # Transform the polygons (which were read in as unprojected geographic
> coordinates)
> # to an Albers Equal Area projection.
> #
> IRLProj = spTransform(IRLBase,CRS("+proj=aea +ellps=GRS80 +datum=WGS84"))
> #
> # Convert to a PolygonSet for compatability with PBSmapping package
> routines.
> #
> IRLProjPS = SpatialPolygons2PolySet(IRLProj)
> #
> plotPolys(IRLProjPS, proj =
> TRUE,col="wheat1",xlab="longitude",ylab="latitude")
> #
> # polygon area calculations
> #
> print("Calculating Ireland polygon areas...")
> attr(IRLProjPS, "projection") <- "LL"
> IRLPolyAreas = calcArea(IRLProjPS,rollup=1)
*
> Error in calcArea(IRLProjPS, rollup = 1) :
> To calculate the areas of polygons defined by longitude-latitude
> coordinates, this routine first projects them in UTM.
>
> Attempted to automatically calculate the missing 'zone' attribute, but
> that failed because the mean longitude falls outside the range
> -180 < x <= 180.
*
>
> Thanks,
> Matt
>
> Roger Bivand wrote
>> On Mon, 25 Aug 2014, frankma wrote:
>>
>>> Hi,
>>>
>>> I'm having the same issue. Did you manage to solve this?
>>
>> Please always quote the problem verbatim, providing a reproducible
>> example. Even if anyone cares to open nabble to find the link, nobody can
>> actually know whether your problem is the same. The reason we ask for
>> reproducible examples is that the effort needed by the questioner to
>> create a reproducible example means that he/she has to look more closely
>> at - here "I’m following a tutorial found in the web" - what they are
>> trying to do, and very often see their mistake themselves. Learning to
>> debug one's own work is not easy, takes time, but wastes less of other
>> peoples' time than asking lazy questions - see the posting guide:
>>
>> http://www.r-project.org/posting-guide.html
>>
>> It is also very helpful to see an affiliation, so that anyone caring to
>> answer knows where to start. This is especially important when the
>> sender's address is non-institutional.
>>
>> In the original question:
>>
>> https://stat.ethz.ch/pipermail/r-sig-geo/2014-April/020764.html
>>
>> the obvious error was to think that the PolySet class in PBSmapping
>> understands other projections than UTM:
>>
>> # library(maptools)
>> # zm.* not available
>>> input = readShapePoly("zm.shp", proj4string=CRS("+proj=lcc +ellps=GRS80
>> +datum=WGS84"))
>>> projectedpolygons = spTransform(input, CRS("+proj=lcc +ellps=GRS80
>> +datum=WGS84"))
>>> zmprojPS = SpatialPolygons2PolySet(projectedpolygons)
>>> attr(zmprojPS, "projection") <- "LL"
>>
>> I assume that no answer was posted because the error message was totally
>> obvious, that is that lcc is Lambert conformal conic, but LL is the code
>> for geographical, unprojected, coordinates, so PBSmapping could not
>> project to UTM to calculate areas as:
>>
>> "the mean longitude falls outside the range -180 < x <= 180."
>>
>> If you are also mixing up projections, then this also explains what you
>> are seeing.
>>
>> Hope this clarifies,
>>
>> Roger
>>
>>>
>>> Thanks,
>>> Matt
>>>
>>>
>>>
>>> --
>>> View this message in context:
>>> http://r-sig-geo.2731867.n2.nabble.com/Error-message-when-dissolving-polygons-tp7586109p7586979.html
>>> Sent from the R-sig-geo mailing list archive at Nabble.com.
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>>
>> R-sig-Geo@
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>
>>
>> --
>> 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@
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo@
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
> Roger Bivand wrote
>> On Mon, 25 Aug 2014, frankma wrote:
>>
>>> Hi,
>>>
>>> I'm having the same issue. Did you manage to solve this?
>>
>> Please always quote the problem verbatim, providing a reproducible
>> example. Even if anyone cares to open nabble to find the link, nobody can
>> actually know whether your problem is the same. The reason we ask for
>> reproducible examples is that the effort needed by the questioner to
>> create a reproducible example means that he/she has to look more closely
>> at - here "I’m following a tutorial found in the web" - what they are
>> trying to do, and very often see their mistake themselves. Learning to
>> debug one's own work is not easy, takes time, but wastes less of other
>> peoples' time than asking lazy questions - see the posting guide:
>>
>> http://www.r-project.org/posting-guide.html
>>
>> It is also very helpful to see an affiliation, so that anyone caring to
>> answer knows where to start. This is especially important when the
>> sender's address is non-institutional.
>>
>> In the original question:
>>
>> https://stat.ethz.ch/pipermail/r-sig-geo/2014-April/020764.html
>>
>> the obvious error was to think that the PolySet class in PBSmapping
>> understands other projections than UTM:
>>
>> # library(maptools)
>> # zm.* not available
>>> input = readShapePoly("zm.shp", proj4string=CRS("+proj=lcc +ellps=GRS80
>> +datum=WGS84"))
>>> projectedpolygons = spTransform(input, CRS("+proj=lcc +ellps=GRS80
>> +datum=WGS84"))
>>> zmprojPS = SpatialPolygons2PolySet(projectedpolygons)
>>> attr(zmprojPS, "projection") <- "LL"
>>
>> I assume that no answer was posted because the error message was totally
>> obvious, that is that lcc is Lambert conformal conic, but LL is the code
>> for geographical, unprojected, coordinates, so PBSmapping could not
>> project to UTM to calculate areas as:
>>
>> "the mean longitude falls outside the range -180 < x <= 180."
>>
>> If you are also mixing up projections, then this also explains what you
>> are seeing.
>>
>> Hope this clarifies,
>>
>> Roger
>>
>>>
>>> Thanks,
>>> Matt
>>>
>>>
>>>
>>> --
>>> View this message in context:
>>> http://r-sig-geo.2731867.n2.nabble.com/Error-message-when-dissolving-polygons-tp7586109p7586979.html
>>> Sent from the R-sig-geo mailing list archive at Nabble.com.
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>>
>> R-sig-Geo@
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>
>>
>> --
>> 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@
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo@
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
-----
Matthew Franklin
School of Mathematics and Statistics
University of Sheffield
--
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