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Displaying Overlapping Lines

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Kris Johnson

Kris Johnson


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I have a map with about 50,000 lines running across the globe. When many of them converge or overlap it creates a "fill in" effect. I desire to create more of a intensity or transparency effect when the lines are overlapping in large numbers. See the maps below for some examples. Any advice will be much appreciated.

I'm currently working in Illustrator CS4, but I would welcome other suggestions that make use of free drawing packages as well.






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I have done a similar thing by setting the transparency of each line. But it would get very computational expensive! Note that you can't do it for a whole layer or group in Illy, you would have to make sure you select the individual lines.

Another way is to rasterize the lines and work in either GIS (easier) or Photoshop, not sure how one would work there.
Hugo Ahlenius
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Hi Kris,

I believe in the two examples you give the arcs aren't actually transparent, but just colour coded for length, with lighter routes in lighter colours; since many short flights (e.g., between European cities) would be in places of high air traffic, that might evoke their lightness as being a function of number of crossings, but is actually based on something else.

Don't know much about achieving that sort of effect with CS4, but if you were programming this, you could have lines check for intersection with other lines, count the frequency of crossings, and modify the colour alpha value the line takes with that number. Not sure if that's much help though.


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