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I have a vector line made of different pieces of lines with different directions. I merged the lines to one. But this merged line still have different direction in its "invisible" parts. I want to plot a profile, so I need ONE direction. Any idea?

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Both qgis and GRASS have tools to switch line direction. In QGIS you need the (experimental) plugin Swap Line direction. In GRASS you can use the v.edit module with "tool-flip" However, in both cases, you'll have to manually select which line segments to flip, and which to leave as they are. I don't think there's any way around that.

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Not sure if it will work for you, but can you create a topology with a sink at the end of the line in the direction you want the line to run?

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My apologies for bumping an old thread. – Barrett May 13 '13 at 18:10

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