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

[Peter Ellis (Wellington)]

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?

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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