[R-sig-Geo] 'nowrapSpatialPolygons' doesn't remove horizontal scratches

[Roger Bivand]

On Fri, 7 Jan 2011, Kevin Ummel wrote:

> Thanks, Roger. I still need the extent of 'out1' to match the extent of 
> 'orig'. i.e. I need to subtract 360 from each out1 x-coordinate greater 
> than 180, but is there an easy way to directly access and overwrite the 
> polygon coordinates? Indexing through the structure -- at least judging 
> by str(out1) -- looks potentially messy.

No, you can either have geographical coordinates -180 to 180 or 0 to 360, 
but not both. You cannot join -180 == 180 on a planar display. So either 
you suffer the reat of the world getting in the way, or you convert all 
the other objects to 0 to 360 (which looks easier to do - if rasters, 
substitute the GridTopology in the underlying SpatialGridData Frame. If 
you cannot, then you can either analyse or display, but not both.

Roger

>
>> extent(orig)
> class       : Extent
> xmin        : -180
> xmax        : 180
> ymin        : -68.65256
> ymax        : 22.47224
>
>> extent(out1)
> class       : Extent
> xmin        : 60.87917
> xmax        : 221.1910
> ymin        : -68.65256
> ymax        : 22.47224
>
> On Jan 7, 2011, at 7:21 PM, Roger Bivand wrote:
>
>> On Fri, 7 Jan 2011, Kevin Ummel wrote:
>>
>>> Hello,
>>>
>>> I have a shapefile with polygons located in Oceania, some of which straddle longitude =-180/0. When plotted, horizontal scratches result. I assumed that use of 'nowrapSpatialPolygons' would do the trick, but the plot looks the same before and after. Clearly, I'm not utilizing it properly. Could someone look at the example code below Ultimately, I need to successfully rasterize the data onto a -180/180 grid, so maybe another solution exists...
>>>
>>> Thanks!
>>> Kevin
>>>
>>> The folder (WDPA Oceania) with the example shapefile can be downloaded here: http://dl.dropbox.com/u/14314000/WDPA%20Oceania.zip
>>>
>>> orig=readShapeSpatial("sql_statement",CRS("+proj=longlat"))
>>>
>>> plot(orig)
>>>
>>> out=nowrapSpatialPolygons(orig,offset=180)
>>
>> No, you mean:
>>
>> out <- recenter(orig)
>> out1 <- SpatialPolygonsDataFrame(out, data=slot(orig, "data"))
>> plot(out1, axes=TRUE)
>>
>> If you need to merge the input polygons that were split by 180 degrees, and now appear as touching polygons, you'll need to identify them somehow.
>>
>> Hope this helps,
>>
>> Roger
>>
>>>
>>> plot(out)
>>>
>>>
>>>
>>>
>>>
>>>
>>> 	[[alternative HTML version deleted]]
>>>
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>>
>> --
>> 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
>>
>
>

-- 
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