[R-sig-Geo] Large PDF files when using tiff raster images as background

[Roger Bivand]

On Mon, 4 May 2009, dominik baenninger wrote:

> Dear list,
>
> When plotting spatial informaion it is common to use a pixel map as 
> background. Therefore I used the following commands (in brief)
>
> library(sp)
> library(rgdal)
>
> pdf()
> readGDAL(geotiff-file)
> image()
> plot(...,add=T)
> dev.off()
>
> This works fine, however, the original tiff file has a size of about 600kb, 
> the pdf 20MB even I do not add  any further information.

You have (at least) two choices - making the choice depends on which 
output graphics device you want to use. All R graphics draw raster cells 
as filled rectangles, so that on a PDF you will get many filled 
rectangles, hence the size of the output file. To reduce the size, 
decimate the raster with the output.dim= argument in readGDAL - you lose 
resolution, but reduce the number of filled rectangles.

An alternative is to use a bitmapped device such as png(), possibly trying 
to set the width and height to suit the raster object - see GE_SpatialGrid 
in maptools for examples of the manipulations needed for making Google 
Earth PNG image overlays. A compressed bitmapped device will yield an 
image not dissimilar to the input raster in size.

Hope this helps,

Roger

> Since I like to add point and line information with the plot command I 
> would like to generate some kind of vecotrbased graphics. Any idea how I 
> can the geotiff-background without producing large files?
>
> Thanks for any hint in advance.
>
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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