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I was looking at some maps like: Map from Ship Logs Map

or the Facebook worldwide friendships Mapped (via the Flowing Data blog) alt text

These Maps look good because single lines are almost transparent, and as more and more lines are drawn, the opacity goes up, and you get patterns only with lines where multiple lines are present.

I have some data of Bus Routes in shapefile format, and would like to do something similar from this.

Which software will allow me to do this? Which software can create this effect?

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    I think you can do it in QGIS using a conjuntion of layer transparency, layer blending mode and symbol transparency. But I would have to try it.
    – Alexandre Neto
    Commented Jan 13, 2014 at 14:55

4 Answers 4

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You can do it in QGIS using symbol transparency, feature blending mode and symbol color.

Notice the difference between Layer Transparency and blending mode(that will be applied to all features) and the symbol transparency and feature blending mode that will stack with other features in the same layer.

All seetings are available in Layers Properties > Style.

enter image description here

With the value of 95% of symbol transparency, the chosen color for the line will be totally opaque only when at least 20 features overlay each other. This number is limited to a 100 features overlay (tranparency 99%).

UPDATE: Following Nathan advice, using different blending modes (like multiply and addition) other effects can be achieved.

enter image description here

Duplicating the layer, with different line colors and using modes like dodge in the layer blending, can give some nice effects.

enter image description here

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  • I'm still thinking about how to do the second image, going from blue (or other color) to white. Any ideas of how to do it in QGIS?
    – Alexandre Neto
    Commented Jan 14, 2014 at 9:48
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    Nope, layer and feature level. Feature blending mode. Even in your first screenshot :)
    – Nathan W
    Commented Jan 14, 2014 at 10:55
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    Go with 50% blue and turn on subtract blending mode.
    – Nathan W
    Commented Jan 14, 2014 at 10:56
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    With a dark background and Addition you will get this: i.imgur.com/tuGiKpc.png
    – Nathan W
    Commented Jan 14, 2014 at 11:01
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    Resuming, +1 for QGIS for having lots of available tools for creating "increasing opacity maps" in a very straightforward way.
    – Alexandre Neto
    Commented Jan 14, 2014 at 11:18
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I'm not sure for the first example, but the second one is an image of the line density (a raster data created based on a kernel that produces an weighted sum of the lines in a neighborhood).

in practice, you can do it with arcgis line density (spatial analyst) and you will find open source hints here

EDIT: the first example was thus well lines with a feature transparency. This is not feasible in ArcGIS (you can set feature transparency for polygons, but it does not add to each other for overlapping polygons of the same layer. In theory, of course, you could make one layer per line...). So +1 for QGIS...

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You can also do this in Tilemill using the following: 

line-comp-op: plus;
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This can be done in ArcGIS, albeit without advanced blending modes: Use "Symbol Levels" under "Advanced" in the "Symbology" tab and use a field for transparency.

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