[R-sig-Geo] Speeding up SpatialPolygonsDataFrame - possibly by making it less detailed

[Roger Bivand]

On Fri, 15 Jun 2012, Peter Ellis (Wellington) wrote:

> Hi
>
> I am using the digital boundaries for New Zealand shapefiles available 
> from Statistics New Zealand at 
> http://www.stats.govt.nz/browse_for_stats/people_and_communities/Geographic-areas/digital-boundary-files.aspx.
>
> The aim is to use these to produce choropleth maps.
>
> I have no problem in doing this using code based on Hadley Wickham's 
> https://github.com/hadley/ggplot2/wiki/plotting-polygon-shapefiles . 
> However, manipulating the shapefiles into something I can merge with 
> statistics is painfully slow.  For example, to draw the 20 or so 
> regional council boundaries with the code below involves creating a data 
> frame (REGC12.df) with 3.2 million rows.  It produces a lovely looking 
> graphic I could expand up to very large size, but is a pain in the neck 
> for playing around with.
>
> Is there a function in R that can take an object of class 
> sp::SpatialPolygonsDataFrame and reduce the fine level of detail, while 
> keeping the basic shapes of the polygons to some specified degree of 
> approximation?  Or is this a job for a specialist GIS package?

?gSimplify in rgeos should do this, once you find a suitable value for the 
tol= argument.

Roger

>
> Example code:
>
> REGC12 <- readOGR(dsn=".", layer="REGC12")
> REGC12 at data$id <- rownames(REGC12 at data)
> REGC12.points <- fortify(REGC12, region="id")
> REGC12.df <- join(REGC12.points, REGC12 at data, by="id")
>
> ggplot(REGC12.df) +
> 	aes(long, lat, group=group, fill=NAME) +
> 	geom_polygon()
>
> thanks
> PE
>
> Peter Ellis | Manager Research and Evaluation 
> Tourism, Events and Consumer Affairs Branch
> Ministry of Economic Development | Manutū Ōhanga
> 33 Bowen Street | PO Box 1473 | Wellington 6011 | New Zealand
> p. 04 498 7448| mb. 029 771 0135 | www.tourismresearch.govt.nz | www.med.govt.nz
>  
>
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-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
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