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Best way to depict overlapping territories (polygons)

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#1
Erin John LeFevre

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I'm working on a set of maps that depict territories that overlap. The convention seems to be to use a solid fill on the lowest polygon in the stacking order and then apply a hatched fill to overlaps so that the underlying polygon is still visible. This approach works when you only have one overlap,  but what about when you have three or more overlaps in different combinations? My initial reaction is to create a unique style for each scenario of three or more polygons. This approach generates more classes but reduces overlaps and clutter. Thoughts?

 

If anyone has any guidance or examples on how they approach this issue it would be greatly appreciated. Hopefully, my description makes sense. If it doesn't I can post a sample image and try again.

 

Cheers,

Erin





#2
Kate Leroux

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I often refer back to this elegant map by Gretchen Peterson as a good example of overlapping polygons/lines: http://www.petersong...om/#gallery_2-8



#3
David Medeiros

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I've never been a fan of pattern overlays in maps. I usually use various modes of color transparency (typically multiply) to show overlaps. The unique color combinations create tier own sub symbols in a way, but I leave the main map legend at the regular set of symbols. As yo mention, this can be problematic for many overlaps, or in places where some polygons are fully overlapped and never show their real symbol color.

 

You could look into using a bivariate  color symbology and grid legend. Take a look at the example from Joshua Stevens linked below. 

 

http://www.joshuaste...choropleth-map/

 

Patterns can work, but I think look best with single overlaps and with bold pattern styles (no dots or fine complex textures). 


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#4
David Medeiros

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Another option, assuming what you have are sections with coincident and overlapping boundaries is to offset the boundaries so each shows up in its own symbology as opposed to creating mixed symbols.

 

That map from Petterson could have done that and I think it might have looked a little better (eliminating the muddy looking "overlapping boundaries" class).

http://res.cloudinar...work_riqwui.jpg


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#5
Dennis McClendon

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The first thing I would try is having a horizontal hatching for the second area, and a vertical or slanted one for the third.  Subtle colors, of course, just barely visible if you're not looking for them.  Or use hatching for the second area, and stippling for the third.

 

 

8eWcCiu.png


Dennis McClendon, Chicago CartoGraphics
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#6
Charles Syrett

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This posting is a little late - I'm sure you found a solution long ago - but the suggestions here all look good to me. For existing examples, I immediately thought of historical maps, which often need to show areas of multiple overlap. Here's a scan of a recent National Geographic map, published in "Inside The Medieval World" (which appears not to be available online any more). As you can see, they've used a combination of overlapping fills, boundary "ribbons", and hard boundary lines. In the middle of this map, where you see the text "LOMBARDS", there's an area overlapped by Lombards (light purple ribbon), Avars (light green vignette fill), and Kingdom of the Ostrogoths (solid dark line). The scan doesn't do justice to the original, but you get the idea.

Attached Thumbnails

  • NatGeoOverlapAreasSample4.jpg





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