I've spent the last 5 days trying to figure out how to overlay the  
positions of geographic data on top of a map. I've scoured the man  
pages for the maps, mapproj, maptools, mapdata, and the huge sp  
package. I've experimented with countless permutations of commands.  
So far I've learned a lot about map projections, but I still have no  
ideas how to display my data the way I want! (sigh)

The data are the latitude-longitude coordinates of 1,599 survey sites  
located across the US. I want to plot these sites on a map of the US.  
Seems like a simple task. I've tried converting them to a  
SpatialPoints object and worked out how to project the sites using a  
given map projection (i.e. Bonne). But how do I correctly overlay  
(i.e. display) the data points onto a similarly projected map of the  
US? It might be possible to do it using overlay {sp} but for that it  
seems I would need the coordinates of the geopolitical boundary for  
the map of the US. Can these values be extracted somehow from the US  
map?

Here's how I generated the SpatialPoints object:

#x = longitude values in decimal degrees
#y = latitude values in decimal degrees
#create SpatialPoints object using CRS-class Bonne projection  
centered on longitude 100
S <- SpatialPoints(cbind(x,y),CRS("+proj=bonne +lon_0=100"))

#this next part plots just the data points (cloud of points)
plot(S,pch=20)

How do I overlay the same points on top of a map of the US such that  
they are correctly positioned wrt coordinates?

Also, the R-sig-geo info page does show an URL for a searchable  
archive of R-sig-geo. Does one exist?

Many thanks for your time.
Mike
______________________________
Michael M Fuller, Ph.D.
The Institute for Environmental Modeling
University of Tennessee
1416 Circle Drive
Knoxville, TN 37996-1610




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