I've been playing around with some of the Geo* features in Mathematica 10, and I've come across something that seems like it should be simple to do, but I can't quite see how.
If you want to see a map of all the countries in the European Union, it's straightforward:
This will produce a nice map of all the countries in the EU:
However, I'd like to see a map with the polygons joined anywhere they touch. That is, most of continental Europe should be one large polygon, no visible border between Sweden and Finland, etc etc. Is there a simple way to accomplish this, or am I doomed to have internal borders?
I've found a solution to my issue, though I'm ashamed to say I didn't find it sooner. It's not an obvious solution, but it is provided under "Possible Issues" in the GeoGraphics Mathematica 10 documentation. To combine polygons:
multipoly = Polygon[GeoPosition[
Join @@ (EntityValue[EntityClass["Country", "Europe"],
"Polygon"] /. Polygon[GeoPosition[x_]] :> x)]];
Glad to see a previously-unsolved issue is now a one-liner in the documentation.
Do you have example code with output? The snippet doesn't get to the goal.
No matter what you plot the underlying image has borders. You can suppress the edges and hype up the color to obscure the underlying map. Your taste may vary.
GeoListPlot[EntityClass["Country", "EuropeanUnion"], PlotStyle -> Directive[EdgeForm, Opacity[.9]]]
I could add an overall edge if I had documentation on how to add my own "country".
I'd look into GeoGroup.
Otherwise you can form a union (sic) of any two polygon regions with a common vertex and delete all common edges.