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    <p>To plot multiple <code>SpatialPolygons</code> objects on the same device, one approach is to specify the geographic extent you wish to plot first, and then using <code>plot(..., add=TRUE)</code>. This will add to the map only those points that are of interest.</p> <p>Plotting using a projection, (e.g. a polyconic projection) requires first using the <code>spTransform()</code> function in the <code>rgdal</code> package to make sure all the layers are in the same projection. </p> <pre><code>## Specify a geographic extent for the map ## by defining the top-left and bottom-right geographic coordinates mapExtent &lt;- rbind(c(-156, 80), c(-68, 19)) ## Specify the required projection using a proj4 string ## Use http://www.spatialreference.org/ to find the required string ## Polyconic for North America newProj &lt;- CRS("+proj=poly +lat_0=0 +lon_0=-100 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs") ## Project the map extent (first need to specify that it is longlat) mapExtentPr &lt;- spTransform(SpatialPoints(mapExtent, proj4string=CRS("+proj=longlat")), newProj) ## Project other layers can1Pr &lt;- spTransform(can1, newProj) us1Pr &lt;- spTransform(us1, newProj) ## Plot each projected layer, beginning with the projected extent plot(mapExtentPr, pch=NA) plot(can1Pr, border="white", col="lightgrey", add=TRUE) plot(us1Pr, border="white", col="lightgrey", add=TRUE) </code></pre> <p>Adding other features to the map, such as highlighting jurisdictions of interest, can easily be done using the same approach: </p> <pre><code>## Highlight provinces and states of interest theseJurisdictions &lt;- c("British Columbia", "Yukon", "Northwest Territories", "Alberta", "Montana", "Alaska") plot(can1Pr[can1Pr$NAME_1 %in% theseJurisdictions, ], border="white", col="pink", add=TRUE) plot(us1Pr[us1Pr$NAME_1 %in% theseJurisdictions, ], border="white", col="pink", add=TRUE) </code></pre> <p>Here is the result:</p> <p><img src="https://i.stack.imgur.com/t1x2A.jpg" alt="enter image description here"></p> <p>Add grid-lines when a projection is used is sufficiently complex that it requires another post, I think. Looks as if @Mark Miller as added it below! </p>
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    1. COThanks for the detailed example. Do you have any idea why the GADM maps don't show the Great Lakes, but instead just show a polygon northeast of Wisconsin?
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      1. USKevin Wright
 

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