On 23/09/2014 3:27 a.m., Alain Dubreuil wrote:
Hi. I have a requirement to plot a series of points on a map of Canada along
with boundaries defining search and rescue (SAR) regions. I have been
successful in plotting the map of Canada (Lambert projection) and the points,
but I have been unable thus far to plot the SAR regions on top of the map. I'm
at the point now where I need help to resolve the issue.
To plot the map of Canada, I have used the following line of code:
map(database= "worldHires","Canada", ylim=c(39,90), xlim=c(-150,-25),
col=alpha("grey90",0.5), fill=TRUE, projection="lambert", param=c(50,65))
Note that the ylim and xlim limits go wider that the actual coordinates of
Canada, but that is necessary because the SAR regions go out to sea quite a
distance. Also, I need the map to go all the way to the North Pole.
To plot the points, I have used a "dummy" list of points which I will
eventually replace with my real data. I convert the points to the lambert projection on
the fly using the following lines of code:
lon <- c(-60, -60, -80, -80.1, -90, -105, -140) #a test longitude vector
lat <- c(42.5, 42.6, 54.6, 54.4, 75, 68.3, 60) #a test latitude vector
coord <- mapproject(lon, lat, proj="lambert", param=c(50,65)) #convert
points to projected lat/long
points(coord, add=TRUE, pch=20, cex=1.2, col=alpha("red", 0.5)) #plot
converted points
As stated, plotting the SAR regions has not worked thus far. The best I have ever
gotten is a square box around the map. I have data files that list the coordinates
of the SAR regions, which is a succession of up to 12100 lat & long points. A
colleague converted those data files into shapefiles defining polygons, with the
coordinates already projected to Lambert. I have tried various options to plot the
regions, but none have worked.
Using readOGR:
region <- readOGR(dsn="C:/myRfolder",layer="mySARshapefile")
plot(region, add=TRUE, xlim=c(-150,-25),ylim=c(39,90),
col=alpha("lightgreen", 0.6), border=TRUE)
You don't state which package readOGR() comes from, but it is not part
of the maps package, which doesn't understand shapefiles, so just using
plot() on top of a (projected) map() is unlikely to succeed.
I believe what you have to do is go back to your lat/long pairs for your
SAR regions and use mapproject() to convert them to the coordinates used
by the plotted projection. Note that you don't need the proj="lambert"
option when you call mapproject() after a call to map() with a
projection because the most recent projection (and its parameters) are
"remembered". Also I suspect (though untested) is that if you put NA
pairs in between your lists of projected SAR regions, then you can just
use lines() to draw them all at once.
Hope this helps,
Ray Brownrigg
Using read.shp and draw.shp:
region <- read.shp("C:/myRfolder/mySARshapefile.shp")
draw.shape(shape=region, type="poly", col="red")
Using readShapePoly:
region <- readShapePoly("C:/ myRfolder/mySARshapefile.shp")
plot(halRegion, add=TRUE, xlim=c(-150,-25),ylim=c(39,90),
col=alpha("lightgreen", 0.6), border=TRUE)
Using readShapeLines after converting the region coordinates to a Lines
shapefile instead of a Polygon shapefile:
region <- readShapeLines("C:/myRfolder/mySARshapefile_lines.shp")
lines(region, col=alpha("black", 0.6))
I have tried playing with spplot, but I haven't quite understood how this one works yet (gives me
an error message: "Error in stack.SpatialPointsDataFrame(as(data,
"SpatialPointsDataFrame"), : all factors should have identical levels")
I would appreciate any help or insight that you could provide to help me get
those boundaries drawn on-top of the country map.
Thanks
Alain Dubreuil
Ottawa, Canada
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