R help - Jul 2007 - Plot SpatialLinesDataFrame with xlim & ylim

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

_______________________________________________________
Michael Folkes
Salmon Stock Assessment
Canadian Dept. of Fisheries & Oceans     
Pacific Biological Station
3190 Hammond Bay Rd.
Nanaimo, B.C., Canada
V9T-6N7
Ph (250) 756-7264 Fax (250) 756-7053  folkesm@pac.dfo-mpo.gc.ca


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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))|.
--

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

I've succeeded doing a subset within the panel function of xyplot - if
I'm subsetting based on either the value of 'x' or 'y' (e.g.
below).  However, I wish to subset based on the value of another variable and
colour that one plotted point.  It's not working.  Either it doesn't
plot the coloured data point, or if I sort the data differently it colours one
datapoint, but the wrong one.   I assume this means it's not getting the
right subscripts?    Finally I can sort of see the light as if I remove the
conditioning variable (week) and subset before the xyplot (e.g. week==1) to get
just one panel, it plots the correct data including the correct single red
point.
Where am I erring?
_______________________________________
print(xyplot(yval~xval|week,data=mydata,
 panel=function(x,y,subscripts){
	#panel.xyplot(x,y,type='p',col=1,cex=.5)
	panel.xyplot(x[y<=40],y[y<=40],type='p',col=2,cex=.5)  # 
<-----this works
	panel.xyplot(x[mydata$yr==2005],y[mydata$yr==2005],type='p',pch=16,col=2,cex=.5)
#  <-----sometimes this won't work or it colours wrong datapoint
}))
___________________________

Thanks very much!
Michael Folkes
_______________________________________________________
Michael Folkes
Salmon Stock Assessment
Canadian Dept. of Fisheries & Oceans     
Pacific Biological Station
3190 Hammond Bay Rd.
Nanaimo, B.C., Canada
V9T-6N7
Ph (250) 756-7264 Fax (250) 756-7053  folkesm@pac.dfo-mpo.gc.ca


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