[R] [R-sig-Geo] Plot SpatialLinesDataFrame with xlim & ylim

[Roger Bivand]

On Wed, 11 Jul 2007, Edzer J. Pebesma wrote:

> Michael,
>
> The plot method for SpatialLinesDataFrame objects resides in package sp,
> and questions regarding it are easier noticed on the r-sig-geo mailing
> list.
>
> The reason why they are plotted with aspect ratio 1 is that they are
> assumed to be spatial (geographical) data, and assume that 1 m north
> equals 1 m west -- think of a map. The exception is when the projection
> argument is set to longlat data (i.e. decimal degrees North/East), where
> the aspect ratio is computed differently, such that the argument above
> more or less holds.
>
> You should be able to override the default aspect setting by explicitly
> passing the e.g. asp=0.5 argument to plot.
>
> Here's the comment in the documentation of plot for Spatial objects
> (such as SpatialLinesDataFrame):
>
> The default aspect for map plots is 1; if however data are not projected
> (coordinates are longlat), the aspect is by default set to 1/cos(My *
> pi)/180) with My the y coordinate of the middle of the map (the mean of
> ylim, which defaults to the y range of bounding box).
>
> The argument |setParUsrBB| may be used to pass the logical value |TRUE|
> to functions within |plot.Spatial|. When set to |TRUE|, par(?usr?) will
> be overwritten with |c(xlim, ylim)|, which defaults to the bounding box
> of the spatial object. This is only needed in the particular context of
> graphic output to a specified device with given width and height, to be
> matched to the spatial object, when using par(?xaxs?) and par(?yaxs?) in
> addition to |par(mar=c(0,0,0,0))|.


Yes, if you look at how the GE_SpatialGrid() function in maptools works - 
you'll see how it meddles with the actual regional extents and the device 
size. I think Michael's data should also have been set to longlat:

proj4string(tmp) <- CRS("+proj=longlat")

The key is realising that the axes are driven by the device shape, not by 
the xlim/ylim as such, as Edzer says.

Roger


> --
>
> Edzer
>
> I'm running windows xp, R 2.3.1 with maptools 0.6-6, I guess.
> When plotting from a large SpatialLinesDataFrame and using xlim & ylim to reduce the area, the plot axes automatically have the same scale size, even if xlim and ylim ranges differ.
> E.g.:
> tmp <- readShapeLines(filepath)
> plot(tmp,xlim=c(-126,-119),ylim=c(50,51))
>
> The y-axis range is actually 47-54, same range as the x-axis.  What am I doing wrong?  Should I be using a different object for simple coastline & river data?
> Thanks in advance!
> Michael
>
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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