Combining Geo Polygons?

Questions
1

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:

GeoListPlot[EntityClass["Country", "EuropeanUnion"]]

This will produce a nice map of all the countries in the EU:

EU.jpg
EU.jpg360×349 25.2 KB

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?

2

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.

3

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”.

EU.jpg
EU.jpg613×594 105 KB

4

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.

5

Do you have example code with output? The snippet doesn’t get to the goal.